Stabilization of two smallest possible diastereomeric β-hairpins in a water soluble tetrapeptide containing non-coded α-amino isobutyric acid (Aib) and m-amino benzoic acid

Article abstract of DOI:10.1016/j.molstruc.2009.03.027

Single crystal X-ray diffraction study reveals that the water soluble tetrapeptide H₂N-Ile-Aib-Leu-m-ABA-CO₂H, containing non-coded Aib (α-amino isobutyric acid) and m-ABA (meta-amino benzoic acid), crystallizes with two smallest pos

Full text of DOI:10.1016/j.molstruc.2009.03.027

Journal of Molecular Structure 928 (2009) 138–143
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Stabilization of two smallest possible diastereomeric b-hairpins in a water
soluble tetrapeptide containing non-coded a-amino isobutyric acid (Aib)
and m-amino benzoic acid
Anita Dutt a, Arpita Dutta a, Sudeshna Kar a, Pradyot Koley a, Michael G.B. Drew b, Animesh Pramanik a,
*
a Department of Chemistry, University of Calcutta, 92, A.P.C. Road, Kolkata 700 009, India
b School of Chemistry, The University of Reading, Whiteknights, Reading RG6 6AD, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Single crystal X-ray diffraction study reveals that the water soluble tetrapeptide H2N–Ile–Aib–Leu–m-
ABA–CO2H, containing non-coded Aib ( -amino isobutyric acid) and m-ABA (meta-amino benzoic
acid), crystallizes with two smallest possible diastereomeric b-hairpin molecules in the asymmetric
unit. Although in both of the molecules the chiralities at Ile(1) and Leu(3) are S, a conformational
reversal in the back bone chain is observed to produce the b-hairpins with b-turn conformations of
type II and II0. Interestingly Aib which is known to adopt helical conformation, adopts unusual
Received 8 February 2009
Received in revised form 16 March 2009
Accepted 21 March 2009
Available online 2 April 2009
a
Keywords:
Peptide
Hairpin
semi-extended conformation with /: ꢀ49.5(5)°,
w: 135.2(5)° in type II and /: 50.6(6)°, w:
ꢀ137.0(4)° in type II0 for occupying the i + 1 position of the b-turns. The two hairpin molecules are
further interlocked through intermolecular hydrogen bonds and electrostatic interactions between –
a-Amino isobutyric acid
COꢀ2 and ꢀ NH3 groups to form dimeric supramolecular b-hairpin aggregate in the crystal state.
+
m-Amino benzoic acid
The CD measurement and 2D NMR study of the peptide in aqueous medium support the existence
of b-hairpin structure in water.
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
designs, the strand regions contain hydrophobic residues, while
the D-Pro-Gly segment, which in principle can adopt both II0 and
b-Hairpins are known to participate in many important biolog-
ical recognition processes such as protein–protein [1], protein–
RNA [2,3], and protein–DNA interactions [4,5]. Alzheimer’s disease
[6–8], prion diseases [9–11], the IgE-mediated allergic response
[12], the interaction of bacterial cell surface-associated protein
with IgG [13–15], and HIV gp 120 binding to human T-cell surface
protein CD4 [16,17] are some examples involving the b-hairpin
secondary structure element in biological processes. In spite of
its significance, the principles underlying hairpin stability are still
unresolved, perhaps because of the general difficulty of investigat-
ing hairpins, due to their low solubility and high propensity to
aggregate [18–23]. Despite these obstacles, several examples of
naturally occurring b-hairpins and hairpin mimetics have been re-
ported [24–28].
Various factors such as the type of b-turn [16,20,23], the
b-sheet-forming tendencies of amino acids [29–31], the length of
the peptide strands [32–34], steric factors [35], the role of inter-
chain hydrogen bonds [36–38] and electrostatic interactions
control the hairpin stability [20,39]. But the relative importance
of these factors is not fully understood as yet. In most b-hairpin
I0 turns, is most often used to form the tight turn [40–43]. To a les-
ser extent segments like Asn-Gly and Aib-D-Ala have also been
used to facilitate the formation of I0 turn in b-hairpin designs
[44–47]. Solution phase studies by Kunwar et al. show that the
b-hairpins of terminally protected octapeptides tolerate insertion
of m-amino benzoic acid and also permits accomodation of both
enantiomers of Pro-Gly turn motifs [48,49]. Although there are
several examples of large b-hairpin designs in the literature
[40–49] stabilization of b-hairpin in tetrapeptide is rare. Designing
the smallest possible b-hairpin by inserting m-amino benzoic acid,
a substituted
c-amino butyric acid with an all trans extended
configuration and also without using D-Pro-Gly segment or any ri-
gid turn mimetic molecules will provide more insights about the
various cooperative weak interactions responsible for b-hairpin
nucleation and stabilization. Therefore our attempt was to design
and synthesize a terminally deprotected water-soluble tetrapep-
tide H2N–Ile(1)–Aib(2)–Leu(3)–m-ABA(4)–CO2H(I) (Aib,
a-amino
isobutyric acid; m-ABA, meta-amino benzoic acid) to examine
the hairpin formation both in the crystal state and in water
(Fig. 1). The centrally placed Aib(2)–Leu(3) fragment, where the
achiral Aib is a overwhelmingly constrained residue to adopt
helical conformation [50–57], is expected to produce the turn
required for a b-hairpin nucleation.
* Corresponding author. Tel.: +91 33 2484 1647; fax: +91 33 2351 9755.
E-mail address: animesh_in2001@yahoo.co.in (A. Pramanik).
0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2009.03.027

A. Dutt et al. / Journal of Molecular Structure 928 (2009) 138–143
139
evaporated. The residue obtained was diluted with water and
washed with diethyl ether. The aqueous layer was cooled in ice
and neutralised by 2 M HCl and extracted with ethyl acetate. The
solvent was evaporated in vacuo to give a white solid.
Yield: 0.87 g (90.0%). Anal. Calcd. for C28H44N4O7 (548.664): C,
61.29; H, 8.08; N, 10.21%. Found: C, 61.42; H, 8.25; N, 10.38%.
O
H
O
O
H
O
H
N
H
N
H
N
H
OH
N
H
2.1.3. H2N-Ile(1)-Aib(2)-Leu(3)-m-ABA(4)-CO2H(Peptide I)
Fig. 1. Schematic representation of peptide I.
To 0.6 g (1.09 mmol) of 2, 2 ml of TFA was added at 0 °C. The
reaction mixture was then stirred at room temperature and the
progress of deprotection of Boc-group was determined by thin
layer chromatography (TLC). After 6 h TFA was removed com-
pletely under vacuo and the residue was taken in water, washed
with diethyl ether. The aqueous part was then dried under vacuo
to yield I as a white solid. Purification was done using silica-gel
as the stationary phase and chloroform–methanol mixture as the
eluent. Single crystals of the compound were grown from water–
ethanol mixture by slow evaporation.
2. Experimental
2.1. Synthesis of peptides
The protected tetrapeptide was synthesized by conventional
solution phase methods using racemization free fragment conden-
sation strategy [58]. The Boc group was used for N-terminal pro-
tection and the C-terminus was protected as methyl ester.
Couplings were mediated by N-N0-dicyclohexylcarbodiimide/1-
hydroxybenzotriazole (DCC/HOBt) [58]. Deprotection of methyl es-
ter was carried out using saponification method and the Boc group
was deprotected by TFA (trifluoroacetic acid). The final product
was purified by column chromatograghy using silicagel (100–200
mesh) as stationary phase and chloroform methanol mixture as
the eluent. All the intermediates and purified final compound have
been fully characterised by various spectroscopic techniques like
NMR, IR and mass spectrometry. Single crystal suitable for X-ray
diffraction study of peptide I was obtained from water–ethanol
solution by slow evaporation.
Yield:
0.49 g
(90.0%).
;
Mp = 204–206 °C;
IR
(KBr):
3333,1666,1664 cmꢀ1
1H NMR 300 MHz (d6-DMSO, d ppm): 9.77
(m-ABA(4) NH, 1H, s); 8.55 (Leu(3) NH,1H, bs); 8.41 (m-ABA(4)
Ha, 1H, s), 8.00 (Aib(2) NH, 1H, s), 7.94(m-ABA(4) Hd, 1H, d,
J = 7.5 Hz), 7.60 (m-ABA(4) Hb, 1H, d, J = 7.2 Hz), 7.32 (m-ABA(4)
a
Hc, 1H, t, J = 7.8 Hz), 4.33–4.35 (C Hs of Ile(1) & Leu(3), 2H, m),
1.55–1.80 (CbHs of Ile(1) and Leu(3) 2H, m), 1.37–1.46 (CbHs of
c
Aib(2), 6H, s), 1.03–1.30 (C Hs of Ile(1) and Leu(3), 3H, m), 0.81–
c
0.85 (C Hs of Ile(1) and CdHs of Ile(1) & Leu(3), 12H, m); Anal.
Calcd. for C23H36N4O5 (448.54): C, 61.58; H, 8.09; N, 12.49%. Found:
C, 61.42; H, 7.92; N, 12.33%. HR–MS (M+Na+) = 471.29, Mcalcd
(M+Na)+ = 471.55.
2.1.1. Boc–Ile–Aib–Leu–m-ABA–OMe (1)
2.2. FT-IR spectroscopy
Boc–Ile–Aib–Leu–OH [52] (0.96 g, 2.23 mmol) was dissolved in
DMF (3 ml). m-ABA–OMe (0.61 g, 4.46 mmol) obtained from its
hydrochloride was added followed by DCC (0.69 g, 3.35 mmol)
and HOBt (0.36 g, 2.68 mmol). The reaction mixture was stirred
at room temperature for 5 days. The precipitated N-N0-dicyclohex-
ylurea (DCU) was filtered and diluted with ethyl acetate. The or-
ganic layer was washed with excess of water, 1 M HCl
(3 ꢁ 30 ml), 1 M Na2CO3 solution (3 ꢁ 30 ml) and again with water.
The solvent was then dried over anhydrous Na2SO4 and evaporated
in vacuo, giving a light yellow solid. Purification was done using sil-
ica gel as stationary phase and ethyl acetate–petroleum ether mix-
ture as the eluent.
IR spectra were examined using a Perkin Elmer – 782 model
spectrophotometer. The solid-state FT-IR measurements were per-
formed using the KBr disk technique.
2.3. NMR experiments
The 1H NMR study was recorded on a Bruker Avance 500 model
spectrometer operating at 300 MHz, respectively. The 2D experi-
ment was carried out in D2O on a Bruker DRX 500 MHz equipped
with a 5 mm broadband inverse probe head. The peptide concen-
tration was in the range 5–10 mM in D2O for 1H NMR and 20–
30 mM for 2D NMR measurements.
Yield: 1.11 g (94.0%). Mp = 110–111 °C; IR (KBr): 3306, 1723,
1665, 1528 cmꢀ1 1H NMR 300 MHz (CDCl3, d ppm): 9.17 (m-
;
ABA(4) NH, 1H, s); 8.52 (m-ABA(4) Ha, 1H, s); 8.15 (m-ABA(4)
Hd, 1H, d, J = 7.9 Hz); 7.71 (m-ABA(4) Hb, 1H, d, J = 7.7 Hz); 7.33
(m-ABA(4) Hc, 1H, t, J = 7.9 Hz); 7.24 (Leu(3) NH, 1H, d,
J = 7.8 Hz); 6.70 (Aib(2) NH, 1H, s); 5.21 (Ile(1) NH, 1H, d); 4.54
2.4. Circular dichroism spectroscopy
Aqueous solution of peptide I (1.5 mM) was used for obtaining
the CD spectrum. Far-UV CD measurements were recorded at 25 °C
with a 0.5 s averaging time, a scan speed of 50 nm/min, using a
JASCO spectropolarimeter (J 720 model) equipped with a 0.1 cm
pathlength cuvette. The measurements were taken at 0.2 nm
wavelength intervals, 2.0 nm spectral bandwidth and five sequen-
tial scans were recorded for the sample.
a
a
(C H of Leu(3), 1H, m); 3.86 (–OCH3, 3H, s); 3.80 (C H of Ile(1),
1H, m); 1.84–1.89 (CbHs of Ile(1) and Leu(3) 2H, m); 1.59 (CbHs
c
of Aib(2), 6H, s); 1.49 (Boc–CH3s, 9H, s); 1.23–1.30 (C Hs of
c
Ile(1), 3H, m); 0.92–0.93 (C Hs of Ile(1) and CdHs of Ile(1) &
Leu(3), 9H, m); 13C NMR 75 MHz (CDCl3, d ppm): 173.80, 171.84,
171.30, 167.09, 156.95, 139.09, 130.41, 128.53, 124.75, 124.52,
121.15, 81.28, 60.96, 57.02, 52.82, 51.92, 40.04, 36.49, 28.10,
27.40, 25.52, 25.11, 23.78, 23.25, 22.57, 20.76, 15.63, 11.51; Anal.
Calcd. for C29H46N4O7 (562.69): C, 61.89; H, 8.24; N, 9.95%. Found:
C, 61.75; H, 8.08; N, 9.80%.
2.5. Mass spectrometry
Mass spectra of peptide I was recorded on HEWLETT PACKARD
Series 1100MSD and Micromass Qtof Micro YA263 mass spectrom-
eters by positive mode electro spray ionization.
2.1.2. Boc–Ile–Aib–Leu–m-ABA–OH (2)
1 (1.0 g, 1.77 mmol) was dissolved in methanol (15 ml) and 2 M
NaOH (8 ml) was added. The reaction mixture was stirred at room
temperature for 2 days. The progress of the reaction was monitered
by TLC. After completion of the reaction the methanol was
2.6. Crystal data for peptide I
C46H90N8O14.5, M = 987.19, monoclinic, spacegroup C2, Z = 4,
a = 30.820(3) Å, b = 8.603(1) Å, c = 20.994(5) Å, b = 109.41(2)°,

140
A. Dutt et al. / Journal of Molecular Structure 928 (2009) 138–143
Fig. 2. The hairpin structure of peptide I with ellipsoids at 25% probability. Two hairpin molecules A and B are interlocked to form dimeric supramolecular b-hairpin
aggregate. Hydrogen bonds are shown as dotted lines. For clarity solvent molecules are omitted.
U = 5249.9(14) Å3, dcalcd = 1.238 g cmꢀ3. The crystal was positioned
at 50 mm from the CCD. Three hundred and twenty-one frames
were measured with a counting time of 20 s. Data analysis was car-
ried out with the Crysalis program [59]. The structure was solved
using direct methods with the Shelxs97 program [60–63]. The
non-hydrogen atoms were refined with anisotropic thermal
parameters. The hydrogen atoms bonded to carbon and nitrogen
were included in geometric positions and given thermal parame-
ters equivalent to 1.2 times (1.5 times for methyl groups) those
of the atom to which they were attached. The structure was refined
on F2 using Shelxl97 to R1 0.0771, wR2 0.1367 for 4535 reflections
respectively (Table 1), which deviate significantly from the ideal
values for a type II b-turn /1: ꢀ60°, 1: 120° and /2: 80°, 2: 0°.
As result weak intramolecular hydrogen bond between
w
w
a
a
Ile(1)–CO and m-ABA(4)–NH (N1A–H1Aꢂ ꢂ ꢂO8A, 2.20 Å, Table 2) is
observed. Interestingly another intramolecular hydrogen bond be-
tween Ile(1)–+NH3 and m-ABA(4)–COꢀ2 (N9A–H9A2ꢂ ꢂ ꢂO28A, 2.02 Å,
Table 2) is built up to produce the smallest possible b-hairpin
structure. The planar and extended geometry of m-amino benzoic
acid helps to form interstrand hydrogen bonds. The w1 value at
Ile(1) is found to be 151.1(4)°, which corresponds to an extended
structure.
with I > 2r(I). The data have been deposited at Cambridge Crystal-
The second molecule in the asymmetric unit B adopts a type II0
b-turn structure, diastereomeric to A. The torsion angles at Aib(2)
lographic Data Center with reference number CCDC 674779.
and Leu(3) were found to be /2: 50.6(6),
w
2: ꢀ137.0(4) and
/3:ꢀ97.4(4), 3: 14.7(2), respectively (Table 1). This molecule B
w
3. Results and discussion
also contains the same two intramolecular hydrogen bonds that
were observed in molecule A as listed in Table 2 and illustrated
in Fig. 2. Planar m-amino benzoic acid modulates the loop confor-
mation to a type II0 turn which is more flattened in nature com-
pared to the twisted I0 counterpart [47]. As a result Aib which is
known to adopt helical conformation [50–57], adopts unusual
3.1. Peptide conformations in the crystal state
The crystal structure of peptide I contains two molecules (A and
B) in the asymmetric unit with different conformations. In both the
molecules the chiralities at Ile(1) and Leu(3) are S. The molecules A
and B are joined together by two intermolecular hydrogen bonds
to form a molecular dimer (Fig. 2). Molecule A adopts a folded con-
formation corresponding to a slightly distorted type II b-turn struc-
ture with Aib(2) and Leu(3) occupying the i + 1 and i + 2 positions,
respectively. The torsion angles at Aib(2) and Leu(3) were found to
semi-extended conformation with /2: ꢀ49.5(5)°,
w2:135.2(5)° in
2: ꢀ137.0(4)° in type II b-turns. Although
0
type II and /2: 50.6(6)°,
w
in both of the molecules A and B the chiralities at Ile(1) and Leu(3)
Table 2
be /2: ꢀ49.5(5)°,
w
2: 135.2(5)° and /3: 69.1(5)°,
w
3: 14.4(5)°,
Selected intra and intermolecular hydrogen bonding parameters for I.
ꢀ
ꢀ
D–Hꢂ ꢂ ꢂA
Hꢂ ꢂ ꢂA/Aꢁ
Dꢂ ꢂ ꢂA/Aꢁ
D–Hꢂ ꢂ ꢂA/°
N9A–H9A1ꢂ ꢂ ꢂO29B
N9A–H9A2ꢂ ꢂ ꢂO28A
N9B–H9B1ꢂ ꢂ ꢂO29B
N9B–H9B3ꢂ ꢂ ꢂO28A
N1B–H1Bꢂ ꢂ ꢂO8B
1.85
2.02
2.20
1.86
2.30
2.20
1.96
2.15
2.12
2.24
2.20
2.00
2.724
2.792
2.946
2.740
3.086
3.041
2.831
2.893
2.874
3.041
3.008
2.854
170
143
142
179
153
149
165
144
146
149
156
175
Table 1
Selected torsion angles (°) for peptide I.
Molecule A
Molecule B
N9–C9–C8–N7(w1
C9–C8–N7–C6(x1
C8–N7–C6–C5(/2)
N7–C6–C5–N4(w2
C6–C5–N4–C3(x2
C5–N4–C3–C2(/3)
N4–C3–C2–N1(w3
C3–C2–N1–C21(x3
C2–N1–C21–C22
)
)
151.1(4)
176.5(4)
ꢀ49.5(5)
135.2(5)
174.3(4)
69.1(5)
145.2(4)
168.0(4)
50.6(19)
ꢀ137.0(4)
ꢀ172.8(4)
ꢀ97.4(4)
14.7(2)
175.5(5)
ꢀ161.3(4)
9.9(8)
N1A–H1Aꢂ ꢂ ꢂO8A
N9A–H9A3ꢂ ꢂ ꢂO1(w)a
N7A–H7Aꢂ ꢂ ꢂO2(w)b
N4A–H4Aꢂ ꢂ ꢂO2Ac
N9B–H9B2ꢂ ꢂ ꢂO5(w)d
N4B–H4Bꢂ ꢂ ꢂO2Be
N7B–H7Bꢂ ꢂ ꢂO29Af
)
)
)
14.4(5)
)
174.2(4)
169.2(4)
ꢀ14.7(7)
Symmetry elements: a x, y + 1, z; b ꢀx + 1, y + 1, ꢀz + 1; c ꢀx + 1/2, y + 1/2, ꢀz + 1; d x,
C22–C23–C27–O28
e
f
y + 1, z; ꢀx + 3/2, y ꢀ 1/2, ꢀz + 2; ꢀx + 1, y, ꢀz + 2.

A. Dutt et al. / Journal of Molecular Structure 928 (2009) 138–143
141
b
c
a
Fig. 3. Packing arrangement of peptide I showing the formation of supramolecular b-hairpin structure. Side chains of amino acids and solvent water molecules are omitted
for clarity. Hydrogen bonds are shown as dotted line.
are S, a conformational reversal in the main chain is observed to
produce the b-hairpins with b-turn conformations of type II and
II0. Generation of two diastereomeric type II0 and II turns by the
same hairpin sequence has not been observed previously. The pres-
ent result also reveals that interstrand electrostatic interactions are
important for stabilization of small b-hairpin structure.
In the asymmetric unit the two b-hairpin molecules A and B are
interlocked through intermolecular hydrogen bonds (N9A–
H9A1ꢂ ꢂ ꢂO29B, N9B–H9B3ꢂ ꢂ ꢂO28A, Table 2) and electrostatic inter-
actions between –COꢀ2 and –+NH3 groups to form a dimeric supra-
molecular b-hairpin structure (Fig. 2). It is not clear whether the
individual hairpins are stable enough to exist separately or that
the stabilization provided by the dimeric assembly is necessary.
In the crystal the dimers are further hydrogen bonded (N4A–
H4Aꢂ ꢂ ꢂO2A, N4B–H4Bꢂ ꢂ ꢂO2B and N7B–H7Bꢂ ꢂ ꢂO29A) to form a cor-
rugated b-sheet like structure (Fig. 3, Table 2). The crystal structure
contains several solvent water molecules. These form a few inter-
molecular hydrogen bonds (Table 2), and facilitate the formation of
the supramolecular b-sheet structure.
mely low field (<8.0 ppm) is indicative of a hairpin conformation
in which the backbone protons are expected to be deshielded [64].
Further attempts to characterize the conformation in D2O reveal
that peptide I exhibits some important NOEs, namely Aib(2)NH -
a
M Aib(2)CbHs, Leu(3)NH M Aib(2)CbHs and Ile(1)C H M m-ABA(4)
Ha indicative of a b-hairpin conformation nucleated by a b-turn
with Aib–Leu as the corner residues (Figs. 4 and 5). Moreover the
existence of the interstrand NOE between the alpha protons of iso-
a
leucine and m-ABA Ha proton (Ile(1)C H M m-ABA(4) Ha) is diag-
nostic of a hairpin conformation in which the strand regions are
occupied by isoleucine and m-amino benzoic acid having high b-
sheet propensity (Fig. 5) [65].
The conformation of peptide I was probed further in solution
phase by far-UV CD measurement in water. The CD pattern of pep-
tide I is presented in Fig. 6. Notably the spectrum of the water sol-
uble peptide reveals a broad band at 210–225 nm with a distinct
negative maximum at 217 nm and a cross-over point at nearly
205 nm, differing slightly from that observed for conventional b-
hairpin peptides which shows a negative maximum at 220 nm. In-
deed such anamolous CD patterns are observed for m-ABA contain-
ing peptides [48–49]. Thus such anamolous behaviour may be
attributed due to the presence of m-ABA which can contribute to
the CD with its aromatic chromophore. The self aggregation of hai-
pins in water may also cause some changes in the CD pattern. Nev-
ertheless, in addition to the NMR experiment, the CD data too
strongly favor the conclusion that peptide I adopts a hairpin con-
formation in solution. Due to anamolous behaviour of the CD spec-
tra of peptide I the co-existence of both the hairpin isomers A and
B cannot be established in the solution phase unequivocally.
3.2. Solution phase conformations
Sequence specific assignments of peptide I were readily achieved
using a combination of DQF COSY and NOESY methods in D2O [63].
The appearance of most of the NH resonances of peptide I at extre-
H
N
4. Conclusions
HN
O
O
H
H N
O
The present study establishes that it is possible to stabilize a
smallest possible b-hairpin structure in a water soluble tetrapep-
+
Ha
tide containing non-coded
a-amino isobutyric acid (Aib) and
N
meta-amino benzoic acid. The result indicates that Aib–Leu seg-
ment can be utilized to generate turns for b-hairpins. The all trans
extended configuration of m-amino benzoic acid generates flat
non-twisted strand in the hairpin which is responsible for modu-
lating the turn conformations in the hairpin. As a consequence
Aib which is known to adopt helical conformation, adopts unusual
H
H
H
O
O
Fig. 4. Schematic representation of the proposed b-hairpin structure in peptide I.
The hydrogen bonds are shown as dotted lines. Observed NOEs are highlighted by
double edged arrows in water (500 MHz).
semi-extended conformation with /: ꢀ49.5(5)°,
w: 135.2(5)° in
type II and /: 50.6(6)°,
w
: ꢀ137.0(4)° in type II0 b-turns of the

142
A. Dutt et al. / Journal of Molecular Structure 928 (2009) 138–143
Fig. 5. NOESY (500 MHz) spectrum of peptide I in D2O (peptide concentration: 1 ꢁ 10ꢀ2 M) exhibiting the cross peaks diagnostic of small hairpin conformation.
Appendix A. Supplementary data
30
Supplementary data associated with this article can be found, in
20
10
the online version, at doi:10.1016/j.molstruc.2009.03.027.
Supplementary data contains a capped stick model of hairpin
structure, 1H NMR spectrum, Mass spectrum, an enlarged NOESY
spectrum and crystallographic data in CIF format.
0
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Acknowledgements
AD thanks UGC, New Delhi, India, for a Senior research fellow-
ship (SRF). AD and SK wishes to thank CSIR for SRF. The financial
assistance of UGC, New Delhi is acknowledged [Major Research
Project, No.32-190/2006(SR)]. We also thank EPSRC and the Uni-
versity of Reading, UK for funds for Oxford Diffraction X-Calibur
CCDC diffractometer.

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