Vol. 3 INTERNATIONAL JOURNAL OF PHOTOENERGY 2001

Laser production of gas phase complexes of metalα-aminophosphonic acid mixtures and their role

in chiral recognition

A. Paladini,1 C. Calcagni,1 T. Di Palma,2 M. Satta,1 M. Speranza,3 D. Scuderi,1,3

A. Laganà,1 G. Fago,1 and A. Giardini Guidoni1,2

1Dipartimento di Chimica, Università di Roma “La Sapienza”, pl. A. Moro 5, I-00185 Roma, Italy2CNR-Istituto Materiali Speciali, I-85050 Tito Scalo (Pz), Italy

3Facoltà di Farmacia, Dipt. di Chimica e Tecnologia delle Sostanze Biologicamente Attive,

Università di Roma “La Sapienza”, pl. A. Moro 5, I-00185 Roma, Italy

Abstract. Clusters between first-group metal ions and chiral α-aminophosphonic acids have been read-ily generated by Pulsed Laser Ablation (PLA) and by Electrospray Ionization (ESI) and their fragmentationinvestigated by mass spectrometry. The complexes studied have the general formula [Me(I)CL2]+, whereMe(I) is H, Li, Na, or K, C is (R)-(−)-(1-aminoethyl) phosphonic acid (ER) and (S)-(+)-(1-aminoethyl) phospho-nic acid (ES), (1R)-(+)-(1-amino-2-methylpropyl) phosphonic acid (PR) and (1S)-(−)-(1-amino-2-methylpropyl)phosphonic acid (PS), (1R)-(−)-(1-amino-hexyl) phosphonic acid (HR) and (1S)-(+)-(1-amino-hexyl) phosphonicacid (HS), o-phospho-L-serine (SS) and o-phospho-D-serine (SR), and L is a reference α-aminophosphonic acid(E, P, H or S) of defined configuration. Collision induced dissociation (CID) of diastereomeric [Me(I)CL2]+complexes leads to fragmentation patterns characterized by [Me(I)CL]+/[Me(I)L2]+ abundance ratios whichdepend upon the configuration of solute C. These different spectral features were correlated to the differentstability of the diastereomeric [Me(I)CRL]+ and [Me(I)CSL]+ complexes in the gas phase.

1. INTRODUCTION

Interest in the analysis of biomolecules is rising every-day in molecular biology, biochemistry, pharmaceuti-cal and chemical industries. Considering that the mostof these molecules are chiral, the specificity of inter-molecular interactions in these systems can lead to apronounced enantioselectivity in a number of reactiveprocesses. Variousmethodologies have been developedto discriminate between enantiomers in solid, liquidand gaseous phase. Recently attention has been paidto studies of the physicochemical properties of chi-ral systems in solvent free ambient by resonant laserspectroscopy [1, 2] and mass spectrometry under suit-able conditions [3]. Both techniques rely on measuringthe different stabilities of the diastereomers formedby interactions of enantiomers with a chiral reagent.However most of the molecules of biological interestare non-volatile and thermally labile compounds, whicheasily decompose upon heating. This problemhasmadedifficult to study these molecules in the gas phase:many efforts have been made to find new vaporizationmethods. Among them, Pulsed Laser Ablation (PLA) hasbeen demonstrated to be suitable for vaporizing chiralamino acids [4], but no evidence has been reported todate of PLA formation of their diastereomeric adducts.An alternative methodology is ESI-CID-MS2 mass

spectrometry which may provide information on thestability of chiral clusters simply through the measure-ment of the abundances of fragments peaks coming

from the competitive dissociation of tetrameric clusterion, according to the Cooks’ kinetic method [5].In this paper we report on the application of mass

resolved PLA and ESI-CID-MS2 techniques to the enan-tiodiscrimination of several α-aminophosphonic acids.α-Aminophosphonic acids are the analogues of natu-ral α-aminocarboxylic acids in biological systems. Theyare used as pharmacological (antiviral and antibiotic)agents, enzyme inhibitors, herbicides and pesticides,catalysts in organic transformations, and finally theyare the key building block of phosphono-peptides [6].The biological activity of these compounds is relatedto their stereochemistry and is strictly dependent upontheir affinity towards metal centers. For these reasons,we have investigated the affinity ofα-aminophosphonicacids towards first-group metals in the gas phase andhow this is affected by solute configuration.

2. EXPERIMENTAL

The investigated molecules are (R)-(−)-(1-aminoethyl)phosphonic acid (ER) and (S)-(+)-(1-aminoethyl) phos-phonic acid (ES), (1R)-(+)-(1-amino-2-methylpropyl)phosphonic acid (PR) and (1S)-(−)-(1-amino-2-methylpropyl) phosphonic acid (PS), (1R)-(−)-(1-amino-hexyl) phosphonic acid (HR) and (1S)-(+)-(1-amino-hexyl) phosphonic acid (HS), o-phospho-L-serine (SS) and o-phospho-D-serine (SR). R and Sα-aminophosphonic acids were all commercially

218 A. Paladini et al. Vol. 3

available from Sigma Aldrich.

2.1. Mass resolved pulsed laser ablation (PLA).The experiments have been carried out using a com-mercial LAMMA 500 instrument from Leybold-Heraeus.A very intense pulse from a ND:YAG laser (τ = 6ns) isfrequency quadrupled (λ = 266nm) and tightly focusedon the sample. Solid samples of α-aminophosphonicacids were dissolved in water/methanol solution andspread on an electron microscope metal grid. After sol-vent evaporation the grid was inserted into the samplestage and covered with a quartz window. The irradia-tion of the samples was performed in the vacuum cham-ber at a pressure of 10−8 torr and at a laser power in therange 108–109 W/cm−2. The resolved mass signals aredetected by an electron multiplier, sent to a digital os-cilloscope and stored on a PC. The data reported hereconcern positive ions, averaged over several spectra.

2.2. Electrospray ionization mass spectrometry(ESI-MS). The electrospray experiments were per-formed using a commercial API 100/300 triple-quadrupolemass spectrometer from Perkin Elmer SciexInstruments, equipped with an ESI source and a sy-ringe pump. Operating conditions for the ESI sourcewere as follows: spray voltage, 3.8KV; capillary temper-ature, 298K; sheath gas (N2)flow rate, 30 units (roughly0.75L/min). The experiments were conducted in thepositive ion mode. Reported spectra represent the av-erage of about 150 scans, each requiring 0.1 seconds.The sample was infused via a syringe pump at a flowrate of 10µl/min. In the full scan MS-MS-MS mode theparent cluster ion was first isolated and then subjectedto a CID process, in an energy range of 2–18eV in thelaboratory system.The selected gas phase complexes were gener-

ated by electrospraying 50/50 water/methanol so-lutions containing a mixture of optically pure α-aminophosphonic acids, 10µM each, and 2.5µM metalsulphate. Complexes between H+ or Na+ and the se-lected α-aminophosphonic acids are observed in plainwater/methanol solutions also in the absence of addedsalts.

3. RESULTS AND DISCUSSION

Mixtures of α-aminophosphonic acids were studied byLaser Microprobe Mass Analysis (LAMMA). In the ab-sence of a metal source, the solid α-aminophosphonicacid mixtures produces mainly the ionized parentmolecules and their fragments. Under these conditions,formation of ionised clusters is negligible. When ametal is added to the α-aminophosphonic acid in theablation region, signals up to quaternary complexescontaining the metal appear in the mass spectrum.This indicates that the metal ion efficiently coordinatesthe α-aminophosphonic acid and that the complexes

formed are stable at the ablation temperature. Mix-tures of two optically pure α-aminophosphonic acidswith first-group metals were studied. It is convenientto define the cluster ions as “homo” when the two α-aminophosphonic acids have the same configurationand “hetero” in the opposite case. A typical LAMMAspectrum of ES and HS mixed with potassium is re-ported in Figure 1. Similar spectra have beenmonitoredby replacingHS withHR . The ion abundances of the het-ero and homo clusters do not differ within the 4% exper-imental errors, thus suggesting a very small differencein their stability.

0

10

20

30

40

50

150 200 250 300 350 400 450 500 550

m/e

Intensity(a.u.)

solid mixture of ES, HS and Kλ = 266nmElaser = 130µJ/imp[K

HS]+

[K(ES) 2]+

[KE SHS]+

[K(ES) 3]+

[K(ES) 2HS]+

[KE S(H

S) 2]+

Figure 1. Laser ablation mass spectrum of a solid mixture

containing ES, HS, and potassium. The spectrum was taken

at λ=266nm and is the average of several spectra collected

at Elaser =130µJ/pulse, in positive mode acquisition.

More insights into the relative stability of heteroand homo clusters is obtained by using the ESI-CID-MS2 technique. The ESI-MS spectrum of ES and PR inplain water/methanol solutions is reported in Figure 2.The clusters [H(PR)n]+, [Na(PR)n]+, [H(ES)n]+, and[Na(ES)n]+ (n = 1–4) are observed, together with allfamilies of mixed clusters, including [Na(PR)(ES)2]+

and [H(PR)(ES)2]+. Analogous results are found for theother α-aminophosphonic mixtures investigated. TheCID experiments have been carried out on the ionictetrameric clusters [Me(I)CL2]+, where Me = H, Li, Na,or K, C is the solute α-aminophosphonic acid (the S andR enantiomers of E, P, H and S), and L is a referenceα-aminophosphonic acid of defined configuration (theS or R enantiomer of E, P, H and S). Their fragmenta-tion leads essentially to the [Me(I)CL]+ and [Me(I)L2]+

species. This is illustrated in Figure 3(a),(b) reportingthe CID fragmentation patterns of [NaPS(ES)2]+ and[NaPR(ES)2]+, respectively.According to the Cooks’ quotient ratio method [5],

the ion abundance RS = [Me(I)CSL]+/[Me(I)L2]+ andRR = [Me(I)CRL]+/[Me(I)L2]+ ratios of the fragmentsarising from energetic collision of [Me(I)CSL2]+ and[Me(I)CRL2]+ with a target gas under the same condi-

Vol. 3 Laser production of gas phase complexes of metal α-aminophosphonic … 219

5

0

5

0

5

30

35

200 300 400 500 600m/z

126.0

154.0

149.0

176.0

251.0

301

317.0329.0

340.0

376.0

404

426.0470.0

493.0 529.0539.0

557.0

567.0595.0

623.0654.0

682.0

Figure 2. ESI-MS spectrum of a methanol/water 1 : 1 solution containing ES and PR , 10µM each. The spectrum is the result

of 150 scans.

tions provides a measure of the relative stability of the[Me(I)CSL]+ or [Me(I)CRL]+ complexes and, therefore,a tool for enantiodiscriminating the C solute. Dissocia-tions occur as follows:

[Me(I)CS(L)]+

[Me(I)CS(L)2]+(kC)S

kL [Me(I)(L)2]+

[Me(I)CR(L)]+

[Me(I)CR(L)2]+(kC)R

kL [Me(I)(L)2]+

and (kC)S/kL = RS, (kC)R/kL = RR.The relative rates of the two competitive dissocia-

tions can be expressed as RR/RS ratio, which is definedas the chiral resolution factor Rchiral. When L has theS configuration, a value Rchiral > 1 indicates that theheterochiral [Me(I)CRL]+ complex is more stable thanthe homochiral [Me(I)CSL]+ one and vice versa. ForRchiral = 1 there are no stability differences and chiraldiscrimination is unattainable by this method.Within the reasonable assumption that entropy ef-

fects on fragmentation are negligible, the difference be-tween the bond dissociation energies of the trimericclusters [Me(I)CSL]+ and [Me(I)(L)2]+ can be ex-pressed as: ln[(kC)S/kL] = lnRS = (∆H0)S/RTeff. A sim-ilar equation can be written for the other enantiomerCR : ln [(kC)R/kL] = lnRR = (∆H0)R/RTeff. Accord-

ingly, the difference in the binding energies ∆(∆H0)R−Sbetween the hetero [Me(I)CRL]+ and homo [Me(I)CSL]+

clusters can be expressed in terms of Rchiral: (∆H0)R −(∆H0)S = ∆(∆H0)R−S = RTeff lnRchiral, as shown inFigure 4.

The chiral resolution factors Rchiral for a num-ber of diastereomeric [Me(I)CSL2]+ and [Me(I)CRL2]+

complexes has been measured. The reproducibil-ity of the method was investigated by changingthe chirality of the reference and the referencewith the solute. Heat of formation of the clus-ters [Me(I)CSLS]+ and [Me(I)CRLR]+, one mirror im-age of the other, are the same. As a consequencethe chiral factors for the clusters [Me(I)CLS]+ and[Me(I)CLR]+ are one the reciprocal of the other (i.e.,Rchiral[NaH(SR)2]+ = 1/Rchiral[NaH(SS)2]+). Similarlyit can be shown that exchanging the reference with thesolute and viceversa the Rchiral value doesn’t change(i.e., RchiralNaH(SR)2]+ = Rchiral[NaS(HR)2]+) [7]. Tak-ing into account these considerations average valuesof Rchiral can be determined for each cluster studied.

In Table 1 the average values of Rchiral are reportedfor some systems, together with the corresponding∆(∆H0)R−S terms at an effective temperature Teff =950K [3]. It can be seen that the stability differencesbetween metal ionized diastereomeric clusters can beestimated as ranging from about 0 to 3.2KJ/mol. Thesevalues are generally lower than thosemeasured for neu-tral and ionized diastereomeric clusters of chiral alco-hols in gaseous phase at temperatures of few Kelvin [1].

220 A. Paladini et al. Vol. 3

Intensity(a.u.)

Intensity(a.u.)

(a)

(b)

m/z

10

20

30

40

50

60

70

80

90

10

20

30

40

50

60

70

80

90

250 300 350 400

250 300 350 400

224.0273.0

301.0

377.0

426.0

[NaPS(ES)2]+

[NaPSES]+

[Na(ES)2]+

208.0

273.0

301.0

344.0 365.0

426.0

[Na(ES)2]+

[NaPRES]+[NaPR(ES)2]+

m/z

Figure 3. ESI-CID-MS2 spectrum of [NaPS(ES)2]+ (a)

and [NaPR(ES)2]+ (b). The spectra are the resulting of

300 scans collected at 8 eV.

[Me(I)CR(L)2]+

[Me(I)CS(L)2]+

[Me(I)CR(L)]+

[Me(I)CS(L)]+

[Me(I)(L)2]+

∆(∆H0)R−S

Figure 4. Potential energy levels of the [Me(I)CSL2]+ and

[Me(I)CRL2]+ fragmentation.

From the table it appears that the ionized heterochiralcomplexes coordinated by a metal are generally morestable than the homochiral analogues. The ionic radiusof the complexing metal doesn’t appear to have an in-fluence on the stability of the clusters.

Table 1. Enantiodifferentiation of chiral α-amino-phosphonic acids by ESI-CID-MS2 fragmentation of

their diastereomeric [Me(I)C(L)2]+.

C/L Me(I) Rchiral = RR/RS ∆(∆H0)R−S Cluster Stability(KJ mol−1)(a) Order

PH+ 1.22± 0.12 +1.6± 0.7 hetero>homoLi+ 0.80± 0.08 −1.8± 0.7 homo>hetero

ENa+ 1.27± 0.13 +1.9± 0.7 hetero>homoK+ 1.10± 0.11 +0.8± 0.7 hetero>homo

SH+ 0.90± 0.09 −1.0± 0.7 homo>heteroLi+ 1.28± 0.13 +1.9± 0.7 hetero>homo

ENa+ 1.30± 0.13 +2.0± 0.7 hetero>homoK+ 1.24± 0.12 +1.7± 0.7 hetero>homo

HH+ 0.86± 0.09 −1.2± 0.7 homo>heteroLi+ 1.0± 0.1 - hetero≈homo

ENa+ 1.1± 0.11 +0.8± 0.7 hetero>homoK+ 1.50± 0.11 +3.2± 0.7 hetero>homo

(a) Assuming Teff = 950K.

4. CONCLUSIONS

In this paper we have reported the study of directpulsed laser ablation of solid metal-aminophosphonicacid mixtures and shown that gas phase ionized metal-aminophosphonic acid complexes can be generated.The relative abundance of the pureα-aminophosphonicacid complexes with respect to the metal bonded clus-ters are generally very low, indicating that themetal canenhance theα-aminophosphonic acid coordination andthat the complexes formed are stable at the ablationtemperature. Being very important the role played in so-lutions by metal ion-biomolecules complexes in manyaspects of everyday life, it is evident how important issuch a result to get information on the capability ofmetals to bond one or more α-aminophosphonic acidmolecules in gaseous phase.Furthermore metal-aminophosphonic acid com-

plexes have shown to be a useful mean to helpdiscrimination of R and S α-aminophosphonic acid.Tetrameric [Me(I)C(L)2]+ complexes, when in theCID process undergo fragmentation, give the corre-sponding trimeric forms by loss of an acidic ligand(either C or L). The relative rates of the two competingfragmentations, i.e. [Me(I)C(L)2]+ → [Me(I)CL]+ + L;[Me(I)C(L)2]+ → [Me(I)(L)2]+ + C, are measuredfrom the corresponding [Me(I)CL]+/[Me(I)(L)2]+

ion abundance ratios. If optically active α-aminophosphonic acids (CR and CS) are employed,the comparison of the [Me(I)CRL]+/[Me(I)(L)2]+ and[Me(I)CSL]+/[Me(I)(L)2]+ ratios provides a measureof the relative stability of the homochiral [Me(I)CSL]+

vs the heterochiral [Me(I)CRL]+ adducts and, there-fore, a means for enantiodiscriminating the chiral CRand CS solutes. The ionized heterochiral complexescoordinated by a metal are generally more stable thanthe homochiral analogues. The ionic radius of the

Vol. 3 Laser production of gas phase complexes of metal α-aminophosphonic … 221

complexing metal doesn’t appear to have an influenceon the stability of the clusters.From the reported results it can be inferred that

the ESI-CID-MS2 technique proves suitable for detect-ing the abundance of chiral molecules of large bio-chemical interest, such as the α-aminophosphonicacids, in complex matrices. In the frame of the kineticmethod, the measured fragmentation intensities arecorrelated to the termochemical properties of the α-aminophosphonic acids.

ACKNOWLEDGEMENTS

This work was supported by the Ministerodell’Università e della Ricerca Scientifica e Tecno-logica (MURST) and the Consiglio Nazionale delleRicerche (CNR): Progetto Finalizzato MSTA II.

References

[1] S. Piccirillo, C. Bosman, D. Toja, A. Giardini Guidoni,M. Pierini, A. Troiani, and M. Speranza, Angew.Chem. Int. Ed. Engl. 36 (1997), 1729.

[2] A. R. Al-Rabaa, E. Bréhéret, F. Lahmani, and A.Zehnacker, Chem. Phys. Lett. 237 (1995), 480.

[3] K. Vékey and G. Czira, Anal. Chem. 69 (1997), 1700.[4] F. Piuzzi, I. Dimicoli, M. Mons, B. tardivel, and Q.

Zhao, Chem. Phys. Lett. 320 (2000), 282; E. Nir, L.Grace, B. Brauer, and M. S. de Vries, J. Am. Chem.Soc. 121 (1999), 4896.

[5] W. A. Tao, D. Zhang, F. Wang, P. Thomas, and R. G.Cooks, Anal. Chem. 71 (1999), 4427.

[6] C. E. Mc Kenna, L. A. Khawli, A. Bapat, V.Harutunian, and Y-C. Cheng, Biochemical Pharma-cology 36 (1987), 3103.

[7] Cristina Calcagni, Thesis in Chemistry, A. A. 1999–2000.

Submit your manuscripts athttp://www.hindawi.com

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Inorganic ChemistryInternational Journal of

Hindawi Publishing Corporation http://www.hindawi.com Volume 2014

International Journal ofPhotoenergy

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Carbohydrate Chemistry

International Journal of

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Journal of

Chemistry

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Advances in

Physical Chemistry

Hindawi Publishing Corporationhttp://www.hindawi.com

Analytical Methods in Chemistry

Journal of

Volume 2014

Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

SpectroscopyInternational Journal of

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

The Scientific World JournalHindawi Publishing Corporation http://www.hindawi.com Volume 2014

Medicinal ChemistryInternational Journal of

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Chromatography Research International

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Applied ChemistryJournal of

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Theoretical ChemistryJournal of

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Journal of

Spectroscopy

Analytical ChemistryInternational Journal of

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Journal of

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Quantum Chemistry

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Organic Chemistry International

ElectrochemistryInternational Journal of

Hindawi Publishing Corporation http://www.hindawi.com Volume 2014

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

CatalystsJournal of