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Geochimica et Cosmochimica Acta 133 (2014) 142–155

Adsorption of L-glutamic acid and L-aspartic acid to c-Al2O3

Edward Greiner a,b, Kartik Kumar c, Madhuresh Sumit d,e, Anthony Giuffre a,f,Weilong Zhao h, Joel Pedersen c,g, Nita Sahai h,i,⇑

a Department of Geoscience, 1215 W. Dayton Street, University of Wisconsin, Madison, WI 53706, United Statesb NASA Astrobiology Institute, 1215 W. Dayton Street, University of Wisconsin, Madison, WI 53706, United States

c Molecular and Environmental Toxicology Program, 1300 University Avenue, University of Wisconsin, Madison, WI, 53706, United Statesd Khorana Program, University of Wisconsin, Madison, WI 53706, United States

e Program in Biomedical Sciences, University of Michigan, Ann Arbor, MI 48109, United Statesf Department of Geological Sciences, VirginiaTech, Blacksburg, VA 24060, United States

g Department of Soil Science, 1525 Observatory Drive, University of Wisconsin, Madison, WI 53706-1299, United Statesh Department of Polymer Science, 170 University Avenue, University of Akron, Akron, OH 44325-3909, United States

i NASA Astrobiology Institute, 170 University Avenue, University of Akron, Akron, OH 44325-3909, United States

Received 24 July 2013; accepted in revised form 8 January 2014; Available online 28 January 2014

Abstract

The interactions of amino acids with mineral surfaces have potential relevance for processes ranging from pre-bioticchemistry to biomineralization to protein adsorption on biomedical implants in vivo. Here, we report the results of experi-ments investigating the adsorption of L-glutamic (Glu) and L-aspartic (Asp) acids to c-Al2O3. We examined the extent ofGlu and Asp coverage as a function of pH and solution concentration (pH edges and isotherms) in solution-depletion exper-iments and used in situ Attenuated Total Refkectance Fourier Transform Infrared (ATR-FTIR) spectroscopy to estimate themolecular conformations of the adsorbed molecules. Glu and Asp exhibited similar adsorption behavior on c-Al2O3 withrespect to pH and solution concentration. In general, adsorption decreased as pH increased. At low and high amino acid con-centrations, the isotherms exhibited two apparent saturation coverages, which could be interpreted as 1:4 or 1:2 ratios ofadsorbed molecule/surface Al sites. Tetradentate tetranuclear and bidentate binuclear species were the dominant conforma-tions inferred independently from FTIR spectra. In these conformations, both carboxylate groups are involved in bonding toeither four or to two Al surface atoms, through direct covalent bonds or via H-bonds. An outer sphere species, in which onecarboxylate group interacts with a surface Al atom, could not be ruled out based on the FTIR spectra.� 2014 Elsevier Ltd. All rights reserved.

1. INTRODUCTION

The interactions of amino acids, peptides, nucleobases,lipids and extracellular polymeric substances with mineralsurfaces play a key role in many biological andbiogeochemical processes such as protein adsorption to

http://dx.doi.org/10.1016/j.gca.2014.01.004

0016-7037/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: Department of Polymer Science, 170University Avenue, University of Akron, Akron, OH 44325-3909,United States. Tel.: +1 330 972 5795.

E-mail address: sahai@uakron.edu (N. Sahai).

biomedical implant devices, biomineralization, and,potentially, pre-biotic chemistry, and the origin and earlyevolution of life (Goldschmidt, 1952; Oparin, 1924; Bernal,1951; Cairns-Smith, 1986; Lowenstam and Weiner, 1989;Cleaves et al., 2012; Sahai, 2002a; Xu et al., 2009, 2012,2013; Oleson and Sahai, 2008, 2010; Oleson et al., 2010,2012). Of relevance to pre-biotic chemistry, in particular,L-amino acids are enriched in carbonaceous chondriticmeteorites (Cronin and Pizzarello, 1983; Glavin andDworkin, 2009), and mineral surfaces may facilitate thepolymerization of amino acids into peptides (Bernal,

E. Greiner et al. / Geochimica et Cosmochimica Acta 133 (2014) 142–155 143

1951; Orgel, 1998; Lambert, 2008) and of activated RNAmonomers into oligomers (Ferris et al., 1996).

Among the amino acids, most previous studies havefocused on the adsorption of glutamic (Glu) acid, aspartic(Asp) acid and lysine at oxyhydroxide surfaces, includingCr(OH)3, c-Al2O3, amorphous Fe(OH)3 or hydrous ferricoxide (HFO), goethite (a-FeOOH), lepidocrocite (c-FeO-OH), maghemite (c-Fe2O3), rutile (a-TiO2), anatase (b-TiO2), amorphous TiO2, and amorphous SiO2. Theseamino acids have negatively and positively charged sidechains and may be expected to adsorb to inorganic solidsurfaces of the opposite surface charge and to show limitedadsorption on negatively charged oxide surfaces(Kumanomido et al.; 1978; Davis and Leckie, 1978;Giacomelli et al., 1995; Roddick-Lanzilotta et al., 1998;Roddick-Lanzilotta and McQuillan, 1999; Fitts et al.,1999; Sousa et al., 2001; Rubim et al., 2001; Ojamaeet al., 2006; Sverjensky et al., 2008; Paszti and Guczi,2009; Paszti et al., 2008; Lopes et al., 2009; Parikh et al.,2011; Jonsson et al., 2009, 2010; Bouchoucha et al.,2011). A few workers have examined adsorption ofnon-polar and aromatic amino acids, such as glycine,alanine and tryptophan (Kumanomido et al., 1978; Gaoet al., 2009; Lopes et al., 2009; Ojamae et al., 2006).

The adsorption of amino acids on c-Al2O3 has not beenstudied, to date, in any detail. Oxide and oxyhydroxideminerals have been used for decades to study adsorption,because they commonly occur as coatings on aluminosili-cate minerals as secondary weathering products. Many re-cent studies have focused on rutile, even though it isusually only a minor accessory phase in most rocks andsoils. Aluminum and ion oxides and hydroxides are highlyinsoluble, so their amorphous or poorly crystalline formsare especially common, and hence, relevant to the fieldsof earth sciences and astrobiology. Thus, a gap exists inthe literature with regard to amino acid adsorption onc-Al2O3. Furthermore, rutile crystallizes in the tetragonalcrystal system compared to the cubic crystal system ofc-Al2O3, so the adsorption behavior of molecules may beexpected to differ on the two surfaces. The binding ofCu(II)-glutamate complexes was examined at the c-Al2O3

surface by X-ray absorption spectroscopy and FTIR in

air (Fitts et al., 1999). The emphasis of that study was onunderstanding Cu binding to c-Al2O3 as modified by thepresence of glutamate, without an emphasis on glutamatealone. Thus, no systematic study utilizing in situ spectros-copy exists to date for Glu or Asp adsorption to c-Al2O3,nor detailed solution adsorption experiments, includingboth isotherms and “pH edges,” and no combined analysisof both spectroscopic and solution experiments.

It is also worthwhile noting that the adsorption of ami-no acids or other small molecules, such as carboxylic acidsand L-DOPA, to metal oxide surfaces is usually studiedeither by pH-dependence in bulk solution (pH-edges) orby spectroscopic techniques such as ATR–FTIR Raman,nuclear magnetic resonance or X-ray absorption spectros-copy (Kumanomido et al.; 1978; Davis and Leckie, 1978;Yost et al., 1990; Fitts et al., 1999; Dobson and McQuillan,1999; Sousa et al., 2001; Rubim et al., 2001; Johnson et al.,2004a,b; Rotzinger et al., 2004; Persson and Axe, 2005; Hug

and Bahnemann, 2006; Hwang et al., 2007; Sverjenskyet al., 2008; Lopes et al., 2009; Parikh et al., 2011; Jonssonet al., 2009, 2010; Bahri et al., 2011), but isotherms

are rarely measured in combination with pH-edges and

spectroscopy. Adsorption isotherm data provide indepen-dent information on maximum surface coverage and reac-tive site density for interpretation of FTIR spectra andfor surface complexation modeling. In the present study,therefore, we examined the interactions of L-Glu and L-Asp on c-Al2O3 as a function of solution concentrationand pH (isotherms and pH edges) and characterized thegeometry of the adsorbed species using ATR-FTIR. Themolecular structures of Glu and Asp are very similar, differ-ing only by the presence of a single additional methylene(�CH2�) group in the side chain of Glu and their proton-ation reactions occur at similar pH values (Table 1), leadingto the expectation of similar adsorption behavior.

2. METHODS

2.1. Characterization of c-Al2O3 powder

c-Al2O3 powder (Sigma) was dialyzed in 18 MX cmresistivity deionized water (Nanopore UV, Barnstead,Dubuque, IA, USA) for 3 days to remove any impurities.The manufacture provided primary particle size is <50 nmand sub-spherical secondary aggregates with hydrodynamicdiameters of approximately 180 nm were observed by dy-namic light scattering (Zetasizer NanoZS, Malvern, UK),and a surface area of 26.6 ± 0.1 m2 g�1 was determinedby a multi-point B.E.T. N2 gas adsorption isotherm(Micromeritics, Gemini).

2.2. Bulk adsorption experiments

All glassware was cleaned in a 15% HCl bath for 48 hand rinsed three times with 18 MX cm water prior to use.Solutions of L-Glu and L-Asp were prepared from crystal-line powder (Sigma, 99% purity) without purification. Theionic strength of the solutions was set to 100 mM usingNaCl (Sigma), and the pH was adjusted using 1.0 M HCland NaOH (Fisher). Experiments were conducted in1.5 mL polypropylene microcentrifuge tubes at a solid con-centration of 20 mg mL�1, and amino acid concentrationsranging from 0.01 to 100 mM. Suspensions were shakenwith a vortexer for �12 h at ambient temperature(�22 �C). This incubation time is typical of adsorptionstudies for amino acids on oxide phases. The pH was mea-sured before and after the adsorption reaction and wasfound to be stable within 0.2 units over the reaction time.Samples were centrifuged in microcentrifuge tubes at30,000g for 35 min, and the supernatent was analyzed todetermine amino acid concentration. Each adsorptionexperiment was conducted in triplicate.

2.3. Amino acid analysis

For amino acid quantification, ninhydrin reagent (2%)in 75% dimethyl sulfoxide and lithium acetate buffer, pH5.2 (Sigma), and ethanol (Fisher, 200 proof) were used

Table 1Acid base properties of c-Al2O3 and the aqueous amino acids. Surface acidity constants (pKs

a1 and pKsa2) and the pH of the point of zero

charge (pHpzc) are provided for the c-Al2O3 surface. Aqueous acidity constants (pKa1 and pKa2) and the isoelectric point (IEP) are providedfor the amino acids. The corresponding values in D2O calculated by Eq. (1) are also provided to facilitate interpretation of the FTIR spectraobtained in D2O.

pKsa1 pKs

a2 pHpzc

c-Al2O3a,b 6.3, 6.3 9.3, 9.6 8.0, 8.2

pKa1 or pKa, a-COOH in H2O,D2O

pKa2 or pKa, R-COOH in H2O, D2O pKa3 or pKa; NHþ3

in H2O,D2O

IEP in H2O, D2O

L-Aspc 1.9, 1.6 3.9, 3.7 10.0, 10.3 2.9, 2.7L-Glud 2.2, 1.9 4.3, 4.2 9.9, 10.2 3.2, 2.9

a Values from: Sverjensky and Sahai (1996).b Sprycha (1989).c DeRobertis et al. (1991).d Smith and Martell (2004).

144 E. Greiner et al. / Geochimica et Cosmochimica Acta 133 (2014) 142–155

without further treatment. Each amino acid sample wasderivatized with ninhydrin (McCaldin, 1960; Moore,1968; Lamoth and McCormick, 1972) and analyzed byUV–visible absorbance spectrophotometry. For derivatiza-tion, one part (500 lL) sample was added to two parts nin-hydrin reagent, and heated for 15 min at 100 �C to form thecompound Ruhemann’s purple. After 15 min, three partsethanol were added to quench the reaction, and the vialwas cooled in a water bath prior to measuring absorbanceat k = 570 nm (UV-1700 spectrophotometer, Shimadzu).Each triplicate sample was analyzed twice. The quantityof amino acid was calculated using a standard curve ofknown amino acid concentrations analyzed at the sametime as the samples. The amount of amino acid adsorbedto the c-Al2O3 surface was calculated as the difference be-tween the total initial amount added and the final amountremaining in solution (“equilibrium concentration”) afterreaction with c-Al2O3. Sorbent-free blanks were run to ac-count for any loss of Glu or Asp to microcentrifuge tubewalls.

2.4. Attenuated Total Reflectance-Fourier Transform

Infrared Spectroscopy

Solutions of L-Glu and L-Asp were prepared in D2O forATR-FTIR experiments (99.8% purity, Acros) to shiftsolvent peaks away from the spectral region of interestfor carboxyl vibrations (1700–1400 cm�1). Solution pDwas adjusted by addition of 1.0 M DCl or 1.0 M NaOD,(99.8% purity, Acros). The relationship between pKa inH2O and in D2O is given by the equation (Krezel andBal, 2003):

pKa;H2O ¼ 0:929pKa;D2O þ 0:42 ð1Þ

Vibrational spectra were recorded on a Digilab FTS7000infrared spectrophotometer, equipped with a mid-IRsource, a KBr beam splitter and a deuterated triglycine sul-fate detector. A 10-bounce germanium (Ge) internal reflec-tance element (IRE) with a face angle of 45� in a horizontaltrough (Pike Technologies, Inc.) was employed for theseexperiments. Spectra were collected with Digilab WinIR

Pro software package. Each spectrum represents 100 scansat 4 cm�1 resolution using triangular apodization.

Spectra of amino acids in solution and associated withthe c-Al2O3 film (see below) were acquired using a Ge crys-tal and c-Al2O3-coated Ge crystal background, respectively,with subsequent subtraction of spectra of solvent or sol-vated c-Al2O3 at the appropriate pD (100 mM NaCl inD2O; peaks centered at 2800 cm�1 and 1200 cm�1 (O�Dstretching and D�O�D bending vibrations) used for sub-traction; subtraction factor = 0.6 � 1.0). Any residualwater and water vapor introduced due to atmospheric dif-fusion during sample reaction time was subsequently sub-tracted. The subtracted spectra were smoothed (5–7 pointbinomial function), normalized, and, when necessary,slightly baseline corrected (2-point, applied between 3500and 600 cm�1).

Solutions of 100 mM NaCl and aqueous amino acids atspecific concentrations and pH values were deposited viapipette (200 lL) onto the IRE surface, and allowed to standundisturbed for 30 min followed by acquisition of spectra.The chamber was constantly purged with N2 gas and thesample was maintained at 25 �C.

The IRE surface was coated with a thin film of c-Al2O3

as follows. A 100 mg mL�1 ethanolic suspension of c-Al2O3

was centrifuged in 2 mL microcentrifuge tubes at 100g for5 min to sediment particles larger than 100 nm accordingto Stokes’ Law. The suspension (25 lL cm�2) was pipettedonto the IRE surface. The ethanol was allowed to evapo-rate for 30 min, and residual ethanol was removed by rins-ing three times with 500 lL D2O. The rapid evaporation ofethanol prevented visible aggregates of c-Al2O3 from form-ing during film deposition. The coating was stable over thetime period needed to collect multiple scans to produce thespectra of the bound species. In contrast, coatings formedfrom aqueous suspensions contained visible aggregates ofalumina, were unstable, and flaked off during multiplescans, compromising reproducibility. The concentrationsof L-Glu and L-Asp in solution were below the limit ofdetection when an uncoated IRE was employed and, there-fore, did not contribute to the spectra of the species ad-sorbed to the c-Al2O3 film.

E. Greiner et al. / Geochimica et Cosmochimica Acta 133 (2014) 142–155 145

3. RESULTS

3.1. Adsorption (Solution Depletion) experiments

3.1.1. L-Glu adsorbed to c-Al2O3

Adsorption of L-Glu to c-Al2O3 as a function of pH (pHedges) at total initial ligand concentrations of 1.0, 0.1, and0.01 mM in solution is shown in Fig. 1a (data reported inAppendix A.1). At the two lower concentrations, the pHedges showed the largest amount of adsorption at lowerpH values and adsorption decreased up to pH � 7–8(Fig. 1a). These results were consistent with expectationsfor negatively charged ligands adsorbing on a positivelycharged surface. At the highest concentration, pH depen-dence of adsorption was minimal. The apparent small in-crease in adsorption at the lower concentrations as thepD increased above the pHpzc � 8–9 was within statisticalerror and were not considered to be significant. A muchhigher percent of total ligand concentration was adsorbedat lower equilibrium concentrations compared to the frac-tion adsorbed at higher concentration.

Adsorption isotherms for L-Glu to c-Al2O3 at pH 4.2and 6.0 are shown as a function of the final equilibriumconcentrations of ligands in solution (Fig. 1b; AppendixA.2). Within statistical error, no pH effect was observedon adsorption isotherms within the pH and concentration

Fig. 1. L-Glu adsorption on c-Al2O3 at room temperature (a) pH-edges at [L-Glu] = 1.0 mM (j), 0.1 mM ( ) and 0.01 mM ( ), and(b) isotherms at pH 4.2 ( ) and 6 ( ). Particle load-ing = 20 mg�mL�1 and solution ionic strength = 0.1 M NaCl.Error bars indicate standard deviation of triplicate samples.

ranges studied. This result is consistent with the pH edgedata, and is expected because c-Al2O3 is positively chargedat both pHs. Adsorption increased as the equilibrium con-centration increased, and a subtle plateau in the data wasdiscernible at �1 lmoles�m�2, which corresponds to0.5 molecule�nm�2; a further increase was then observedto a final saturation coverage of �1.7 lmoles�m�2 corre-sponding to one Glu molecule�nm�2 (Fig. 1b).

3.1.2. L-Asp adsorbed to c-Al2O3

The pH edges and isotherms for L-Asp on c-Al2O3 areshown in Fig. 2 (data reported in Appendix B). Adsorp-tion behavior obtained for Asp was very similar to thatfor L-Glu. At lower initial concentrations, the pH edgesshowed higher adsorption at lower pH and decreased withincreasing pH until the isoelectric point of c-Al2O3 atpH � 8–9 (Fig. 2a). As with L-Glu, the slight increase inadsorption at the lower concentrations above the IEP werewithin statistical error, and no pH dependence was ob-served at the initial concentration of 1 mM. The isothermsalso showed no pH-dependence. Two adsorption plateauxat �1.0 and 1.6 lmoles�m�2, corresponding, respectively,were observed to 0.5 molecule�nm�2 and 1 molecule�nm�2

(Fig. 2b). The initial plateau was more evident for Aspthan for Glu.

Fig. 2. L-Asp adsorption on c-Al2O3 at room temperature (a) pH-edges at [Asp] = 1.0 mM (j), 0.1 mM ( ) and 0.01 mM ( ), and(b) isotherms at pH 3.8 ( ) and 6 ( ). Particle load-ing = 20 mg mL�1 and solution ionic strength = 0.1 M NaCl.Error bars indicate standard deviation of triplicate samples.

146 E. Greiner et al. / Geochimica et Cosmochimica Acta 133 (2014) 142–155

3.2. ATR-FTIR spectra

3.2.1. Aqueous glutamate species

The ATR-FTIR spectra of 100 mM L-Glu in 0.1 MNaCl solution and at various pD values are shown inFig. 3a. The pD values encompassed the pKa values ofthe primary (-a) and side chain (-R) carboxylate groupsand the amine group (Table 1). The observed peak posi-tions were consistent with the vibrational modes of carbox-ylate functional groups (Pearson and Slifkin, 1972;Mehrotra and Bohra, 1983; Yost et al., 1990; Dobson andMcQuillan, 1999; Fitts et al., 1999; Roddick-Lanzillottaand McQuillan, 2000; Borer et al., 2009; Parikh et al.,2011; Jonsson et al., 2009, 2010). A schematic representa-tion of the infrared vibrational modes of the carboxylategroup is provided in Appendix D.

The FTIR spectra are interpreted based largely on pre-viously published studies of amino acids in solution (Pearsonand Slifkin, 1972; Roddick-Lanzilotta and McQuillan,2000). Peak assignments are summarized in Table 2. Thelowest pD of 1.7 was below the pKa values of both –COOHgroups in D2O. However, pD 1.7 was very close to the pKa

in D2O of 1.9 for the a-COOH group, and �39% of thea-COO� groups would be deprotonated. The peak at1700 cm�1 was attributed to the combined carbonylstretches of both –COOH groups (mC@O,COOH). Deprotona-tion of the a-carboxyl group results in peaks at 1610 and�1400 cm�1, assigned to the asymmetric and symmetricstretching modes of the deprotonated a-COO� group(masy,a-COO� and msym,a-COO�). The masy,a-COO� is usuallyexpected at �1550 cm�1, but may have been shifted to ahigher wavenumber (1610 cm�1), because of the proximityof the amine (�NHþ3 ) group (Pearson and Slifkin, 1972;Roddick-Lanzilotta and McQuillan, 2000). The asymmetricand symmetric stretching modes of the amine (masy;NHþ

3and

Fig. 3. Infrared spectra of L-Glu at various pD values (a) in solution ac-Al2O3film. The solution spectrum at 100 mM concentration and pD =Table 2.

msym;NHþ3) occur at 1620 and 1530 cm�1. A small contribu-

tion from masy;NHþ3

was perhaps present in the shoulder ofthe 1610 cm�1 band, but it was difficult to discern, andwe did not see the 1530 cm�1 band. These bands were alsonot observed in previous studies of glutamic acid (Pearsonand Slifkin, 1972; Roddick-Lanzilotta and McQuillan,2000). The bands at 1450 and 1350 cm�1 arose from thebending (dCH2

) and wagging (xCH2) modes of the methylene

(�CH2) backbone groups. As expected, these peaks did notchange much over the pD range studied.

At pD 2.8, the a-carboxy group was almost completelydeprotonated, whereas the side chain group (R-COOH)was still protonated, because its pKa = 4.2 in D2O. Thus,at this pD, the 1700 cm�1 band was predominantly due tomsym,C@O,COOH for the R-COOH group. The assignmentof the other bands was the same as at pD 1.7.

At pD > 4.2, the R-carboxyl group was increasinglydeprotonated, and correspondingly, the intensity of the1710 cm�1 band decreased. A new peak appeared at�1550 cm�1 attributed to masy,R-COO�. The msym,R-COO� alsocontributed to the 1400 cm�1 peak. The intensity of both the1550 and 1400 cm�1 stretching modes increased withincreasing pD. The relative intensity of the masy,COO� forboth carboxyl groups (at 1610 and 1550 cm�1) decreased rel-ative to that of the mixed symmetric stretch msym,COO� at1400 cm�1, because the carboxyl groups were increasinglydeprotonated. Also, at pD 10.3, approximately half of the�NHþ3 groups were deprotonated at pD = pKa3 = 10.2 inD2O, thus reducing the intensity of the 1610 cm�1 peak evenfurther. It is also possible that any small contribution at1620 cm�1 from masy;NHþ

3was diminished.

3.2.2. Glutamate surface complexes on c-Al2O3

Spectra for L-Glu adsorbed on c-Al2O3 in 100 mM NaClsolution are shown in Fig. 3b at [L-Glu] = 3.0 mM and pD

t concentration = 100 mM, and (b) adsorbed to the surface of the4.9 is shown in (b) for reference. Peak assignments are provided in

Table 2Vibrational frequency (cm�1) assignments for 100 mM L-Glu in aqueous solution and adsorbed on c-Al2O3. The solution conditions for thesurface complexes were chosen to correspond to the low and high concentration conditions of the adsorption isotherms.

Assigned Mode, a,b aqueous L-Glu peaks

pD 1.7 pD 2.8 pD 4.9 pD 7.1 pD 9.2 pD 10.3

mC@O, COOH 1715 1704 1705masy,a-COO� 1610 1612 1607 1610 1605 1610masy,R-COO� 1555 1557 1555 1560dCH2

1454 1454 1450 1450 1450 1451msym,COO� 1406 1407 1400 1403 1401 1400xCH2

1350 1350 1352 1354 1354 1350

Assigned Mode, a,b adsorbed 7 mM L-Glu 3 mM L-Glu

pD 2.8 pD 6.0 pD 3.8

masy,a-COO� 1640 1639 1639masy,a-COO� 1606 1606 1606masy,b-COO� 1560 1560 1560dCH2

1452 1450 1450msym,a-COO�; msym,R-COO� 1408 1408 1411xCH2

1322 1330 1331

a Vibration mode: m = stretching, d = bending, x = wagging.b Vibrational band assignments based on previous studies; Pearson and Slifkin (1972), Roddick-Lanzilotta and McQuillan (1999), and

Parikh et al. (2011).

E. Greiner et al. / Geochimica et Cosmochimica Acta 133 (2014) 142–155 147

3.8, and at [L-Glu] = 7.0 mM and pD 2.8 and 6.0. Thesesolution conditions were chosen for comparison of the spec-tra with the low and high concentration regions and bothpH values of the bulk adsorption isotherms. The goal wasto determine the conformations of the surface complexesat these different surface loadings. The spectrum for100 mM L-Glu at pD 4.9 in 100 mM NaCl solution isincluded in Fig. 3b to facilitate comparison with thespectra of the adsorbed species. The spectra of adsorbedL-Glu at solution conditions studied were similar, exceptthat peak intensities at 3.0 mM L-Glu were weaker asanticipated for a lower concentration. Spectra at pD 10.3,could not be obtained likely due to the dissolution of thec-Al2O3 thin film on the IRE before accurate scans couldbe acquired.

The FTIR spectra were interpreted based largely on pre-viously published studies of amino acids in solution and ad-sorbed at TiO2 surfaces (Pearson and Slifkin, 1972;Roddick-Lanzilotta and McQuillan, 2000; Parikh et al.,2011). Peak assignments are summarized in Table 2. Thespectra of the adsorbed species were similar under all thesolution conditions examined. The peaks at 1606, 1560,1452, 1408 and 1330 were assigned in the same way as forthe aqueous species.

However, the spectra of adsorbed Glu also exhibitedsome differences compared to the solution spectra at corre-sponding pD values. First, the bands for adsorbed specieswere broader than those for the corresponding solutionspecies. The peak centered at �1610 cm�1 correspondingto masy,a-COO� was still present and represented a bondingstructure of the adsorbed species that was similar to thatin aqueous solution. The band was broader, however,than in aqueous solution and a new shoulder was observed

at �1640 cm�1. The �1640 cm�1 band was assigned tomasy,a-COO�, for a population of molecules present in adifferent coordination structure.

The 1560 cm�1 peak corresponding to masym,R-COO� ap-peared at pD 2.8, but was not seen until pD 4.9 in the aque-ous species. Similarly, the peak at 1710 cm�1 assigned to thesymmetric stretch for both �COOH groups (mC@O,COOH)was missing at pD 2.8 in the spectrum of the adsorbedspecies.

Peaks at similar wavenumbers were obtained in previ-ous studies of glutamate and aspartate adsorption onamorphous TiO2 and rutile (Roddick-Lanzilotta andMcQuillan, 2000; Jonsson et al., 2009, 2010; Parikhet al., 2011), Cu-glutamate complexes on c-Al2O3 (Fittset al., 1999), and carboxylic acids on hematite, goethite,lepidocrocite, rutile and anatase (Hug and Sulzburger,1994; Persson and Axe, 2005; Hug and Bahnemann,2006; Hwang et al., 2007).

3.2.3. Aspartate aqueous and surface complexes on c-Al2O3

Spectra for L-Asp in 100 mM NaCl solution at variouspD values are shown in Fig. 4a and for L-Asp adsorbedon c-Al2O3 in 100 mM NaCl solution at different concen-tration and pH values are shown in Fig. 4b. Peak assign-ments for the solution and surface species of L-Asp aresummarized in Table 3. The vibrational features of theaqueous L-Asp aqueous species and the L-Asp surface com-plexes were very similar to the corresponding L-Glu spectra.The masym,R-COO� and dCH2 bands for the adsorbed Asp spe-cies were less well defined than for the corresponding Gluspecies, but the peak positions were similar. Similar to theGlu system, adsorbed L-Asp spectra could not be obtainedat pD 10.3.

Fig. 4. Infrared spectra of L-Asp at various pD values (a) in solution at concentration = 100 mM, and (b) adsorbed to the surface of thec-Al2O3film. The solution spectrum at 100 mM concentration and pD = 4.9 is shown in (b) for reference. Peak assignments are provided inTable 3.

Table 3Vibrational frequency (cm�1) assignments for L-Asp in D2O and L-Asp adsorbed to c-Al2O3. The solution conditions for the surfacecomplexes were chosen to correspond to the low and high concentration conditions of the adsorption isotherms.

Assigned Mode,a,b aqueous L-Asp peaks

pD 1.7 pD 2.8 pD 4.9 pD 7.1 pD 9.2 pD 10.3

mC@O,–COOH 1709 1707 1704masy,a-COO� 1612 1609 1610 1611 1613 1607masy,b-COO� 1570 1565 1553 1566 1567 1567dCH2

1413 1419 1425 1423 1427 1419msym, a-COO�; msym,b-COO� 1401 1400 1397 1396 1395 1397xCH2

1345 1347 1349 1351 1347 1350dCH2

1301 1297 1307 1310 1309 1312

Assigned Mode, a,b adsorbed 7 mM L-Asp 3 mM L-Asp

pD 2.8 pD 6.0 pD 3.8

masy,a-COO� 1635 1634 1634masy,a-COO� 1600 1605 1602masy,b-COO� 1576 1578 1578dCH2

1413 1412 1420msym, a-COO�; msym,b-COO� 1399 1405 1402xCH2

1365 1354 1367

a Vibration mode: m = stretching, d = bending, x = wagging.b Band assignments are based on previous studies; Pearson and Slifkin (1972), Roddick-Lanzilotta and McQuillan (1999), and Parikh et al.

(2011).

148 E. Greiner et al. / Geochimica et Cosmochimica Acta 133 (2014) 142–155

4. DISCUSSION

4.1. Potential binding geometries of L-Glu to c-Al2O3 surface

The structures of the possible the surface-complexes ofL-Glu to the surface of c-Al2O3 are shown in Fig. 5. Anal-ogous structures are expected for L-Asp. We have adoptedthe IUPAC nomenclature for the bonding structures in

Fig. 5 (Muller, 1994). The number of bonds from the ligandto the surface defines the “denticity” of the ligand as mono-dentate, bidentate, tridentate, tetradentate, etc., and thenumber of surface Al atoms involved in the complex definesthe “nuclearity” of the surface complex. If both amino acidcarboxylate groups are involved in binding to the surface,they are also called “bridging” (e.g., Fig. 5e–h). Structureswith direct bonds between the amino acid and the

Fig. 5. Possible geometries of L-Glu carboxylate functional group binding to c-Al2O3. (a) Monodentate mononuclear geometry, attached byone bond from ligand to one surface metal atom surface site; (b) bidentate mononuclear or bidentate chelate, attached by two bonds fromligand to one surface metal atom; (c) bidentate binuclear, attached by two bonds from ligand to two surface metal atoms; (d) H-bondedbidentate binuclear; (e) bidentate bridging binuclear, attached by one bond from each carboxyl group to one surface metal atom each; (f)tridentate bridging trinuclear, attached by two bonds from one carboxyl group to two surface metal atoms and by one bond from the othercarboxyl group to a surface metal atom; (g) tetradentate tetranuclear or bidentate bridging tetranuclear; (h) H-bonded tetradentatetetranuclear; (i) outer sphere, with water molecule separating ligand from surface metal atom; (j) electrostatic interactions between ammoniumfunctional group and deprotonated surface > AlO� group. Modified from Roddick-Lanzilotta and McQuillan (2000).

E. Greiner et al. / Geochimica et Cosmochimica Acta 133 (2014) 142–155 149

c-Al2O3 surface are referred to as inner sphere or covalentlybonded surface complexes (Fig. 5a–c, e–g). Hydrogen-bonded complexes refer to those species where a H-bondexists between the surface >AlOH sites and the ligand(Fig. 5d and h). Outer sphere surface complexes interactelectrostatically with the surface charge (Fig. 5i and j).We note that different nomenclatures have been used inthe literature for bonding structures, which hinders com-parison of results among studies, so we have provided acomparison of nomenclature from our study with thoseused in two key studies (Mehrotra and Bohra, 1983; Rod-dick-Lanzilotta and McQuillan, 2000) (Appendix C).

4.2. ATR-FTIR spectra of L-Glu and L-Asp adsorbed to

c-Al2O3

A comparison of the adsorbed and aqueous amino acidspectra showed that the �1715–1704 cm�1 peak

(mC@O,COOH) was missing at all pD values on the adsorbedspecies spectra, and the 1560 cm�1 peak (masym,R-COO-) ap-peared at pD 2.8 which is lower than the R-COO� group’spKa in D2O for both amino acids. These observations indi-cated that both carboxyl groups were deprotonated uponadsorption (Figs. 3 and 4; Tables 2 and 3). These resultsare consistent with those of previous studies examiningadsorption on other oxide surfaces under solution condi-tions where those oxides were positively charged; for exam-ple, amino acids to rutile or c-Al2O3 (Fitts et al., 1999;Parikh et al., 2011) and carboxylic acids to hematite(Hwang et al., 2007), goethite (Yost et al., 1990; Perssonand Axe, 2005), rutile, anatase and lepidocrocite (Hugand Sulzburger, 1994; Hug and Bahnemann, 2006).

Furthermore, little or no adsorption was observedpreviously for amino acids with neutrally charged sidechains or for carboxylic acids possessing only a singlecarboxyl group, such as glycine and glutamine on rutile

150 E. Greiner et al. / Geochimica et Cosmochimica Acta 133 (2014) 142–155

(Roddick-Lanzilotta and McQuillan, 2000), and acetic acidon rutile (Dobson and McQuillan, 1999). Thus, the pres-ence of the R-carboxyl group was key for the adsorptionof L-Glu on oxides such as c-Al2O3.

As noted previously by Roddick-Lanzilotta and McQui-llan (2000), vibrational peaks of adsorbed amino acids werebroader than those of the aqueous species. This phenome-non was expected because the adsorbed species were likelypresent in a number of different bonding environments.Thus, the broadening of the masym,R-COO� peak centered at�1610 cm�1 and the appearance of the shoulder at�1640 cm�1 (Figs. 3 and 4; Tables 2 and 3) were ascribedto the presence of amino acids adsorbed in two differentbonding structures. Previous work has also shown that in-ner sphere coordination between surface metal cationsand carboxyl oxygens induced the appearance of this new,shifted peak at �1640 cm�1 for asymmetric stretchingmodes, for amino acid adsorption at solution conditionswhere other oxide surfaces were also positively charged;for example, c-Al2O3 (Persson et al., 1998), goethite (Yostet al., 1990), lepidocrocite (Borer et al., 2009), and rutile(Parikh et al., 2011). Such interactions could perhaps havealso accounted for the 1610 band-broadening and new1640 cm�1 peak in our study, but this inference needed tobe verified based on further analysis of the FTIR spectra,as discussed below.

The deprotonated a-carboxyl or R-carboxyl group mayhave formed covalent, inner sphere bonds or H-bonds withsurface sites in multiple configurations (Fig. 5a–h). Outer-sphere or electrostatic interactions (Fig. 5i and j) involvingthe carboxyl and amine functional group with charged sur-face sites were also possible. To distinguish between thesepossibilities, we used an approach developed previouslyfor aqueous systems in which the stretching bands foruncomplexed carboxylic acid were compared to those fora carboxylic acid complexed to an aqueous metal ion. Weextended this approach in our study to compare the carbox-ylate stretching bands for aqueous L-Glu and L-Asp to thebands for the adsorbed species where the amino acid wascomplexed to the surface Al atoms.

In detail, the difference between the asymmetric andsymmetric stretching frequencies of carboxylate groupswere defined previously as DmL,aq = masym � msym, wherethe subscript, “L, aq” refers to the uncomplexed ligand insolution (Mehrotra and Bohra, 1983). The correspondingvalue for the metal–ligand aqueous complex was definedas, DmML,aq = masy � msym. A comparison of DmL,aq andDmML,aq in solution could yield insight into bonding modesof aqueous metal–carboxylate complexes (Mehrotra andBohra, 1983). We note that the nomenclature used byMehrotra and Bohra (1983) differs slightly from ours; theydefined ionic or electrostatic, monodentate, bidentate che-late, bidentate bridging structures in solution correspond-ing, respectively, to our surface complexes in Fig. 5i, a–c.We used the IUPAC nomenclature defined above inSection 4.1.

The value of DmL,aq is �150–215 cm�1 for many carbox-ylate ligands (Mehrotra and Bohra, 1983), consistent withour result of DmML,aq = 152 and 205 cm�1 for aqueous L-Glu or L-Asp (Table 4). In monodentate binding, the

carbonyl groups have a lower symmetry compared to theuncomplexed ligand, with an increase in the asymmetricstretching frequency and a decrease in the symmetricstretching frequency, and therefore, DmML,aq is much largerrelative to DmL,aq. A large DmML,aq value of �300–550 cm�1

indicates a monodentate binding geometry, such as inFig. 5a. However, when bonded to a metal ion as an innersphere (covalent), bidentate bridging species (Fig. 5c), thesymmetry of the carboxyl group is similar to that of theuncomplexed carboxylate. In such scenarios, DmML,aq andDmL,aq have similar values. Finally, bidentate chelation(Fig. 5b) may result in a reduction of the O–C–O bond an-gle with a corresponding small value of DmML,aq < �60–90 cm�1, although they caution that this is not always thecase (Mehrotra and Bohra, 1983).

At all pD values and concentrations studied here,adsorbed Glu and Asp species showed DmML,ads = 149–176 cm�1 for the R-carboxyl group and the Dm ML,ads -� 195–236 cm�1 for the a-carboxyl group (Table 4). TheseDmML,ads values were much smaller than �300–550 cm�1

and much larger than �60–90 cm�1, allowing us to ruleout monodentate mononuclear (Fig. 5a) and bidentatemononuclear (or chelate) binding (Fig. 5b). ExtendingMehrotra and Bohra’s concept to the structures shown inFig. 5e, f, we would expect a large DmML, ads value, becauseof the asymmetry of the structure. Thus, we also ruled outstructures 5e and 5f. Rather, the Dm ML, ads range obtainedfor the adsorbed complexes was similar to the DmL,aq valuesfor the uncomplexed amino acids, indicating structureswhich involve one or both carboxylate groups and two orfour surface Al atoms (Fig. 5c, d, g, and h). Thus, the infer-ence of two different bonding structures for the a-COO�

group from the FTIR spectra probably reflected bidentatebinuclear (Fig. 5c) and tetradentate tetranuclear (Fig. 5g).We note that based on the FTIR spectra we were not ableto discriminate between covalently bonded and H-bondedspecies of the same denticity and nuclearity (e.g., Fig. 5cversus Fig. 5d; or Fig. 5g versus Fig. 5h).

We also expected that the carboxyl group symmetry inthe outer sphere surface complex and in the electrostati-cally-bonded species involving the amine functional group(Fig. 5j) would be similar to that of aqueous, uncomplexedamino acids, resulting in similar Dm values. Based on theabsence of the �NHþ3 stretching band, previous studies ofL-Glu and L-Asp adsorption on rutile from pH 2.2–9.8 con-cluded that the amine group did not interact with thesurface (Roddick-Lanzillotta and McQuillan, 1999;Roddick-Lanzillotta et al., 1998; Parikh et al., 2011). Wealso did not observe this band for the adsorbed species.c-Al2O3 maintains a net positive surface charge over thispH range, but the surface charge of rutile changes from po-sitive to negative, yet no interaction of the amine functionalgroup with negatively charged >TiO� sites was previouslyreported.

4.3. Relating crystal structure to the geometry of adsorbed

species

c-Al2O3 has a defect spinel structure in the cubic crystalsystem and the (110) face dominates the morphology

Table 4Values of DmL, aq and DmML, ads (cm�1), respectively for uncomplexed, aqueous amino acids in D2O and complexed to Al ions at the c-Al2O3

surface. These values are shown for both the a- and the R-carboxylate groups.

Conditions L-Glu L-Asp

DmL, aq (a) DmL, aq (R) DmL, aq (a) DmL, aq (R)

100 mM, pD 3.8 205 152 205 152

DmML, ads (a) DmML, ads (R) DmML, ads (a) DmML, ads (R)7 mM, pD 2.8 198, 232 152 201, 236 1777 mM, pD 6.0 198, 231 152 200, 231 1733 mM, pD 3.8 195, 228 149 200, 232 176

E. Greiner et al. / Geochimica et Cosmochimica Acta 133 (2014) 142–155 151

(Kovarik et al., 2013). Reactive site densities in the range of1–3 sites�nm�2, for surface protonatable (>AlOH) or Lewisacid (>Al) sites, have been estimated from IR, potentiomet-ric titration, tritium-exchange and pyridine adsorptionexperiments (Peri, 1966; Kummert and Stumm, 1980; Tariet al., 2000; Metivier et al., 2003; Lefevre et al., 2004;Sanchez-Valente et al., 2004). In the analysis below, weassumed the average value of 2 active sites�nm�2. We note,however, that values of 8 to as high as 19–25 sites�nm�2

have also been reported in the literature based on theamount of methane generated by a Grignard reagentreaction with c-Al2O3, potentiometric titration and 3H-ex-change methods and from crystal structures (Tamuraet al., 1999; Halter, 1999; Kummert and Stumm, 1980;Sahai and Sverjensky, 1997a,b; Koretsky et al., 1998).

The two adsorption plateaux obtained for L-Glu orL-Asp correspond to 0.5 molecule�nm�2 and 1 mole-cule�nm�2. Using the active site density of 2 sites nm�2

Fig. 6. The structure of the (110) crystal face of c-Al2O3 in ball-and-stickatoms as red balls, and the unit cell is shown by the black box. The greenatoms roughly match oxygen spacing within a carboxylate group, and thinterpretation of the references to colour in this figure legend, the reader

yielded 1 molecule per 4 Al sites and 1 molecule per 2 Alsites, respectively. Interestingly, these molecule/surface Alactive site ratios were identical to those for tetradentate tet-ranuclear or H-bonded tetradentate tetranuclear (Fig. 5gand h) and for bidentate binuclear or H-bonded bidentatebinuclear (Fig. 5c and d). Furthermore, it was reasonablethat the tetradenate tetranuclear species occured at the low-er surface coverages where sufficient surface area was avail-able and the molecule configuration changed to a bidentatebinuclear configuration at higher surface coverages due tosteric crowding. Thus, only 2–4 Al surface atoms wereoccupied out of �10–15 total Al atoms�nm�2 (as estimatedfrom crystal structure). A Langmuir model could befit to the isotherms of both amino acids yieldingKads � 0.2 lmol�m�2 and a maximum site occupancy of�3 lmol�m�2 or 2 molecules�nm�2 with r2 = 0.9. However,the Langmuir isotherm assumes one adsorbate molecule peractive surface a 1:1 (mononuclear geometry), and a model

representation. The Al atoms are rendered as blue balls, the oxygenarrows indicate interatomic distances. Distances between surface Ale distance between a- and R-carboxyl groups of Glu and Asp. (For

is referred to the web version of this article.)

152 E. Greiner et al. / Geochimica et Cosmochimica Acta 133 (2014) 142–155

fit does not necessarily confirm a 1:1 adsorbate/active sur-face site ratio.

Because of the similar protonation constants and struc-tures of Glu and Asp, except that Asp has one less –CH2–group, the adsorption behavior of both species wasexpected to be similar. The FTIR spectra indicated thepresence of multiple types of surface species for both aminoacids, in particular, bidentate binuclear where one carboxylgroup binds to two Al atoms; tetradentate tetranuclearwhere both carboxyl groups bonded to four Al atoms,and perhaps, also outer sphere and electrostatically bondedspecies. The inference of multinuclear species was consistentwith the results of the adsorption isotherms.

The bonding modes of the amino acids could be relatedto the atomic structure of c-Al2O3. The (110) and (111)faces are believed to be dominant for plate-like syntheticcrystals of c-Al2O3 (Kovarik et al., 2013), but we were un-able to find any information on the dominant facets ofspherical synthetic crystals used in the present study (Xuet al., 2012, 2013). The Al–Al distances of approximately2.81, 3.29, 5.16–5.61 and 7.94–8.50 A are found on the(100), (110) and (111) faces of c-Al2O3 (Fig. 6). The dis-tance between the two O atoms in a planar carboxyl groupis �2.0–2.2 A, and the distance between the two carboxylgroups is �5–6 A for Asp and Glu. Thus, the Al atomicpositions on the c-Al2O3 surfaces provided appropriate dis-tances for both amino acids to form the structures shown inFig. 1c, d, g and h.

The adsorption of Glu and Asp to hydrous ferric oxideand TiO2 phases has been studied in detail, so we comparedthe results of the present study to those obtained for theother two other phases for these two oxides. Using theExtended Triple Layer Surface Complexation Model,Sverjensky et al. (2008) fit pH edge data for L-Glu adsorp-tion on hydrous ferric oxide (Davis and Leckie, 1978) witha tetradentate tetranuclear complex at low surface coveragesand with a bidentate binuclear configuration at high surfacecoverage. Their results were similar to those obtained in thepresent study for both Glu and Asp on c-Al2O3.

As seen above, both amino acids behaved similarly toeach other at all surface coverages on c-Al2O3, and thesegeometries corresponded to the low surface coverage geom-etries on the TiO2 phases. Glu and Asp behaved similar toeach other at low coverages (tetradentate tetranucleargemoetry) on TiO2, but showed different binding geome-tries compared to each other at high surface coverage. Inthe latter case, Glu attached to the TiO2 surface in a com-bination of inner sphere complexes, outer sphere complexesor H-bonded complexes; in contrast, Asp attached as outersphere or H-bonded species (Roddick-Lanzilotta andMcQuillan, 2000; Jonsson et al., 2009, 2010; Parikh et al.,2011). The difference between Glu and Asp adsorption onrutile was attributed to steric strain in the surface complex,which is associated with the shorter side chain of Asp (Rod-dick-Lanzilotta and McQuillan, 2000). The Ti–Ti spacingson the (101) face of rutile are 2.96 A, 3.60 A, and 5.36 A(Parikh et al., 2011; Jonsson et al., 2009; 2010). Thus, it islikely that the Asp molecule, in which the oxygen atomsof the two carboxyl groups are separated by more than5 A, was unable to bridge the Ti–Ti distances as surface

coverage increased. In the case of c-Al2O3, Asp was ableto form bridging bidentate species, because a shorter Al–Al distance of �5.16 A exists on the (111) face.

Another potential reason for the different behavior ofthe amino acids at the two oxide surfaces could beexplained, at least partially, on the basis of differences ininterfacial solvation. The change in interfacial solvation en-ergy (DGsolv,intf) is an unfavorable contribution to the over-all Gibbs free energy of adsorption (DGads). The DGsolv,intf

scales as the inverse of the dielectric constant (e) of theoxide (Sverjensky and Sahai, 1996; Sahai, 2002b). Thedielectric constants of rutile and c-Al2O3 are 120 and 10,respectively, so DGsolv,intf contributions are on the orderof 0.01 and 0.1. Thus, the unfavorable DGsolv,intf contribu-tion is of a smaller magnitude at the rutile surface than atc-Al2O3. Formation of inner sphere species could then bemore favored at the rutile surface than at c-Al2O3. The sizedifference between the two amino acids may have becomemore important on rutile, because of the potential for innersphere bonding. In contrast, the relative difficulty in desol-vating the c-Al2O3 surface could have favored the forma-tion of H-bonded, outer sphere and electrostaticallybonded surface species over the entire concentration range.These types of species could be less sensitive to differencesin the length of the side chain between Glu and Asp.

4.4. Geochemical Implications

We have examined adsorption of a single amino acid onthe c-Al2O3 surface without competition from other ligandsor the presence of metal cations. In (bio)engineered andgeochemical systems, other ligands and cations may inter-act with both the oxide surface and the amino acid. Theseligands may outcompete the amino acids for surface sitesas the energy difference between bidentate (as in Fig. 5b)and monodentate binding modes of ligands is believed tobe minimal (Dudev and Lim, 2004, 2007; Boehme et al.,2002). Also, naturally occurring ligands can cause the bind-ing mode of carboxylates to change, for example, frombidentate to monodentate for Glu and Asp side chainresidues on peptides (Ryde, 1999; Dudev and Lim, 2007).The results of the present study may contribute to estimatesfor the amount of amino acids associated with meteorites(Pizzarello and Cronin, 2000; Sephton 2002; Glavin andDworkin, 2009) and delivered by meteorite impacts to theearly Earth as a potential source of pre-biotic organicmolecules.

5. CONCLUSIONS

We have interpreted the binding mechanisms of Glu andAsp to the c-Al2O3 surface by analyzing adsorption data inconjunction with ATR–FTIR spectra. Both amino acidsshowed similar binding geometries over the range of con-centrations and pD values examined. Maximum surfacecoverage of 1 molecule/nm2 was obtained, correspondingto one molecule/10 Al surface sites. The data were inter-preted as showing bidentate binuclear and tetradentate tet-ranuclear structures involving two or four Al atoms. Anouter sphere complex involving a single carboxyl group,

E. Greiner et al. / Geochimica et Cosmochimica Acta 133 (2014) 142–155 153

and electrostatically bonded species involving the aminegroup and negatively charged, deprotonated >AlO� siteat high pH could not be ruled out.

ACKNOWLEDGMENTS

The authors are grateful for the assistance provided by Dr. JieXu in characterizing the crystalline c-Al2O3; C.M. Jonsson with theamino acid analysis protocol. We appreciate helpful discussionswith Drs. H. James Cleaves, Jason Dworkin, Jie Xu, Nianli Zhang,Yang Yang, Zhijun Xu and Tim Oleson; Profs. Eric Roden,Dimitri Sverjensky and Huifang Xu; Ms. Chunxiao Zhu and Mr.C. L. Jonsson. Financial support was provided to N. Sahai byNSF EAR CAREER grant EAR 0346689, NASA AstrobiologyInstitute Director’s Discretionary Funds (NAI DDF) grant,“start-up” funds from University of Akron, and the Simons Col-laboration on the Origins of Life Grant from the Simons Founda-tion, NY; and a Week’s Graduate Fellowship to E. Greiner fromthe Department of Geosciences, University of Wisconsin, Madison.

APPENDIX A. SUPPLEMENTARY DATA

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.gca.2014.01.004.

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Associate editor: John Moreau