Self-assembly and nonlinear optical properties of a synthetic dipeptide

Article abstract of DOI:10.1039/c2jm34338j

The self-assembly propensities and nonlinear optical properties of synthetic dipeptides are illustrated. The single crystal X-ray diffraction study of dipeptide 1 containing a p-nitrophenylalanine moiety reveals that the peptide adopts a supramolecular antiparallel β-sheet structure using hydrogen bonding, as well as π-π stacking interactions, in the solid state and the peptide exhibits nonlocal thermal nonlinear refraction due to the thermal lensing effect. The heat dissipation in the dipeptide 1 was a slow process with a millisecond to microsecond time scale. However the peptide 2 containing a p-nitrophenylacetic acid moiety adopts a parallel β-sheet structure and has no thermal lensing effect.

Full text of DOI:10.1039/c2jm34338j

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PAPER
Self-assembly and nonlinear optical properties of a synthetic dipeptide†
a
Suman Kumar Maity, Ravi Kumar, Deepak K. S. Ambast, Bipul Pal and Debasish Haldar
a
b
b
a
*
*
Received 4th July 2012, Accepted 5th September 2012
DOI: 10.1039/c2jm34338j
The self-assembly propensities and nonlinear optical properties of synthetic dipeptides are illustrated.
The single crystal X-ray diffraction study of dipeptide 1 containing a p-nitrophenylalanine moiety
reveals that the peptide adopts a supramolecular antiparallel b-sheet structure using hydrogen bonding,
as well as p–p stacking interactions, in the solid state and the peptide exhibits nonlocal thermal
nonlinear refraction due to the thermal lensing effect. The heat dissipation in the dipeptide 1 was a slow
process with a millisecond to microsecond time scale. However the peptide 2 containing a p-
nitrophenylacetic acid moiety adopts a parallel b-sheet structure and has no thermal lensing effect.
Recently we have reported the photo physical properties of
peptides and peptide–CdS conjugates.14 Herein we present the
formation of supramolecular antiparallel and parallel sheet-like
structures by synthetic peptides through intermolecular
hydrogen bonding and p–p stacking interactions. The peptide 1
containing a p-nitrophenylalanine residue exhibits nonlocal
thermal nonlinear refraction due to the thermal lensing effect.
Although, the peptide 2 containing a p-nitrophenylacetic acid
moiety has no thermal lensing effect.
Introduction
Organic materials which have large nonlinear optical properties
are of great importance due to their potential application in
frequency conversion,1 optical limiting,2 optical modulators3 and
in the investigation of nonlocal effects.4,5 The understanding of
the nonlinear polarization mechanisms and their relation to the
structural characteristics of the materials are highly interesting.
The nonlinear optical properties of organic compounds can be
attributed to light-induced molecular reorientation, incoherent
electronic excitation and the thermal lens effects.6 Depending
upon orientation, temperature and light wavelength some
organic materials change light passing through them and release
one photon of accumulated higher energy. Various types of
organic compounds including polyenes,7 nitroanilines,8 ionic
liquids,9 phthalocyanines10 and polymethines11 have been used to
develop materials presenting large nonlinearities and satisfying
technological requirements for applications by having a fast
response, wide transparency range and high damage threshold.
In most cases, because of their large dipole moment the highly
Results and discussion
p-Nitro substituted phenylalanine, a promising material for
nonlinear optics, was synthesized by a conventional solution
phase nitration of acetyl protected phenylalanine at low
temperature. The peptides 1 and 2 (Fig. 1) containing p-nitro-
phenylalanine and p-nitrophenylacetic acid were synthesized by
conventional solution-phase methodology, purified, character-
ized and studied. The assumption was that the Aib residue in
peptide 1 would enhance the helical propensity and crystallinity15
and the p-nitrophenyl group should impart additional polarity in
the compounds and help to modulate light.16
nonlinear optical molecules are organized in a centrosymmetric
€
7
array. In a recent review, Facke and co-workers discussed the
high macroscopic nonlinearities of organic dyes for development
of the optical-holographic data-storage devices.12 Sol–gel derived
hybrid organic–inorganic materials have also been used to design
materials with good mechanical properties and nonlinear optical
applications.13
aDepartment of Chemical Sciences, Indian Institute of Science Education
and Research Kolkata, Mohanpur, West Bengal 741252, India
bDepartment of Physical Sciences, Indian Institute of Science Education
and Research Kolkata, Mohanpur, West Bengal 741252, India. E-mail:
deba_h76@yahoo.com; bipul@iiserkol.ac.in; Fax: +91 3325873020; Tel:
+91 3325873119
† Electronic supplementary information (ESI) available: 1H NMR, 13C
NMR, solid state FTIR spectra, ESI, Fig. S1–S5 and S1–S11. CCDC
883160 and 883161. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c2jm34338j
Fig. 1 A schematic presentation of the reported peptides 1 and 2.
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Preliminary information on the conformational preferences of
the peptides was obtained from FT-IR studies. For peptide 1
there are two peaks in the 3200–3500 cmꢀ1 region corresponding
to H-bonded NH stretching frequencies. The band at 1739 cmꢀ1
is attributed to the non H-bonded C]O of the methyl ester
moiety and the band at 1637 cmꢀ1 is for the stretching of the
H-bonded C]O of peptide 1 (ESI, Fig. S1†). The bands
observed at 1637 and 3275 cmꢀ1 are typical for b-sheet con-
formation.14a For peptide 2 the short band at 1717 cmꢀ1 is for the
H-bonded ester C]O stretching and the strong band at 1654
cmꢀ1 is for the H-bonded amide C]O stretching. The strong
band at 3429 cmꢀ1 is for the H-bonded OH stretching and at
3239 cmꢀ1 is for the H-bonded NH group (ESI, Fig. S1†). Hence,
the reported peptides have self-assembled through hydrogen
bonding interactions.
1 ꢀ x, ꢀy, 2 ꢀ z) and thereby form a supramolecular antiparallel
b-sheet like structure (Fig. 3) along the crystallographic a
direction.18
The antiparallel b-sheet like columns of peptide 1 is further
self-assembled through various noncovalent interactions in a
higher order assembly (Fig. 4). There also exists strong inter-
molecular p–p interactions (Fig. 4) in parallel staggered geom-
etry between p-nitrophenylalanine aromatic rings (centroid to
ꢀ
centroid distance is 3.681 A).
The solid state conformation of peptide 2 shows that the
peptide adopts a kinked conformation (ESI, Fig. S2†). X-ray
crystallography reveals that the molecules of peptide 2 form a
parallel sheet like structure through intermolecular H-bonding
ꢁ
ꢀ
ꢀ
interactions (N2–H2A/O4, 1.98 A, 2.83(2) A, 171 , 1 + x, y, z)
along the crystallographic a direction (Fig. 5). There also exists a
p–p stacking interaction between nitrophenyl–nitrophenyl
The self-assembly of the reported peptides was further studied
by atomic force microscopy. A solution of the corresponding
peptide in ethyl acetate was drop casted on a clean microscopic
cover slip and allowed to dry under vacuum at 30 ꢁC for 2 days.
Examination by atomic force microscopy revealed a flower like
morphology for peptide 1 (Fig. 2a and b) and a fibrillar structure
for peptide 2 (Fig. 2c and d). The peptide 2 fibrils have a diameter
of ca. 100 nm and are several micrometers in length.
ꢀ
(shortest C–C distance is 3.50 A) functional groups and tyrosine–
ꢀ
tyrosine phenyl rings (shortest C–C distance is 3.76 A). Thus the
interactions between the adjacent molecules of peptide 2 are
quite strong in nature.
Moreover, the parallel sheets, like units of peptide 2, are
interdigited through T-shaped p–p stacking and intermolecular
H-bonding interactions to form a supramolecular nanoporous
architecture (Fig. 6). There are three mutually perpendicular
T-shaped p–p stacking interactions which involve the aromatic
rings of tyrosine moieties of four adjacent molecules of peptide 2.
The C to centroid distance of these T-shaped p–p stacking
The solid state conformations of peptide 1 and 2 have been
studied by X-ray crystallography. Yellow triclinic crystals of
peptide 1 and yellow orthorhombic crystals of peptide 2 suitable
for X-ray diffraction studies, were obtained from their ethyl
acetate–hexane solution and ethyl acetate–toluene solution
respectively by slow evaporation.17 The peptide 1 has a centro-
symmetric space group P1. The solid state conformation of
peptide 1 shows that it has an extended backbone conformation
(ESI, Fig. S2†).18 The individual sub-units of peptide 1 are
themselves regularly inter-linked through two intermolecular
ꢀ
interactions is 4.05 A (Fig. 6). The parallel sheet like units of
peptide 2 are also linked through intermolecular H-bonding
between tyrosine OHꢁ and C]O of methyl ester (O2–H2/O5,
ꢀ
ꢀ
2.00 A, 2.81(2) A, 177 , ½ + x, 3/2 ꢀ y, ꢀz) along the axis parallel
to the crystallographic b direction (Fig. 6).
The TGA-DTH experiments show that the supramolecular
antiparallel and parallel sheet like structures of peptide 1 and
peptide 2 have significant thermal stability. Peptide 1 and 2
exhibit no decomposition, phase transition or mass loss up to 220
ꢀ
ꢀ
ꢁ
hydrogen bonding interactions (N1–H1/O6, 2.08 A, 2.87(4) A,
ꢁ
ꢀ
ꢀ
155 , ꢀx, ꢀy, 2 ꢀ z and N2–H2/O4, 2.05 A, 2.86(4) A, 159 ,
ꢁ
ꢁC and 250 C respectively (ESI, Fig. S3†).
Moreover, we have studied the nonlinear optical properties of
the reported peptides 1 and 2 in CHCl3 by using a time-resolved
Z-scan technique.19 A schematic of the Z-scan setup has been
shown in ESI, Fig. S4.† Fig. 7 shows, schematically, how the
lensing effect may give rise to the characteristic peak-valley
Z-scan curve.
Fig. 2 AFM images showing (a) and (b) the flower like morphology of
peptide 1 and (c) and (d) the fibrillar aggregates of peptide 2.
Fig. 3 Intermolecular hydrogen bonded anti-parallel b-sheet obtained
from peptide 1 in solid state. Hydrogen bonds are shown as dotted lines.
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Fig. 7 The schematic presentation of the peak-valley Z-scan curve
obtained by the lensing effect.
wavelength of 780 nm was chosen to be away from the absorp-
tion band (below 400 nm, as seen in ESI, Fig. S5†) of the sample.
This ensures negligible absorption of the incident laser by the
sample. An average laser power of 100–1000 mW was focused to
a spot of radius r ¼ 30 mm by a 10 cm focal length lens. The
transmitted signal was measured by a detector placed at the far-
field behind a small aperture (closed-aperture Z-scan). The signal
was recorded in real time in a digital oscilloscope synchronized
with chopper. Fig. 8 shows a typical oscilloscope trace (trans-
mitted signal behind aperture versus time) measured respectively
for a pre-focal, far-away and post-focal position of the sample
with respect to the laser focus point. The signals were normalized
with respect to the transmittance at a far-away sample position
and the sample positions were scaled by the beam waist param-
eter (Rayleigh length) z0 ¼ 7.1 mm. The time-dependant change
in transmittance for the pre- and post-focal sample positions is
due to build-up of nonlinear optical effects in the sample. The
slow time-scale (ꢂms) of the process suggests that the origin of
the nonlinearity lies with the thermal lensing effect (it is well-
known that electronic nonlinearity has a time scale in picosec-
onds). The thermal lensing effect was generated because of the
cumulative heating of the sample under irradiation with a high
repetition rate laser (80 MHz in our case). Though the amount of
light absorbed within one laser pulse was negligible, the heating
effect of a laser pulse gets accumulated over many pulses which
impinge on the sample at a time interval of about 12 ns. This
heating by a focused Gaussian laser beam creates a radially
nonuniform temperature profile which leads to a radial variation
refractive index due to a nonlinear refractive index, n ¼ n0 + n2I.
Fig. 4 Packing diagram of peptide 1 along the crystallographic b
direction. Phenyl ring centroids are shown as yellow circles.
Fig. 5 Intermolecular hydrogen bonded parallel b-sheet obtained from
peptide 2 in the solid state. Hydrogen bonds are shown as dotted lines.
Fig. 6 The higher order packing of peptide 2 by intermolecular
hydrogen bonding and p–p stacking interactions. Hydrogen bonds and
p–p stacking interactions are shown as dotted lines. Phenyl ring
centroids are assigned as red circles.
Like a typical Z-scan experiment, an intense laser beam was
focused using a lens and the sample was moved along the beam
propagation axis (z-axis), near the focal position. But, unlike the
usual Z-scan experiment, here the incident laser was in the form
of a square pulse train of pulse duration ꢂ2 ms and pulse sepa-
ration of ꢂ60 ms. The pulse train was created using an asym-
metric duty cycle optical chopper. A mode locked Ti: Sapphire
laser having a pulse width of about 100 fs at a wavelength of 780
nm operating at a pulse repetition rate of 80 MHz was used. A
Fig. 8 The oscilloscope trace for the pre-focal, far-away and post-focal
position of the sample with respect to the laser focus point.
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1
Thus the sample acts like a lens causing observed variation in
transmittance in time.
and 400 MHz H and 13C NMR and the final compounds were
1
fully characterized by 500 MHz and 400 MHz H NMR spec-
We measured the transmission traces (like those shown in
Fig. 8) at many different positions of the sample relative to the
laser focal point. We constructed the steady-state Z-scan curve
from the measured transmissions at a time larger than 1 ms. Two
such curves obtained for average laser power of 200 and 500 mW
are shown in Fig. 9a. The pre-focal peak and post-focal valley in
the experimental data indicate a negative lensing effect. The
peak-to-valley asymmetry is consistent with the nonlocal thermal
origin of the observed nonlinear refraction.
troscopy, 13C NMR spectroscopy, mass spectrometry and FT-IR
spectroscopy. Peptide 1 and 2 were characterized by X-ray
crystallography.
(a) Ac-Phe-OH 4. 4.95 g (30 mmol) of L-phenylalanine was
dissolved in 20 mL pyridine and cooled in an ice-bath. 2.15 mL
(35 mmol) of acetyl chloride was added to the solution drop wise
and stirred for 5 h at room temperature. The pyridine was
evaporated under rotary evaporator and a small amount of water
was added and evaporated again. The product was dissolved in
ethyl acetate and washed with 1 N HCl solution three times and
then with brine solution. The organic layer was dried over
anhydrous sodium sulphate and evaporated under rotary evap-
orator. The crude product was purified by column chromatog-
raphy using silica-gel as the stationary phase and an ethyl
acetate–hexane mixture (1 : 6) as eluent to get a pure white solid
compound 4.
The Z-scan data have been analyzed using the theory of the
thermal lens effect.20 Within this theory, the closed aperture Z-
scan curves have been fitted with the formula
ꢀ
ꢁꢀ1
2xq
q2
TðxÞ ¼ 1 þ
þ
;
(1)
1 þ x2 1 þ x2
x ¼ z/z0 is the scaled position of the sample and the fit parameter
q gives the on axis phase shift which is a measure of the strength
of the thermo-optical nonlinearity. Fig. 9b shows that this
formula fits well to the experimental data with the fit parameter
q ¼ 0.40 ꢃ 0.02.
Yield: 4.32 g (84.6%)
1H NMR (500 MHz, DMSO-d6, d ppm): 12.66 (1H, b, s,
COOH), 8.19–8.17 [1H, d, J ¼ 8 Hz, Phe(1) NH ], 7.29–7.18 (5H,
m, Phe aromatic protons), 4.42–4.37 (m, 1H, CaH), 3.05–3.2.80
(m, 2H, CbH), 1.77 (s, 3H, CH3). 13C NMR (100 MHz, DMSO-
d6, d ppm): 173.14, 169.21, 137.70, 129.02, 128.15, 126.38, 53.47,
36.76, 22.31. Anal. calcd for C11H13NO3 (207.089): C, 63.76; H,
6.32; N, 6.76. Found: C, 63.74; H, 6.34; N, 6.76%.
Peptide 2 did not show such a type of nonlocal thermal
nonlinear optical effect with the Z-scan experiment. The different
optical properties of the peptides with the Z-scan experiment
may be attributed to their self-assembly pattern.
Experimental
(b) Ac-Nitrophe-OH 5. 7 mL H2SO4 and 7 mL of HNO3 was
mixed in a round bottom flask and kept in an ice-bath. 3.9 g (18.9
mmol) of compound 4 was added to theꢁmixture in portions with
stirring. The solution was heated at 60 C for 30 min. Then the
reaction mixture was added slowly to the ice cooled water and the
crude product was extracted with ethyl acetate. The organic layer
was washed with fresh water repeatedly. The organic layer was
then dried over anhydrous Na2SO4 and evaporated in vacuum.
The product was purified by column chromatography using
silica-gel as the stationary phase and an ethyl acetate–hexane
mixture (1 : 3) as eluent to get a pure yellowish oily compound 5.
Yield: 3.52 g (81.7%)
1H NMR (400 MHz, DMSO-d6, d ppm): 12.77 (1H, s,
COOH), 8.27 [1H, d, J ¼ 8 Hz, Phe(1) NH], 8.16–8.14 [2H, d, J ¼
8.52 Hz, Phe aromatic protons], 7.52–7.50 (2H, d, J ¼ 8.56, Phe
aromatic protons), 4.49–4.48 (1H, m, CaH), 3.18–3.00 (1H, m,
CbH), 2.98–2.97 (1H, m, CbH), 1.77 (3H, s, CH3). 13C NMR (100
MHz, DMSO-d6, d ppm): 172.80, 169.38, 146.34, 146.20, 130.51,
123.31, 52.91, 36.21, 22.35. Anal. calcd for C11H12N2O5 (252.07):
C, 52.38; H, 4.80; N, 11.11. Found: C, 52.37; H, 4.81; N, 11.12%.
General methods and materials
All amino acids were purchased from Sigma chemicals. HOBt
(1-hydroxybenzotriazole) and DCC (dicyclohexylcarbodiimide)
were purchased from SRL.
Peptide synthesis
The peptides were synthesized by conventional solution-phase
methodology using fragment condensation strategy. The Boc
group was used for N-terminal protection and the C-terminus
was protected as a methyl ester. Couplings were mediated
by dicyclohexylcarbodiimide/1-hydroxy benzotriazole (DCC/
HOBt). All the intermediates were characterized by 500 MHz
(c) Ac-Nitrophe(1)-Aib(2)-OMe 1. 2.5
g (10 mmol) of
compound 5 was dissolved in DCM and cooled in an ice bath.
H-Aib-OMe was isolated from 3.06 g (20 mmol) of the corre-
sponding methyl ester hydrochloride by neutralization, subse-
quently extracted with ethyl acetate, and concentrated (10 mL),
and this was added to the reaction mixture, followed immediately
by 2.06 g (10 mmol) of dicyclohexylcarbodiimide (DCC) and 1.35
g (10 mmol) of 1-hydroxy benzotriazole (HOBt). The reaction
mixture was allowed to come to room temperature and stirred
for 24 h. DCM was evaporated, and the residue was taken in
Fig. 9 (a) Z-scan experimental results at 200 mW (green) and 500 mW
(red). (b) Experimental result of normalized closed aperture Z-scan
measurement at 200 mW. The solid green line is the theoretical fit using
eqn (1).
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ethyl acetate (60 mL); dicyclohexylurea (DCU) was filtered off.
The organic layer was washed with 2 N HCl (3 ꢄ 50 mL), brine, 1
M sodium carbonate (3 ꢄ 50 mL) and brine (2 ꢄ 50 mL), dried
over anhydrous sodium sulfate, and evaporated under vacuum to
yield dipeptide 1 as a white solid. Purification was done on a silica
gel column (100–200 mesh) using ethyl acetate–hexane (1 : 3) as
the eluent. Yield: 2.85 g (80.4%)
Mass spectrometry
Mass spectra were recorded on a Q-Tof Micro YA263 high-
resolution (Waters Corporation) mass spectrometer by positive-
mode electrospray ionization.
UV/Vis spectroscopy
1H NMR (400 MHz, CDCl3, d ppm): 8.17–8.15 (2H, d, J ¼
8.56, Phe aromatic protons), 7.43–7.40 (2H, d, J ¼ 8.56, Phe
aromatic protons), 6.46 [1H, s, Aib(2) NH], 6.26–6.24 [1H, d, J ¼
7.32 Hz, Phe(1) NH], 4.70–4.68 (1H, m, CaH), 3.73 (3H, s,
OCH3), 3.16–3.12 (2H, m, CbH), 2.00 (3H, s, OCH3), 1.47 [3H, s,
Aib(2) CH3], 1.46 [3H, s, Aib(2) CH3]. 13C NMR (125 MHz,
CDCl3, d ppm): 174.34, 170.06, 169.23, 147.09, 144.26, 130.39,
123.67, 56.79, 53.92, 52.78, 38.22, 29.67, 24.57. FTIR (cmꢀ1):
3432, 3275, 3076, 2925, 2852, 1739, 1637, 1542, 1342, 1244,
1150.25. Mass spectra: [M + Na]+: 374.31, (actual 374.13). Anal.
calcd for C16H21N3O6 (351.14): C, 54.69; H, 6.02; N, 11.96.
Found: C, 54.68; H, 6.04; N, 11.95%.
UV/Vis absorption spectra were recorded on a UV/Vis spectro-
photometer (Hitachi).
FTIR spectroscopy
All reported solid-state FTIR spectra were obtained with a
Perkin Elmer Spectrum RX1 spectrophotometer with the KBr
disk technique.
Thermogravimetric analysis (TGA)
TGA was performed on a Mettler-Toledo TGA/SDTA 851e
instrument. Approximately 10–15 mg of the sample was added to
an aluminium crucible and heated from 25 to 400 ꢁC at a rate of
ꢀ1
(d) Nitropheace-Phe-OMe 2. 0.905 g (5 mmol) of compound 5
was dissolved in DCM and cooled in an ice bath. H-Phe-OMe
was isolated from 2.31 g (10 mmol) of the corresponding
methyl ester hydrochloride by neutralization, subsequently
extracted with ethyl acetate and concentrated (10 mL), and this
was added to the reaction mixture, followed immediately by
1.03 g (5 mmol) of dicyclohexylcarbodiimide (DCC) and
0.675 gm (5 mmol) of 1-hydroxy benzotriazole (HOBt). The
reaction mixture was allowed to come to room temperature
and stirred for 24 h. DCM was evaporated, and the residue
was taken in ethyl acetate (60 mL); dicyclohexylurea (DCU)
was filtered off. The organic layer was washed with 2 N HCl
(3 ꢄ 50 mL), brine, 1 M sodium carbonate (3 ꢄ 50 mL) and
brine (2 ꢄ 50 mL), dried over anhydrous sodium sulfate, and
evaporated under vacuum to yield dipeptide 2 as a white
solid. Purification was done on a silica gel column (100–200
mesh) using ethyl acetate)) : hexane (1 : 3) as the eluent. Yield:
1.61 g (90%).
ꢁ
10 C min under continuous nitrogen purge.
X-ray crystallography
Single crystal X-ray analysis of peptide 1 and 2 were recorded on
a Bruker high resolution X-ray diffractometer instruments.
Atomic force microscopy
The morphology of the reported compound was investigated by
atomic force microscopy (AFM). A drop of the peptide 1 and 2
solutions in ethyl–acetate were placed on a clean microscope
cover glass and then dried by slow evaporation. The material was
then allowed to dry under vacuum at 30 ꢁC for two days. Images
were taken with an NTMDT instrument, model no. AP-0100 in
semi contact-mode.
Conclusions
1H NMR (400 MHz, CDCl3, dppm): 8.17–8.15 (2H, d, J ¼ 8
Hz, Nitrophe aromatic protons), 7.36–7.34 (2H, d, J ¼ 8 Hz,
Nitrophe aromatic protons), 6.81–6.79 (2H, d, J ¼ 8 Hz, Tyr
aromatic protons), 6.65–6.63 (2H, d, J ¼ 8 Hz, Tyr aromatic
protons), 5.88–5.86 (1H, d, J ¼ 8Hz, Tyr NH), 5.33 (1H, s, Tyr
OH), 4.85–4.82 (1H, m, Tyr CaH), 3.76 (3H, s, OCH3), 3.62
(2H, s, Nitropheace CH2), 3.10–3.04 (1H, m, Tyr CbH), 2.97–
2.91 (1H, m, Tyr CbH). 13C NMR (125 MHz, CDCl3, d ppm):
171.78, 168.81, 155.96, 146.43, 142.68, 129.77, 129.72, 126.00,
123.13, 115.09, 53.16, 51.77, 42.18, 36.39. FTIR (cmꢀ1): 3429,
3293, 2963, 2924, 1717, 1654, 1522, 1349, 1260. Mass spectra:
[M + Na]+: 381.21, (actual 381.10). Anal. calcd for C18H18N2O6
(358.11): C, 60.33; H, 5.06; N, 7.82. Found: C, 60.32; H, 5.06; N,
7.84%.
In summary, we have synthesized p-nitro substituted phenylal-
anine, a promising material for nonlinear optics. The dipeptide 1
containing a p-nitrophenylalanine residue adopted a supramo-
lecular antiparallel b-sheet structure using various noncovalent
interactions in the solid state. The peptide has a centrosymmetric
structure and exhibits nonlocal thermal nonlinear refraction due
to the thermal lensing effect. The heat dissipation in the peptide 1
was a slow process with a millisecond to microsecond time scale.
This nonlocal thermal nonlinear refraction of peptide based
materials may have potential application in the development of
photonic devices.
Acknowledgements
We acknowledge the DST, India, for financial assistance Project
no. (SR/FT/CS-041/2009). S. K. Maity acknowledges the
C.S.I.R, India for the research fellowship.
NMR experiments
€
All NMR studies were carried out on a Bruker AVANCE 500
Notes and references
MHz and JEOL 400 MHz spectrometer at 278 K. Compound
concentrations were in the range 1–10 mM in CDCl3 and
(CD3)2SO.
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17 Crystallographic data: peptide 1: C16H21N3O6, Mw ¼ 351.36, triclinic,
ꢀ
space group P1, a ¼ 9.1882(18), b ¼ 10.6127(17), c ¼ 10.8720(17) A, a
3
ꢁ
ꢁ
ꢁ
ꢀ
¼ 97.965(3) , b ¼ 110.138(3) , g ¼ 111.785(3) , V ¼ 880.1(3) A , Z ¼
2, dc ¼ 1.326 Mg mꢀ3, T ¼ 100 K, R1 ¼ 0.0699 and wR2 ¼ 0.2226 for
3641 data with I > 2s(I). Peptide 2: C18H18N2O6, Mw ¼ 358.34,
ꢁ
(e) L. Gonzalez-Urbina, K. Baert, B. Kolaric, J. Perez-Moreno and
K. Clays, Chem. Rev., 2012, 112, 2268.
ꢁ
orthorhombic, space group P212121,
a
¼
ꢀ
4.9118(7),
b
¼
3
ꢀ
12.4813(18), c ¼ 27.4930(4) A, V ¼ 1734.3(4) A , Z ¼ 4, dc ¼ 1.372
Mg mꢀ3, T ¼ 100 K, R1 ¼ 0.0393 and wR2 ¼ 0.1144 for 3657 data
with I > 2s(I). Intensity data were collected with MoKa radiation
for peptide 1 at 296 K and MoKa radiation for peptides 1 and 2 at
100 K using Bruker APEX-2 CCD diffractometer. Data were
processed using the Bruker SAINT package and the structure
solution and refinement procedures were performed using
SHELX97.21 For peptides 1 and 2 non-hydrogen atoms were refined
with anisotropic thermal parameters.
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ꢁ
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ꢁ
ꢁ
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€
€
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This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 22198–22203 | 22203