ORIGINAL ARTICLE

b-Cyclodextrin as a photostabilizer of the plant growth regulator2-(1-naphthyl) acetamide in aqueous solution

Eliana Sousa Da Silva • Hugh D. Burrows •

Pascal Wong-Wah-Chung • Mohamed Sarakha

Received: 1 May 2013 / Accepted: 27 July 2013

� Springer Science+Business Media Dordrecht 2013

Abstract The plant growth regulator 2-(1-naphthyl)

acetamide (NAAm) is susceptible to degradation by sunlight

and UV light in aqueous solution. Its inclusion complex with

b-cyclodextrin (b-CD) was characterized by absorption and

fluorescence spectroscopy and its photodegradation was

compared with that of aqueous solutions of NAAm. The

complex was formed with a stoichiometric ratio of 1:1 with

a binding constant of 651 M-1. The photodegradation

behavior of NAAm in the inclusion complex NAAm:b-CD

was investigated using both UV (k = 254 nm) and simu-

lated solar light (Suntest) irradiation. It was found that the

NAAm:b-CD complex increases NAAm photostability

towards photochemical degradation markedly. In addition,

an influence of b-CD concentration was also observed on

NAAm degradation rate: higher b-CD concentrations lead

to a slower photoinduced transformation. Moreover, some

differences were found in the photoproducts in the presence

and absence of the cyclodextrin, indicating inhibition of

some of the mechanistic pathways. b-CD stabilizes NAAm

photodegradation towards sunlight and UV irradiation,

enhancing its efficient application on formulations for the

treatment of fruits and vegetables.

Keywords 2-(1-Naphthyl) acetamide � Plant growth

regulator � b-Cyclodextrin � Photodegradation �Formulations � Photoprotector

Introduction

2-(1-naphthyl) acetamide (NAAm) (Scheme 1) is a plant

growth regulator that has been employed in agriculture for

several decades [1]. It is used as thinning agent for fruits,

particularly apples and pears, and to prevent premature

fruit fall [2]. NAAm was first commercialized by Amchem

Products Inc. [2], and is present in formulations as Amide-

Thin W (containing 8.2 % w/w NAAm) and Amcotone

(containing 1.2 % w/w NAAm) [3, 4], which are used to

spray on apples for pre-harvest fruit drop control. It is

considered harmful to aquatic organisms with a LC50 of

44 mg a.s/L for fishes and harmful if swallowed based on a

LD50 of 1,655 mg/kg [3].

Under environmental conditions, the interaction of

pesticides with light may influence their activity. Photo-

degradation may lead to loss of activity or even induce the

formation of more toxic compounds. In a previous study

we have shown that NAAm is degraded by sunlight under

environmental conditions, producing primary photoprod-

ucts that are more toxic than the parent compound [5]. This

is of great concern since it can cause adverse effects for

both human health and the environment. Therefore, the

development of methods that allow the stabilization of

formulations of this compound towards photodegradation

is essential for obtaining more efficient application of the

active ingredient with enhanced safety.

E. S. Da Silva (&) � H. D. Burrows

Department of Chemistry, University of Coimbra, Rua Larga,

3004-535 Coimbra, Portugal

e-mail: elianas@qui.uc.pt

E. S. Da Silva � P. Wong-Wah-Chung � M. Sarakha

Clermont Universite, Universite Blaise Pascal, Institut Chimie de

Clermont Ferrand (ICCF) UMR CNRS 6296, Equipe de

Photochimie, BP 80026, 63171 Aubiere Cedex, France

P. Wong-Wah-Chung

Clermont Universite ENSCCF, BP 10448, 63000 Clermont-

Ferrand, France

P. Wong-Wah-Chung � M. Sarakha

CNRS UMR 6505, 63173 Aubiere, France

123

J Incl Phenom Macrocycl Chem

DOI 10.1007/s10847-013-0355-5

Cyclodextrins (CD) are cyclic oligosaccharides com-

posed of more than five D-glucose units, possessing a

hydrophobic internal cavity and a hydrophilic outer surface

containing hydroxyl groups [6]. This toroid-like molecular

structure allows the formation of host/guest inclusion com-

plexes with a great variety of molecules, with consequent

changes in the guest chemistry, including photophysical and

photochemical properties [7–12]. Three main types of CD

are available, a, b and c, which are distinguished by different

central cavity diameters. No covalent bonds are broken or

formed during formation of the inclusion complex [13], and

the inclusion of the guest molecules will depend on their

polarity, size and geometry. b-CD is the most widely used

cyclodextrin. It is non-toxic, biodegradable and relatively

inexpensive. It is formed by seven glucose groups and has an

internal cavity diameter of *0.60–0.65 nm [6]. Several

guest molecules, including pesticides and pharmaceuticals,

have been incorporated in CD to improve water solubility

and biodegradability, to remove pollutants from contami-

nated soils, to increase or stabilize pesticides and pharma-

ceuticals compounds towards degradation, etc. [14–27]. The

possible application of CD to pesticide formulations [28–30]

is gaining importance due to their non-toxic nature, posing

no risks either to humans or to the environment.

We have been interested in investigating the behavior of

the NAAm inclusion complex with b-CD in aqueous

solution and its effect upon photodegradation for potential

application in formulations, and report the study of this

using UV–Vis absorption and fluorescence spectroscopy.

In addition, the fluorescence quenching of the inclusion

complex by several anions was also studied to obtain

information on the nature of any stabilization, while the

NAAm photodegradation in water in presence of b-CD was

investigated under UV and simulated solar light irradiation

as function of b-CD concentration.

Materials and methods

Materials and preparation of solutions

NAAm, b-CD, KBr, KI, KSCN, NaN3, methanol (HPLC

grade) and formic acid were purchased from Sigma-

Aldrich and used as received.

Aqueous stock solution of NAAm (3.0 9 10-4 M) were

prepared by dissolving the respective compound in ultrapure

water (Millipore Milli-Q; resistivity of 18.2 M X cm-1)

with constant stirring till the complete dissolution of the

solid sample. An aqueous NAAm working solution

(1.0 9 10-5 M) (solution 1) was prepared from the previ-

ous stock solution. Another solution composed of b-CD

(1.0 9 10-2 M) ? NAAm (1.0 9 10-5 M) (solution 2) in

water was prepared by dissolving a few mg of b-CD solid in

NAD working solution 1. These solutions were then used in

absorbance and fluorescence measurements. Aliquots of

solution 1 and 2 were mixed in order to obtain solutions with

constant NAAm concentration and varying b-CD concen-

trations in the range 5.0 9 10-4–8.0 9 10-3 M. These

solutions were stirred during 1 h prior to absorbance/fluo-

rescence measurements in order to incorporate NAAm on

the cyclodextrin. For degradation studies, different amounts

(mg) of the solid b-CD were dissolved in NAAm stock

solution (3.0 9 10-4 M) with constant stirring in order to

obtain the final solutions of NAAm with constant concen-

tration (3.0 9 10-4 M) in presence of b-CD with concen-

trations of 1.0 9 10-3, 5.0 9 10-3 and 1.0 9 10-2 M.

For fluorescence quenching, various concentrations of

KBr, KI, KCl, KSCN and NaN3 were added to aqueous

NAAm solution (1.0 9 10-5 M). Stern–Volmer plots were

constructed from relative integrated fluorescence emission

intensities and the Stern–Volmer quenching coefficients,

KSV, were obtained by linear regression according to the

expression (1):

I0

I¼ 1þ KSV½Q� ð1Þ

where I0 and I are the fluorescence intensities in absence

and presence of quencher, respectively, [Q] represents the

quencher concentration and the slope of the linear plot is

equal to the Stern–Volmer constant KSV.

Analytical methods

The UV–Vis absorption spectra measurements were carried

out in 1 cm quartz cuvettes over the range 200–800 nm on

a Cary 3 double-beam UV–Vis spectrometer (Varian).

Corrected steady state fluorescence spectra were mea-

sured using a 1 cm quartz cuvette on a Horiba-Jobin-Yvon

SPEX Fluorolog 3-22 spectrofluorometer equipped with a

300 W Xenon lamp. Fluorescence emission and excitation

spectra were recorded using absorption and emission

maxima wavelength, respectively, with 1 mm slits.

NAAm fluorescence lifetimes were measured using time

correlated single photon counting (TCSPC) with a Horiba-

Jobin-Yvon-IBH NanoLED (282 nm) as excitation source,

a Philips XP2020Q photomultiplier, and Canberra instru-

ments TAC and MCA. Measurements were performed in

de-aerated samples at several emission wavelengths. The

fluorescence decays were analyzed using the modulating

O

NH2

Scheme 1 Structure of 2-(1-

naphthyl) acetamide

J Incl Phenom Macrocycl Chem

123

functions method of Striker with automatic correction for

the photomultiplier ‘‘wavelength shift’’ [31]. The decays

were fitted by using FluoFit Pro version 4.

Irradiation of aqueous NAAm solutions (3.0 9 10-4 M)

in presence and absence of b-CD with simulated solar light

were performed in a Suntest CPS photoreactor (Atlas)

equipped with a xenon lamp and a filter that prevents the

transmission of wavelength below 290 nm. The lamp was

set at the intensity of 425 W m-2. The temperature was

maintained constant by a continuous flow of cold water

through the bottom of the photoreactor. Irradiations at

254 nm of aerated aqueous NAAm solutions

(3.0 9 10-4 M) in the presence and absence of b-CD were

performed in a cylindrical quartz reactor (50 mL) using up

to six germicidal lamps (Mazda TG 15 W).

The transformation of NAAm was monitored by HPLC

using a Waters 540 HPLC chromatography system equipped

with a Waters 996 photodiode array detector. The analysis

wavelength was set at 280 nm. The elution was accomplished

by using a reverse phase Nucleodur column C18

(250 mm 9 4.6 mm, 5 lm) with 50 % water (pH 3.5) and

50 % methanol. The flow rate was set at 1.0 mL min-1 and

the injected volume was 20 lL. Three injections were studied

for each sample in HPLC and the maximum error was 5.0 %.

The comparison of the byproducts was undertaken by using

LC/MS technique as described in our previous work [5].

Determination of the binding constant of the inclusion

complex

The binding constant of the inclusion complex was calcu-

lated using the Benesi–Hildebrand equation [32]. The

equation for 1:1 complex is given by Eq. (2):

1

I� I0

¼ 1

Imax � I0

þ 1

K[Imax � I0�½b� CD]ð2Þ

where I0 is the intensity of NAAm fluorescence without b-

CD, I is the intensity of NAAm fluorescence with a given

concentration of b-CD, Imax is the intensity of NAAm

fluorescence at the maximum concentration of b-CD and K

is the binding constant. A linear plot of 1/(I - I0) versus 1/

[b-CD] corresponds to a 1:1 complex, with the binding

constant given by expression (3):

K ¼ 1=slope Imax � I0ð Þ ð3Þ

Results and discussion

Characterization of the inclusion complex

and determination of the binding constant

Figure 1 presents the fluorescence excitation and emission

spectra of NAAm aqueous solution (1.0 9 10-5 M). The

NAAm excitation spectrum has a broad band with maxi-

mum wavelength at 280 nm while the emission spectra

shows maximum wavelength at 324 nm. Both spectra

clearly show vibronic structure that is typical of naphtha-

lene compounds.

The addition of b-CD (1.0 9 10-2 M) to aqueous

NAAm solution induces changes in both the absorption and

fluorescence emission spectra, as shown in Fig. 2. In the

absorption spectrum there is an increase in NAAm absor-

bance and a 1 nm bathochromic shift in the maximum

wavelength upon b-CD addition (Fig. 2a). This is attrib-

uted to the inclusion of NAAm in cyclodextrin cavity, as is

frequently observed with b-CD [7]. There is also an

increase in the fluorescence emission intensity of NAAm

and a very small bathochromic shift in the emission max-

imum (from 324 to 325 nm, Fig. 2b) upon addition of the

cyclodextrin.

The influence of b-CD concentration over the range

5.0 9 10-4–1.0 9 10-2 M on NAAm fluorescence emis-

sion intensity was studied. As shown in Fig. 3, the fluo-

rescence emission intensity of NAAm was enhanced with

increasing concentration of b-CD, up to a constant value.

Water is known to quench the fluorescence of aromatic

molecules in solution [33], and once NAAm molecules

enter the hydrophobic inner cavity of cyclodextrin this is

diminished, leading to the increase of the fluorescence

emission, reaching a constant value when all NAAm has

been entrapped in the hydrophobic cavity. In agreement

with this, the fluorescence lifetime of NAAm in aqueous

solution was found to be 35 ± 2 ns (v2 = 1.09) while in

presence of b-CD it increased to 41 ± 2 ns (v2 = 1.06).

Cyclodextrin complexes with naphthalene derivatives

have been reported with 1:1, 2:1, 1:2 and 2:2 stoichiometries

260 280 300 320 340 360 380 4000.0

0.2

0.4

0.6

0.8

1.0

No

rmal

ized

inte

nsi

ty/a

.u.

Wavelength/nm

Fig. 1 Normalized fluorescence excitation (dotted line) and emission

(solid line) spectra of aqueous NAAm solution (1.0 9 10-5 M); kex/

em = 280/324 nm

J Incl Phenom Macrocycl Chem

123

[6, 34, 35]. The 2:1 (naphthalene:cyclodextrin) complexes

are characterized by emission from the naphthalene excimer

[35], which was not observed in this case. Considering the

other possibilities, if a 1:2 complex is present a plot of 1/

(I - I0) as a function of of 1/[b-CD]2 should be linear.

Instead, with our system this gave an upward curve. In

contrast, a linear plot (r2 = 0.998) was obtained of 1/(I -

I0) as a function of 1/[b-CD] (Fig. 4), suggesting the for-

mation of a complex with a 1:1 stoichiometric ratio:

b-CDþ NAAm�K

b-CD � NAAm

The binding constant K for the inclusion complex

NAAm:b-CD was determined from the Benesi–Hildebrand

equation [32] based on the fluorescence data presented in

Fig. 3. A value of 651 M-1 was determined, which is

similar to that obtained for naphthalene inclusion in b-CD

(685 M-1) [34] and the naphthalene derivative 2-naph-

thyloxyacetic acid (560 M-1) [9].

Fluorescence quenching by inorganic anions

Previous studies have revealed that NAAm fluorescence is

effectively quenched by the anions I-, Br-, SCN- and N3-

through electron transfer [36]. We were interested to see how

the inclusion of NAAm in b-CD will affect this quenching.

The fluorescence emission of aqueous NAAm solution

(1.0 9 10-5 M) in the presence of b-CD (1.0 9 10-2 M)

was studied by adding different concentrations of the anions

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

240 260 280 300 3200.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

Ab

sorb

ance

Wavelength/nmAb

sorb

ance

Wavelength/nm

(a)

220 240 260 280 300 320 340 360 300 320 340 360 380 400 420 4400.0

8.0x106

1.6x107

2.4x107

3.2x107

4.0x107

Flu

ore

scen

ce e

mis

sio

n in

ten

sity

/a.u

.

Wavelength/nm

(b)

Fig. 2 a UV absorption spectra of aqueous NAAm solution

(1.0 9 10-5 M) in absence (solid line) and presence of b-CD

1.0 9 10-2 M (dotted line). Inset gives the expanded absorption in

the range 230–330 nm. b Fluorescence emission spectra

(kex = 280 nm) of the respective solutions described in (a)

0.0 2.0x10-3

4.0x10-3

6.0x10-3

8.0x10-3

1.0x10-2

3.4x107

3.6x107

3.8x107

4.0x107

4.2x107

4.4x107

Flu

ore

scen

ce e

mis

sio

n in

ten

sity

/a.u

.

[β-CD]/M

Fig. 3 Influence of b-CD concentration on the fluorescence emission

intensity (kex = 280 nm; kem = 325 nm) of aqueous NAAm solution

(1.0 9 10-5 M)

0.0 5.0x102

1.0x103

1.5x103

2.0x103

1.2x10-7

1.8x10-7

2.4x10-7

3.0x10-7

3.6x10-7

4.2x10-7

4.8x10-7

5.4x10-7

1/([β-CD]/M)

1/(I

-I0)

Fig. 4 Benesi–Hildebrand plot for the 1:1 complexation of NAAm in

b-CD (R2 = 0.998)

J Incl Phenom Macrocycl Chem

123

I-, Br-, SCN- and N3- and the results were analyzed using

the Stern–Volmer relationship (Eq. 1).

A comparison between efficiencies of NAAm fluores-

cence quenching in the presence and in absence of b-CD by

the selected anions is summarized in Table 1, while Fig. 5

shows, as an example, the Stern–Volmer plots for quenching

by I- without (a) and with (b) b-CD (1.0 9 10-2 M).

Increasing concentrations of KI induce a decrease on

NAAm fluorescence, both with and without b-CD. Similar

behavior was observed for all the other anions. However, the

quenching appears to be less efficient in the presence of b-

CD. From the fluorescence quenching given in Table 1, it is

possible to note a marked protective effect of the cyclo-

dextrin with all the anions. The ratio of KSV for the systems

(NAAm ? Q)/(NAAm ? Q ? b-CD) presents the highest

value for the anion I-. In this situation, the KSV for NAAm

solution without b-CD is roughly 2.2 times higher

(180 M-1) than in presence of b-CD (82 M-1). This

quenching relation seems to follow the inverse order of the

reduction potentials of the anions [37]. Fluorescence

quenching in these cases is suggested to involve electron

transfer, and these results corroborate the formation of the

inclusion complex between b-CD and NAAm and the pro-

tective effect of the cyclodextrin towards fluorescence

quenching, as previously reported for naphthalene as well as

its derivatives [11, 34].

Influence of b-CD on NAAm photodegradation

under simulated solar light

Aqueous NAAm solutions (3.0 9 10-4 M) were irradiated

in absence and presence of b-CD using two different sys-

tems: solar light simulator (Suntest) and UV irradiation at

254 nm. Figure 6a shows, as an example, the evolution of

the UV absorption spectra of NAAm in presence of b-CD

(1.0 9 10-2 M) upon Suntest irradiation. A decrease in the

absorbance band with maximum centered at 280 nm is

observed along with a simultaneous increase in the absor-

bance for wavelengths higher than 300 nm and smaller

than 260 nm, indicative of NAAm degradation and the

formation of photoproducts that absorb in the same region.

Isosbestic points are also observed in Fig. 6a, which pro-

vides further evidence for a single equilibrium involving

formation of the suggested 1:1 inclusion complex. The

same UV trends were observed either using different

cyclodextrin concentration or the irradiation system at

254 nm. Figure 6b presents the degradation kinetics of

aqueous NAAm solution (3.0 9 10-4 M) without and with

addition of b-CD with different concentrations, upon irra-

diation with the Suntest reactor. All the curves fit first-order

kinetics. In the absence of b-CD, 51 % of NAAm is

transformed after 8 h of irradiation, while in the presence

of b-CD the transformation efficiency is reduced to about

22–27 % after 8 h irradiation. In contrast, in the presence

of b-CD, almost no degradation is observed in the first

Table 1 Stern–Volmer constants for NAAm fluorescence quenching

by I-, Br-, SCN- and N3- in absence and presence of b-CD

(1.0 9 10-2 M)

Quencher (Q) KSV (M-1)

(NAAm?Q)

KSV (M-1)

(NAAm?Q?b-CD)

Br- 18 12

SCN- 123 76

N3- 102 59

I- 180 82

300 320 340 360 380 400 420 4400.0

6.0x106

1.2x107

1.8x107

2.4x107

3.0x107

3.6x107

Flu

ore

scen

ce e

mis

sio

n in

ten

sity

/a.u

.

Wavelength/nm

(a)

300 320 340 360 380 400 420 440

0.0

6.0x106

1.2x107

1.8x107

2.4x107

3.0x107

3.6x107

4.2x107

0.0 2.0x10-2

4.0x10-2

6.0x10-2

8.0x10-2

0

2

4

6

8

10

12

14

no β-CD

I 0/I

[KI]/M

with β-CD

Flu

ore

scen

ce e

mis

sio

n Ii

nte

nsi

ty/a

.u.

Wavelenght/nm

(b)

Fig. 5 Quenching effect of KI on fluorescence intensity

(kex = 280 nm) of aqueous NAAm solution (1.0 9 10-5 M) in

absence (a) and presence (b) of b-CD (1.0 9 10-2 M). Concentra-

tions of KI from top to bottom: 0, 8.0 9 10-4, 3.0 9 10-3,

8.0 9 10-3, 2.0 9 10-2 and 8.0 9 10-2 M. Inset Fig. 5b are repre-

sented the Stern–Volmer plots in absence (black right pointing

triangle) and presence of b-CD (1.0 9 10-2 M) (black circle)

J Incl Phenom Macrocycl Chem

123

120 min of irradiation. Following this inhibition period,

degradation of NAAm occurred, and the rate increased

with decreasing b-CD concentrations in the range

1.0 9 10-2–1.0 9 10-3 M, as shown in Fig. 6b. The

estimated first order rate constants and half-lives obtained

with Suntest in the absence and in the presence of varying

concentrations of b-CD are given in Table 2. The half-life

time of NAAm increased by a factor of eight in the pres-

ence of b-CD (1.0 9 10-2 M), which clearly shows a

stabilizing effect of the cyclodextrin, with NAAm less

prone to transformation with this high concentration of b-

CD. From the NAAm:b-CD binding constant obtained,

with the lowest b-CD concentrations there is still some free

NAAm in solution, and we believe that this is more sus-

ceptible to photodegradation than that included in the

cyclodextrin cavity. Upon increasing b-CD concentration,

virtually all the NAAm is inside CD cavity, leading to a

lower degree of degradation. In addition, studies with

related systems [9] have shown the importance of the

orientation and the total or partial inclusion in the cyclo-

dextrin cavity, and this may affect photoreactivity. For

example, the inclusion of the naphthalene derivatives

2-naphthyloxyacetic (2-NOA) acid and 1-napthylacetic

acid (1-NAA) in b-CD are found to be dependent on the

position of the substituent group [9]. For 1-NAA, a partial

inclusion was observed due to the steric effect of the

substituent group in position 1 while for 2-NOA the total

inclusion of the naphthalene moiety was observed to give a

complex with axial orientation.

Influence of b-CD on NAAm photodegradation

under UV irradiation

An aqueous NAAm solution (3.0 9 10-4 M) was also

irradiated at 254 nm in the absence and in the presence of

b-CD (1.0 9 10-2 M). The kinetic profiles obtained at this

wavelength are presented in Fig. 7. A bi-exponential fit

was observed for the kinetics in absence and presence of

cyclodextrin. In absence of cyclodextrin, NAAm is com-

pletely degraded after 4.5 h irradiation time with a pseudo

first order rate constant of 9.0 9 10-2 min-1 and a half-

life of 7.7 min. When the irradiation of NAAm was carried

out in presence of b-CD (1.0 9 10-2 M), the transforma-

tion still occurs but to a lesser degree. Under these con-

ditions, 74 % of NAAm is transformed after 4.5 h. The

rapid disappearance of NAAm in the early stages of deg-

radation is also observed with an initial estimated first

order rate constant of 4.7 9 10-2 min-1 and a half-life of

14.7 min. This is roughly two times slower than in absence

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 min

120 min

240 min

360 min

480 min

Ab

sorb

ance

Wavelength/nm

(a)

250 300 350 400 0 100 200 300 400 5000.45

0.60

0.75

0.90

1.05

C/C

0

Irradiation time/min

(b)

Fig. 6 a Evolution of UV absorption spectra of aqueous NAAm

solution (3.0 9 10-4 M) in presence of b-CD (1.0 9 10-2 M) upon

irradiation with Suntest. b Kinetics of NAAm degradation

(3.0 9 10-4 M) upon irradiation with Suntest in absence (black

square) and in presence of b-CD with different concentrations:

1.0 9 10-2 M (circle); 5.0 9 10-3 M (black circle) and

1.0 9 10-3 M (down pointing triangle) as function of irradiation

time followed by HPLC (kdet = 280 nm)

Table 2 Pseudo first order rate constants (k) and half-live times (t1/2)

of NAAm (3.0 9 10-4 M) in absence and presence of b-CD upon

irradiation with the Suntest

[b-CD] (M) k (min-1) t1/2 (h)

0 2.4 9 10-3 4.8

1.0 9 10-3 7.0 9 10-4 16.5

5.0 9 10-3 6.0 9 10-4 19

1.0 9 10-2 3.0 9 10-4 38.5

J Incl Phenom Macrocycl Chem

123

of b-CD (1.0 9 10-2 M). Hence, we can say that although

similar behavior was observed using the two different

irradiation systems, the degradation process of NAAm is

faster at 254 nm than with the Suntest probably due to the

high absorbance of NAAm at this wavelength and to the

higher energy produced by the germicidal UV lamps.

Several photoproducts were detected by HPLC upon

NAAm irradiation. All the photoproducts were eluted

before NAAm (24.1 min) indicating the formation of more

polar products compared with the parent compound. The

same products were obtained either using either Suntest or

254 nm irradiation. The main photoproducts of NAAm

formed in presence of b-CD were compared with those

elucidated for direct NAAm degradation [5]. Table 3 pre-

sents the retention time and the chemical structure of the

observed photoproducts. These correspond mainly to

hydroxylated (mono- and di-) and coumarin products. In

contrast with the behavior in the absence of the cyclo-

dextrin, no furanone and di-hydroxylated compounds with

hydroxyl group on the two aromatic rings were detected in

presence of b-CD. This indicates that encapsulation also

affects the photodegradation mechanism.

In conclusion, a stabilizing effect of b-CD on NAAm

photodegradation is observed by both sunlight and UV

irradiation. This suggests that b-CD may be used as

potential additive for NAAm formulations since it is non-

toxic and biodegradable under environmental conditions,

enhancing therefore the efficiency of the active ingredient

when applied on treatment of fruits and vegetables. Thus,

this can be regarded as a more sustainable, economic and

rational use of the plant growth regulator.

Acknowledgments Eliana Sousa Da Silva acknowledges the Por-

tuguese Science Foundation (FCT) for PhD grant (BD/SFRH/BD/

43171/2008) under the framework of the Portuguese POPH/FEDER/

QREN program.

References

1. Gardner, F.E.: Practical applications of plant growth substances

in horticulture. Proc. Fla. State Hort. Soc. 54, 20–26 (1941)

2. Tomlin, C.D.S.: The Pesticide Manual, 12th edn, p. 661. British

Crop Protection Council Publisher, Farnham (2000)

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0 50 100 150 200 2500.00

0.15

0.30

0.45

0.60

0.75

0.90

1.05C

/C0

Irradiation time/min

Fig. 7 Kinetics of NAAm degradation (3.0 9 10-4 M) upon irradi-

ation at 254 nm in absence (black square) and in presence of b-CD

1.0 9 10-2 M (circle) as function of irradiation time followed by

HPLC (kdet = 280 nm)

Table 3 Retention time and proposed structure of NAAm main

photoproducts formed in presence of b-CD

Retention time/

min

Proposed chemical structure

24.1

NH2

O

10.4

19.4

O

NH2

OH

O

NH2

HOHO

O

NH2

11.1

17.0HO

O

NH2

OH

18.5O

O

NH2

O

or OO

O

NH2

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