A synthetic amino acid residue containing a new oligopeptide-based photosensitive fluorescent organogel

Article abstract of DOI:10.1002/asia.201200617

A synthetic amino acid (with a stilbene residue in the main chain) containing a tripeptide-based organogelator has been discovered. This peptide-based synthetic molecule 1 self-assembles in various organic solvents to form an organogel. The gel has been thoroughly characterized by using various microscopic techniques including field-emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), X-ray diffraction (XRD), UV-visible and fluorescence spectroscopy, and rheology. Morphological investigations using FESEM and AFM show a nanofibrillar network structure. Interestingly, the organogel is photoresponsive and a gel-sol transition occurred by irradiating the gel with UV light of 365 nm for 2 h as shown by the UV and fluorescence study. This photoresponsive fluorescent gel holds promise for new peptide-based soft materials with interesting applications. Copyright

Full text of DOI:10.1002/asia.201200617

FULL PAPER
DOI: 10.1002/asia.201200617
A Synthetic Amino Acid Residue Containing A New Oligopeptide-Based
Photosensitive Fluorescent Organogel
Dibakar Kumar Maiti and Arindam Banerjee*[a]
Abstract: A synthetic amino acid (with
a stilbene residue in the main chain)
containing a tripeptide-based organo-
gelator has been discovered. This pep-
tide-based synthetic molecule 1 self-as-
sembles in various organic solvents to
form an organogel. The gel has been
thoroughly characterized by using vari-
ous microscopic techniques including
field-emission scanning electron mi-
croscopy (FESEM), atomic force mi-
croscopy (AFM), X-ray diffraction
(XRD), UV-visible and fluorescence
spectroscopy, and rheology. Morpho-
logical investigations using FESEM
and AFM show a nanofibrillar network
structure. Interestingly, the organogel is
photoresponsive and a gel–sol transi-
tion occurred by irradiating the gel
with UV light of 365 nm for 2 h as
shown by the UV and fluorescence
study. This photoresponsive fluorescent
gel holds promise for new peptide-
based soft materials with interesting
applications.
Keywords: gels
· isomerization ·
peptides · photochemistry · stilbene
Introduction
sensing of explosives like trinitrotoluene.[16g] Ajayaghosh
and co-workers also demonstrated that a fluorescent orga-
nogel can be used as a white-light emitter.[16h] Different pho-
tochromic moieties including the azobenzene system,[17] bis-
thienylethene derivatives,[18] butadiene-containing com-
pounds,[19] and bis(phenylalanine)maleic acid[20] can be intro-
duced as basic building blocks to make smart photosensitive
gels. Shinkai and co-workers and Zhu and co-workers sepa-
rately described the photoresponsive behavior of organoge-
lators derived from anthracene derivatives.[21] Shimizu and
co-workers reported that a thymine-containing bolaamphi-
phile showed a reversible photochemical conversion of self-
assembled helical nanofibers.[22] Recently, Ajayaghosh and
co-workers reported sugar-based functional photoresponsive
gels that selectively removed toxic aromatic solvents from
water.[23]
Stilbene derivatives have become important photochromic
building blocks to attain smart gels.[24] Stilbene derivatives
have several applications as dye[25] and whitening agents.[26]
The scintillation properties of stilbene-based crystals have
been applied for neutron detection.[27] Stilbene derivatives
can also be used as chemosensors[28] and fluorescence
probes for lead ions in live cells and living tissues.[29] In addi-
tion, stilbene derivatives have been documented to be bio-
logically important for their antimalarial[30] and antibacterial
activities.[31] The neuroprotective effects of stilbene deriva-
tives on ischemia or reperfusion brain injury have also been
proved.[32] These stilbene derivatives exhibit interesting non-
linear optical (NLO) properties.[33] Stilbene derivatives can
also be used as efficient light-harvesting and energy-transfer
materials.[34a] Recently, Das and co-workers have reported
a stilbene-based UV-light-controllable nanodevice with tun-
able encapsulating and release properties.[34b] However,
none of these above-mentioned examples of stilbene-con-
taining gelators include a peptide moiety covalently at-
Molecular assembly through various noncovalent interac-
tions is a powerful approach that is used to make new supra-
molecular soft materials. Low-molecular-weight gelators
(LMWGs)[1] have gained increasing interest in current re-
search owing to their various potential applications in pho-
tovoltaics,[2] light-harvesting materials,[3] templates for
making nanoparticles and nanoclusters,[4] controlled drug re-
lease,[5a–g] oil spill recovery,[5h–l] and others.[6] A gelator mole-
cule can self-assemble to form a three-dimensional supra-
molecular network structure with a lot of void space occu-
pied by a large amount of solvent molecules to form a gel
under suitable conditions. Various noncovalent interactions,
including hydrogen bonding,[7] p–p stacking,[8] electrostatic,[9]
and van der Waals,[10] are responsible for the self-association
of the gelator molecule to form a gel. Stimuli-responsive
gels are particularly important owing to their various poten-
tial applications.[6h,11] There are different stimuli-responsive
gels in the literature including photosensitive,[12] pH-respon-
sive,[5b,13] enzyme-responsive,[14] and redox-dependent gels.[15]
For the past few years functional organogels containing
chromophores have drawn significant attention as these soft
materials have found immense interest as novel materials
for optoelectronic and other applications.[16] Other than the
optoelectronic applications, fluorescent organogels are also
important for their use in erasable thermal imaging[16f] and
[a] D. K. Maiti, Prof. A. Banerjee
Department of Biological Chemistry
Indian Association for the Cultivation of Science
Jadavpur, Kolkata-700032 (India)
Fax : (+91)33-2473-2805
E-mail: bcab@iacs.res.in
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201200617.
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Dibakar Kumar Maiti and Arindam Banerjee
Table 1. The gelation properties of stilbene-containing organogelator 1 in
different solvents.
tached to it. So, there is a real need for making a peptide-
appended stilbene-containing gelator molecule that can be
assembled using various noncovalent interactions including
hydrogen bonding and p–p stacking, among others. More-
over, the stilbene moiety is photoresponsive.
Over the course of our examination of the self-assembly
and gelation of peptide-based molecules,[4b,c,5b,35] a synthetic
amino acid residue with a stilbene-moiety-containing oligo-
peptide was found to form a photoresponsive organogel in
various organic solvents. The photosensitivity of the gelator
molecule occurs as a result of a trans–cis isomerization of
the stilbene moiety present in the gelator. The presence of
this UV-light-triggered gel–sol transition holds promise for
the future use of this stimuli-responsive soft material.
Solvent
Property[a]
CGC [mgmLꢀ1
]
chloroform
dichloromethane
ethyl acetate
hexane
S
S
S
–
–
–
–
P
n-octane
P
–
isooctane
S
–
benzene
toluene
dichlorobenzene
chlorobenzene
nitrobenzene
o-xylene
p-xylene
m-xylene
ethanol
TG
TLG
TG
TG
TLG
TG
TG
TG
P
3.75
3.05
3.75
4.50
4.00
3.65
3.55
3.50
–
isobutanol
n-butanol
n-hexanol
n-heptanol
n-octanol
TG
TG
TG
TG
TG
3.60
3.65
4.00
5.38
3.42
Results and Discussion
Gel Formation
The gel was formed from a new gelator based on stilbene-
containing peptide 1 (Figure 1) by dissolving the gelator
molecule in hot (808C) n-octanol to make a clear solution
followed by sonication for 1 min. Then the solution was
[a] S=solution; P=precipitate; TG=transparent gel; TLG=translucent
gel.
Thermodynamic Study
Sol–gel transition temperatures (Tgel) for 1 are plotted
against different gelator concentrations in Figure 2. The gel
melting temperature (Tgel) increases as the concentration of
the gelator molecule increases and finally a plateau appears.
From these results, it is clear that with higher concentrations
Figure 1. Chemical structure of gelator molecule 1 (left). Macroscopic
view of the gel in n-octanol (transparent) and in toluene (translucent)
(right).
cooled down to room temperature, and the transparent or-
ganogel was formed. Gelation was determined by the ab-
sence of flow of the solvent when the tube was inverted. To
measure the critical gelation concentration (CGC), we in-
creased the n-octanol solvent volume by 50 mL every time to
5 mgmLꢀ1 stilbene-mediated organogel until the concentra-
tion was insufficient to form a stable gel.
Figure 2. Tgel plot for the organogel using n-octanol.
Gelation Property of the Stilbene-Mediated Gelator
The gelation properties of 1 in various solvents have been
investigated. The compound could form a gel in most of the
twenty solvents listed in Table 1. The CGC varies from
3.05 mgmLꢀ1 in toluene to 5.38 mgmLꢀ1 in n-heptanol. An-
other interesting feature is that the gelator molecule forms
transparent gels in most of the tested solvents, with a few
exceptions: translucent gels are formed in toluene and nitro-
benzene.
of gelator molecules, the supramolecular gel network is
stronger. The gelation temperature was measured by using
the test-tube inversion method. The thermodynamic param-
eters of gelation have been calculated from Equations (1)–
(3):[5b]
DGꢁ ¼ RT ln f
ð1Þ
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Dibakar Kumar Maiti and Arindam Banerjee
ꢀ
ꢁ
ꢀ
xerogels derived from this organogelator show nanofibrillar
assembly with a high aspect ratio. For the organogel ob-
tained from octanol, the width of the nanofibers are within
the range 30–40 nm; whereas in the case of an organogelator
obtained from toluene, the widths of nanofibers are in the
range of 35–55 nm. Nanofibers are several microns in
length. It is notable that solvent-dependent morphological
transformation has occurred.
Fibers observed in FESEM images using n-octanol gel are
generally straight fibers. Figure 3d (enlarged version of Fig-
ure 3c) shows that the majority of the fibers are straight
with a few exceptions of the occurrence of some bent fibers,
whereas twisted nanofibers are formed in toluene gel (Fig-
ure 3a).
d ln f
RT
DHꢁ ¼ ꢀRT2
ð2Þ
ð3Þ
ꢁ
d ln f
dT
DSꢁ ¼ ꢀR ln f ꢀ RT
in which f (mole fraction of gelator)=a/ACTHNUGRTENUNG(a+b), where a is
the number of moles of gelator, b is the number of moles of
solvent, and T is the corresponding gel-melting temperature.
The graph of lnf versus T [K] has been plotted for the gel
obtained from n-octanol and the slope [dACTHNUGRTENUNG(lnf)/dT] of the
graph was calculated. By using the value of the slope in
these equations, thermodynamic parameters for the gel in n-
octanol were calculated (see the Supporting Information,
Figure S1) and are given in Table 2. The negative value of
the Gibbs free energy (Table 2) shows that the gel formation
is a spontaneous process.
The AFM images of xerogels obtained from organogela-
tor 1 are shown in Figure 4. The AFM image of the toluene
gel obtained from 1 shows a nanofibrillar morphology of
Table 2. The calculated thermodynamic parameters of gelation.
d
G
DG [kJmolꢀ1 Kꢀ1
]
DH [kJmolꢀ1
]
DS [Jmolꢀ1 Kꢀ1
]
ꢀ19.28
ꢀ59.597
ꢀ122.927
Morphological Study
To gain a better insight into the microstructures of the com-
posites, the morphologies of xerogels have been studied
using fluorescence microscopy, field-emission scanning elec-
tron microscopy (FESEM), and atomic force microscopy
(AFM).
The stilbene-containing gelator forms a nanofibrillar net-
work upon gelation in organic solvents. From the fluores-
cence microscopy image (see the Supporting Information,
Figure S2) obtained from the xerogel, a fibrillar morphology
has been obtained for this gel. FESEM images (Figure 3) of
Figure 4. AFM images of the organogel in a) n-octanol and b) toluene.
c) AFM image of a single twisted fiber formed in toluene gel. Height
profile of d) a straight fiber from the organogel in n-octanol and e) a
twisted fiber from the organogel in toluene.
helical twisted fibers of width 35–75 nm, with a pitch of
116 nm. The pitch along a single helical fiber is uniform.
However, the pitch of one helical fiber to another fiber
varies slightly. The AFM image of the n-octanol gel shows
a network of tapelike morphology. The width of the fibers
are within the range 330–430 nm and the height of these
tapes are within the range of 80–100 nm. It is evident from
Figure 4a that the fibers obtained from n-octanol organogels
are generally straight fibers as is indicated by the double-
headed arrow in Figure 4a and the height profile of that
portion is given in Figure 4d, which indicates its straightness.
Some bent fibers are also present and they are observable in
a few cases (Figure 4a). Gel nanofibers and nanotapes are
entangled with each other to form a three-dimensional net-
Figure 3. SEM images of the organogel in (a,b) toluene and (c,d) n-octa-
nol (d is an enlarged version of the area indicated in panel c).
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Dibakar Kumar Maiti and Arindam Banerjee
work. The three-dimensional network structure of the gel
UV-Visible Spectroscopic Study
matrix may be responsible for the entrapment of solvent
molecules to form self-supported organogels. The different
widths of these gel fibers may be a result of the different ex-
perimental procedures that are adopted for different micro-
scopic experiments.[36]
The UV-visible spectroscopic study has been performed
with the sol and gel obtained from 1. In the absorbance
spectrum of the sol, two peaks appeared at wavelengths of
343 and 243 nm with an absorbance maxima at 343 nm.[38]
The strong absorption band at 343 nm with a weak absorp-
tion band at 243 nm can arise owing to the stilbene moiety
in the trans conformation. The gel-phase material also ab-
sorbs at the same wavelength. However, the intensity of
these absorption bands is much higher than those in the sol
state. So, a hyperchromic shift is observed owing to the for-
mation of a supramolecular network structure in the gel
phase. Figure 6 illustrates the UV-visible spectra of the gel
and sol states. The gel material emits blue fluorescence
upon exposure to a UV lamp irradiating at 365 nm.
X-ray Diffraction (XRD) Study
An XRD study of the gel-phase material has been per-
formed to investigate the molecular packing responsible for
the formation of the gel-phase material. The XRD charac-
terization of the xerogel (see the Supporting Information,
Figure S3) shows four strong reflection peaks at 2q=7.87,
15.66, 23.51, and 29.148. Their corresponding d spacings are
d1 =1.122, d2 =0.565, d3 =0.378, and d4 =0.305 nm. The ratio
d1/d2/d3/d4 ꢂ1:1/2:1/3:1/4 clearly indicates the formation of
a layered structure[37] of gelator 1 molecules with a 1.12 nm
long period. The relatively weak peak at 2q=20.58 can be
attributed to p–p stacking with an average distance of
0.43 nm. XRD data of the wet gel (see the Supporting Infor-
mation, Figure S4) shows a strong peak at 2q=20.388 corre-
sponding to d=0.435 nm, which establishes the presence of
p–p stacking in the wet gel also.[37]
Rheological Study
The fundamental mechanical properties of a gel are mea-
sured by using rheology. Rheological measurements provide
the mechanical strength as well as the viscoelastic property
of a gel.
At the low-frequency region, if the storage modulus (G’)
and loss modulus (G’’) are independent of the applied angu-
lar frequency (w) and G’>G’’, then the system is considered
to be a gel.[35a,b] Rheological studies have been carried out at
a constant oscillatory frequency of 1 Hz at room tempera-
ture (258C). In Figure 5, G’ and G’’ are plotted against the
Figure 6. UV spectroscopic study of the UV-light-responsive nature of
the organogel in n-octanol. The upper panel indicates pictures taken in
daylight, the lower panel indicates pictures taken in the dark.
Fluorescence Spectroscopic Study
Fluorescence studies under different conditions of the gela-
tor molecule (in the sol and gel state) have been performed
to probe the changes of the fluorescence pattern in different
states.
The gel-phase material showed an emission peak at
443 nm upon excitation at 394 nm. However, the sol emits at
440 nm with excitation at 398 nm. The fluorescence intensity
is increased for the gel state due to a hyperchromic shift.
The concentrations of the gelator remain the same in the sol
state as well as in the gel state.
A temperature-dependent fluorescence experiment was
performed to obtain the changes in the emission spectra oc-
curring as a result of an increase in temperature. The orga-
nogel was first prepared in n-octanol and fluorescence emis-
sions were recorded at various temperatures ranging from
22 to 708C using a fluorometer attached with a peltier
module. The intensity of the emission peak at 443 nm for
the gel gradually decreased from 22 to 558C and after that
the peak was shifted to 440 nm owing to its transformation
to the sol state (Figure 7).
Figure 5. Rheology of the organogel in n-octanol. Loss modulus=G’’,
storage modulus=G’, angular frequency=w.
angular frequency. Figure 5 shows that G’ and G’’ are almost
parallel to the w axis. It is noticed that the storage modulus
is more than one order of magnitude greater than the loss
modulus, which indicates a rigid-gel-forming behavior of the
self-assembled material.[35a,b]
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Dibakar Kumar Maiti and Arindam Banerjee
Figure 9. SEM study of the UV-light-responsive nature of the organogel
in n-octanol.
After irradiation of UV light for 1 h, the FESEM image of
the gel (Figure 9) shows the disappearance of the network
structure and the presence of isolated nanofibers as well as
nanodots within the system. This indicates that the nanofi-
brillar network structure has started breaking. A noteworthy
feature is that the nanofibers are shorter in length relative
to that of the gel nanofibers. After 2 h of exposure to UV
light, all nanofibers in the gel have disappeared and only
nanodots are visible in the SEM image (Figure 9) and the
gel has been converted to the sol-phase material.
Figure 7. Temperature-dependent fluorescence emission study of the or-
ganogel in n-octanol showing the change of morphology upon the expo-
sure to UV light with respect to time.
Photosensitivity Study
The stilbene-containing peptide-based gelator 1 was dis-
solved in n-octanol with heating to make a clear solution.
Then it was sonicated for a minute to obtain a clear gel.
This gel was irradiated by a simple UV lamp (output 4 W)
at 365 nm for 2 h and the gel finally transformed into the sol
(Figure 8). The distance between the gel and the UV lamp
was about 1 cm and an increase in temperature during the
2 h of irradiation by the UV lamp was only 28C. The gel
melting temperature (at 0.5%w/v) was found to be 528C.
The exact mechanism and characterization for such a type
of photosensitivity have been investigated thoroughly.
UV-Visible and Fluorescence Spectroscopic Study
Irradiation by UV light for 2 h resulted in the transforma-
tion of the gel to the sol state. The sol obtained after the
breaking of the gel through UV irradiation absorbs at
324 nm with a lower intensity of absorption, that is, a blue-
shift has occurred. The other absorption peak of 243 nm
shifted towards a higher wavelength of 252 nm. The reason
for such an observation can be due to the photochemical
transformation of the stilbene-based peptide molecule from
a trans to cis conformation.[38] The exact mechanism behind
such gel melting by irradiation with only UV light will be
discussed in the following NMR spectroscopy section.
However, the sol obtained after UV irradiation of the or-
ganogel is unable to form the original gel state even after
keeping it for a period of 2 months (see the Supporting In-
formation). So, it can be stated that the gel-melting phenom-
enon by using UV light irradiation at 365 nm promotes an
irreversible photochemical change.
A fluorescence spectroscopic study has also been per-
formed to examine the photoresponsiveness of the organo-
gel. The UV irradiation of the organogel for about 2 h also
resulted in a decrease in the intensity of the fluorescence
spectrum (Figure 10) with a slight change of the photolumi-
nescence excitation spectrum accompanied by a shift of the
absorption maxima from 394 to 391 nm (Figure 11).
Figure 8. UV-light-responsive nature of the organogel in n-octanol.
1H NMR Spectroscopy Study
Morphological Study
We were curious to know the exact mechanism that was op-
erating during gel melting upon UV exposure. Thus, we de-
cided to use NMR spectroscopy. The UV radiation will
always be able to operate in the same way in the system
whether chloroform-d or toluene-d8 is used for the study.
Here, we have used chloroform-d as the solvent for the
1H NMR spectroscopy experiment to avoid the broad peaks
A
time-dependent morphological investigation using
FESEM was performed to address the question as to wheth-
er the morphology of the gel-phase material changed upon
slow exposure to UV light at 365 nm. Figure 9 represents
the morphology of the dried gel showing a beautiful nanofi-
brillar network structure before any exposure to UV light.
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Dibakar Kumar Maiti and Arindam Banerjee
Figure 10. Fluorescence emission spectra of the sol and gel in n-octanol
and the UV-light response of the gel.
Scheme 1. Structure and conformation of 1 showing the UV-triggered
trans–cis transition around the C-terminal stilbene moiety.
photochemical cyclization product is ruled out as no new
peak is observed after UV irradiation for 2 h at d=2–3 ppm,
which corresponds to the cyclobutane ring protons of the
[2+2] photochemical cyclization product (see the Supporting
Information).
Conclusion
Figure 11. Fluorescence excitation of the gel, sol, and sol obtained after
irradiation of the gel with UV light.
A new synthetic gelator molecule has been discovered. The
gelator molecule contains a stilbene-containing amino acid
residue in an oligopeptide system and this forms a photosen-
sitive organogel in various organic solvents. One of these
gels has been characterized by using various techniques such
as FESEM, AFM, XRD, UV-visible and fluorescence spec-
troscopy, and rheology. XRD studies indicate the presence
of p–p interactions and a multilamellar-ordered structure in
the gel state. Interestingly, the organogel exhibits photores-
ponsive behavior and the gel breaks under exposure to UV
irradiation at 365 nm, which is accompanied by UV-visible
spectral changes. The exact mechanism responsible for such
a gel-breaking phenomenon is clearly the cis–trans confor-
mational transformation of the double bond of the stilbene
molecule and it has been confirmed by performing
from the toluene-d8 solvent owing to gelation. Molecule
1
1 has a H NMR spectroscopic peak at d=7.042 ppm (HA
and HB) with a coupling constant value of J=17 Hz that cor-
responds to the centrally located double bond of the stilbene
moiety in the trans conformation (Scheme 1). However,
upon exposure to UV light for 2 h, the 1H NMR spectro-
scopic peak is shifted from d=6.99 ppm to d=6.553 and
6.632 ppm (HA and HB) and the coupling constant between
these two protons is 12 Hz (see the Supporting Informa-
tion). This indicates that the double bond across the stilbene
moiety is in the cis conformation (Scheme 1).
So it can be stated from the 1H NMR spectroscopy experi-
ment that the stilbene part of the gelator molecule under-
goes a trans–cis conformation under UV irradiation for 2 h.
This leads to the conclusion that the gel-melting phenomen-
on is accompanied by a trans–cis isomerization of the stil-
bene moiety of the gelator molecule that is triggered by the
UV irradiation for 2 h. The stilbene moiety may also under-
go [2+2] photochemical cyclization under UV irradiation of
the gel. However, the possibility of the formation of a [2+2]
a
1H NMR spectroscopic study. The remarkable photores-
ponsive ability of this organogelator implies its potential
utility in a variety of advanced materials applications.
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Experimental Section
The stilbene-containing peptide-based organogelator 1 has been synthe-
sized by solution-phase coupling of the methyl 4-(4-aminostyryl)benzoate
(5; see the Supporting Information) and Boc-Phe-Phe-OH (9) using dicy-
clohexylcarbodiimide (DCC) and 1-hydroxybenzotriazole (HOBT) cou-
pling reagents followed by purification through silica-gel column chroma-
tography. The solid product obtained was characterized by 1H NMR spec-
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Synthesis of MeO-ST-FF-Boc (1)
Boc-Phe-Phe-OH (9; 0.10 g, 0.243 mmol) was dissolved in a mixture of
N,N-dimethylformamide (DMF; 0.5 mL) in an ice–water bath. Com-
pound 5 (0.06 g, 0.24 mmol) was dissolved in a minimum volume of ethyl
acetate and was added to the reaction mixture, followed by the immedi-
ate addition of DCC (0.05 g, 0.24 mmol). The reaction mixture was al-
lowed to come to room temperature and was stirred for 48 h. Dicyclohex-
ylurea (DCU) was removed by filtration and ethyl acetate was added to
the filtrate. The organic layer was washed with 2m HCl (3ꢁ5 mL), brine
(2ꢁ5 mL), and 1n sodium carbonate (3ꢁ5 mL), and brine (2ꢁ5 mL);
dried over anhydrous sodium sulfate; and evaporated under vacuum to
yield a white solid. The crude product was purified by silica-gel column
chromatography (5% EtOAc/CHCl3) to afford pure 1 as a white solid
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(0.11 g, 71%). ½aꢃ2D5 = +7.548; H NMR (500 MHz, CDCl3): d=8.0 (d, J=
1
8.5 Hz, 2H), 7.59 (m, 2H), 7.54 (d, J=8 Hz, 2H), 7.46–7.43 (m, 2H),
7.34–7.14 (m, 10H), 7.04 (d, J=16 Hz, 1H), 7.02 (d, J=16 Hz, 1H), 4.81
(m, CaH, 2H), 3.91 (s, 3H), 3.04–3.00 (m, CbH, 4H), 1.37 ppm (s, Boc-
CH3, 9H); FTIR: n˜ =3324.48, 3029.36, 2928.21, 2850.48, 1719.41, 1689.3,
1651.37, 1626.61, 1588.57, 1525.17, 1435.89, 1415.66, 1390.42, 1367,
1311.49, 1281.86, 1245.17 cmꢀ1
; elemental analysis calcd (%) for
C39H41N3O6: C 72.31, H 6.38, N 6.49; found: C 72.38, H 6.38, N 6.46;
HRMS: m/z calcd for C39H41N3O6: 670.288 [M+Na]+; found: 670.281.
Acknowledgements
D.K.M. would like to acknowledge CSIR, New Delhi, India for financial
assistance. We gratefully acknowledge the DST project no. SR/S1/OC-73/
2009, Govt. of India, for financial support. We are thankful to anonymous
referees for their valuable and useful comments.
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