Furanoid sugar amino acid based peptidomimetics: Well-defined solution conformations to gel-like structures

Article abstract of DOI:10.1016/S0040-4020(02)00158-8

Furanoid sugar amino acid based peptide and a dimer fold into pseudo β-turn-like structures in DMSO-d₆ with intramolecular H-bonds between LeuNH and sugarOH. In CDCl₃, they display a repeating β-turn-type secondary structure at lower

Full text of DOI:10.1016/S0040-4020(02)00158-8

TETRAHEDRON
Pergamon
Tetrahedron 58 ;2002) 2853±2859
Furanoid sugar amino acid based peptidomimetics: well-de®ned
solution conformations to gel-like structures
T. K. Chakraborty,p S. Jayaprakash, P. Srinivasu, S. S. Madhavendra, A. Ravi Sankar
and A. C. Kunwarp
Organic Division-111, Bioorganic Laboratory, Indian Institute of Chemical Technology, Hyderabad 500 007, India
Received 13 November 2001; revised 9 January 2002; accepted 7 February 2002
AbstractÐFuranoid sugar amino acid based peptide and a dimer fold into pseudo b-turn-like structures in DMSO-d6 with intramolecular
H-bonds between LeuNH and sugarOH. In CDCl3, they display a repeating b-turn-type secondary structure at lower concentrations and start
to form aggregates that gradually turn into excellent organogels as the concentrations are increased, a phenomenon observed for the ®rst time
in sugar amino acid containing peptides. q 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction
excellent organogels as the concentrations were increased.
To the best of our knowledge, this is the ®rst report on the
formation of sugar amino acid based organogels.
In recent years chemists have synthesized a large variety of
oligomeric compounds with an aim to mimic the structures
and functions of biopolymers.1 Rationally designed mono-
meric units are threaded together in speci®c sequences by
iterative synthetic methods leading to many novel homo-
and heteropolymers with architecturally beautiful three
dimensional structures and useful properties. This has led
to an ever-expanding repertoire of structurally diverse
synthetic building blocks that were developed to instill
desirable geometrical biases in the designer molecules.
Sugar amino acids represent an important class of such
conformationally constrained templates that have been
used extensively in recent years in many peptidomimetic
studies2 and have emerged as an important class of synthetic
monomers leading to many de novo oligomeric libraries.3
As part of our ongoing project on sugar amino acid based
molecular designs, we report here detailed structural studies
of two glucose-derived furanoid sugar amino acid [6-amino-
2,5-anhydro-6-deoxy-d-gluconic acid ;1, Gaa)] based pepti-
domimetics, Boc-Gaa-Phe-Leu-OMe 3 and its dimer 4.
Conformational analysis of compounds 3 and 4 by various
NMR spectroscopic techniques revealed that, in a polar
solvent like DMSO-d6, they folded into pseudo b-turn-like
structures with intramolecular H-bonds between LeuNH
and sugarOH, as depicted schematically in structure A. In
a nonpolar solvent like CDCl3, they displayed a repeating
b-turn-type secondary structure at lower concentrations and
started forming aggregates that gradually turned into
2. Results and discussion
2.1. Synthesis of peptidomimetics 3 and 4
Compounds 3 and 4 were synthesized by coupling ®rst
N-Boc-O-benzyl derivative of Gaa ;2) with H2N-Phe-Leu-
OMe under standard solution phase peptide synthesis
conditions using 1-ethyl-3-;3-;dimethylamino)propyl)-
carbodiimide hydrochloride ;EDCI) and 1-hydroxybenzo-
triazole ;HOBt) as coupling agents and amine-free dry
DMF and dry CH2Cl2 as solvents.4 One portion of the result-
ing peptide Boc-Gaa;Bn2)-Phe-Leu-OMe was saponi®ed to
free the acid terminal that was reacted next with H2N-
Gaa;Bn2)-Phe-Leu-OMe, prepared by Boc-deprotection of
the other part, to furnish the protected dimer. Final
debenzylations were carried out by hydrogenation using
Keywords: peptide mimetics; hydrogen bonding; conformation; NMR;
organogels.
p
Corresponding authors. Tel.: 191-40-719-3154; fax: 191-40-716-0757;
e-mail: chakraborty@iict.ap.nic.in
0040±4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020;02)00158-8

2854
T. K. Chakraborty et al. / Tetrahedron 58 )2002) 2853±2859
Table 1. 1H Chemical shifts ;d) in ppm, coupling constants ;J) in Hz and temperature coef®cients of the amide protons ;Dd/DT) in ppb/K of 3 in DMSO-d6
;500 MHz)
Protons
Phe
Leu
NH
CaH
CbH ;pro-S)
CbH ;pro-R)
8.32 ;d, 3JNH±aHˆ9.8 Hz)
4.65 ;ddd)
7.78 ;d, 3JNH±aHˆ8.05 Hz)
4.30 ;ddd)
3.38 ;dd, 3Ja±b;pro-S)ˆ3.60 Hz)
2.85 ;dd, Ja±b;pro-R)ˆ10.8 Hz,
2Jb;pro-R)±b;pro-S)ˆ14.3 Hz)
1.44 ;ddd, 3Ja±b;pro-S)ˆ4.7 Hz)
1.51 ;ddd, 3Ja±bꢀpro-R†ˆ10.5 Hz, 2Jb;pro-R)±b;pro-S)ˆ12.7 Hz)
CgH
CdH
1.71 ;ddt, Jg±b;pro-S)ˆ9.7 Hz, Jg±b;pro-R)ˆ4.9 Hz)
0.82 ;d, CH3;pro-R)), 0.80 ;d, CH3;pro-S)), ;Jg±d;pro-R), Jg±d;pro-R)ˆ6.5 Hz)
Others
2Dd/DT ;ppb/K) for NH
7.15±7.22 ;m, Ar-H)
8.1
1.7
Gaa: NHˆ7.08 ;bt), 5.89 ;d, C3±OH, JH3±OHˆ3.3 Hz), 5.29 ;d, C4±OH, JH4±OHˆ3.4 Hz), 4.27 ;d, C2±H, JH2±H3ˆ3.7 Hz), 4.04 ;C3±H, JH3±H4ˆ3.8 Hz), 3.79
0
;C5±H, JH4±H5ˆ3.1 Hz, JH5±H6ꢀpro-R†ˆ9.7 Hz, JH5±H6ꢀpro-S†ˆ3.0 Hz), 3.76 ;C4±H), 3.46 ;C6±H;pro-R), JH6±NHˆ7.0 Hz), 2.96 ;C6±H;pro-S), JH6 ±NHˆ5.9 Hz);
2Dd=DT ;ppb/K) for GaaNHˆ8.1
Pd;OH)2±C as catalyst in MeOH±EtOAc to furnish 3 and 4
in excellent overall yields.
population with anti relationship between bH;pro-S) and
gH about x2. For Phe also there was very large population
of g2 isomer about x1 ;80%). There was no indication of
other amide protons participating in hydrogen bonding
having very large magnitudes of Dd/DT, 28.1 ppb/K for
both GaaNH and PheNH. The small values of couplings
between the protons in the sugar ring and the nOe between
GaaC2±H and GaaC5±H are consistent with an envelope
conformation of the furanoid ring with GaaC3 being the ¯ap
of the envelope.
2.2. Conformational analysis. NMR spectroscopic
studies
NMR spectroscopic studies of 3 and 4 were carried out both
in polar ;DMSO-d6) and apolar solvents ;CDCl3). The H
1
chemical shifts, coupling constants and temperature coef®-
cients ;Dd/DT ) of the amide protons of 3 and 4 in DMSO-d6
are listed in Tables 1 and 2, respectively. Our earlier
studies2d,4 on larger peptides containing 3 in DMSO-d6
showed a very unusual b-turn like structure as shown in
structure A, with a fairly rigid Leu side chain. It is interest-
ing to note that a smaller peptide 3 also shows a very similar
turn like structure in DMSO-d6, as shown schematically in
Fig. 1, with an intramolecular H-bond between LeuNH and
sugarC3±OH. The structure was supported by rOe cross-
peaks between LeuNH-GaaC3±OH, LeudCH3;pro-R)-
GaaC3±OH, LeubH;pro-R)-GaaC3±OH, LeugH-GaaC6-
H;pro-R), PheNH-LeuNH, LeuaH-LeudCH3;pro-S) and
PheaH-LeuNH. The temperature coef®cient for Leu amide
proton chemical shift ;Dd/DT ) of 21.7 ppb/K, further
con®rmed its participation in intramolecular hydrogen
bond. The Leu side-chain was fairly rigid as re¯ected by
3Ja±bꢀpro-R†ˆ10.5 Hz, 3Ja±b;pro-S)ˆ4.7 Hz, 3Jb;pro-R)±gˆ4.9 Hz
and 3Jb;pro-S)±gˆ9.7 Hz as well as a signi®cant nOe between
LeuaH-LeudCH3;pro-S). Leu side-chain has about 77%
population of g2 isomer about x1 and about 59% rotamer
The peptide 4, in DMSO-d6, showed very similar behavior
for individual Gaa-Phe-Leu units as it was seen in 3.
However, due to overlap of resonances, several of the
details could not be obtained unambiguously and accurately.
Yet the signatures of the turns in the individual Gaa-Phe-
Leu segments were unmistakably seen. The Dd/DT for
Leu;I)NH and Leu;II)NH were 22.7 and 22.2 ppb/K,
respectively, again indicating the presence of hydrogen
bonds of moderate strengths. The assignments of the
resonances to two fragments of Gaa-Phe-Leu were
obtained by the ROESY peaks between Leu;I)aH-
Gaa;II)NH and Leu;I)NH-Gaa;II)NH. The proximities of
both the LeuNH to the corresponding sugar C3±OH in the
ROESY spectrum further con®rmed that LeuNH!GaaC3±
OH hydrogen bonds are formed for both the Gaa-Phe-Leu
units. Additional cross peaks in the ROESY spectrum,
like Leu;II)gH-Gaa;II)C6-H;pro-R), Leu;II)dCH3;pro-R)-
Gaa;II)C6-H;pro-R),
Leu;II)bH-Gaa;II)C3±OH,
Table 2. 1H Chemical shifts ;d) in ppm, coupling constants ;J) in Hz and temperature coef®cients of the amide protons ;Dd/DT) in ppb/K of 4 in DMSO-d6
;500 MHz)
Protons
Phe;I)
Leu;I)
Phe;II)
Leu;II)
NH
CaH
8.12 ;d, 3JNH±aHˆ8.8 Hz)
7.64 ;d, 3JNH±aHˆ7.7 Hz)
8.11 ;d, JNH±aHˆ9.2 Hz)
3Ja±b;pro-R)ˆ9.8 Hz)
2.93 ;2Jb;pro-R)±b;pro-S)ˆ14.2 Hz) 1.50 ;m)
7.86 ;d, JNH±aHˆ8.32 Hz)
4.27 ;dt, 3Ja±b;pro-S)ˆ5.1 Hz,
3Ja±b;pro-R)ˆ8.2 Hz)
1.50 ;m)
3
3
4.63 ;dt, Ja±b;pro-S)ˆ4.0 Hz,
4.36 ;dt, 3Ja±b;pro-S)ˆ4.9 Hz, 4.59 ;dt, Ja±b;pro-S)ˆ3.7 Hz,
3Ja±b;pro-R)ˆ9.4 Hz)
3Ja±b;pro-R)ˆ9.1 Hz)
CbH ;pro-S)
CbH ;pro-R)
CgH
3.27 ;m)
1.45 ;m, Jg±b;pro-S)ˆ9.1 Hz) 3.28 ;dd)
2.94 ;2Jb;pro-R)±b;pro-S)ˆ13.9 Hz) 1.50 ;m, Jgbˆ5.4 Hz)
1.66 ;m, Jgdˆ6.4 Hz)
1.66 ;m, Jgdˆ6.4 Hz)
0.82 ;d, CH3;pro-R)),
0.78 ;d, CH3;pro-S))
3.69 ;s, CO2CH3)
2.2
CdH
0.82 ;d, CH3;pro-R)),
0.79 ;d, CH3;pro-S))
Others
2Dd/DT ;ppb/K) for NH 5.5
7.14±7.28 ;m, Ar-H)
3.68 ;s, CO2CH3)
2.7
7.14±7.28 ;m, Ar-H)
5.5
Gaa;I): NHˆ6.93 ;t, 3Jˆ6.4 Hz), C6±H;pro-R)ˆ3.41, C6±H;pro-S)ˆ3.00, C5±Hˆ3.83, C4±Hˆ3.81, C3±Hˆ4.06, C2±Hˆ4.30, C3±OHˆ5.49,
C4±OHˆ5.18; 2Dd/DT ;ppb/K) for NHˆ7.5
Gaa;II): NHˆ7.77 ;t, 3Jˆ6.8 Hz), C6±H;pro-R)ˆ3.72, C6±H;pro-S)ˆ3.02, C5±Hˆ3.83, C4±Hˆ3.81, C3±Hˆ4.08, C2±Hˆ4.26, C3±OHˆ5.59,
C4±OHˆ5.18; 2Dd/DT ;ppb/K) for NHˆ5.7

T. K. Chakraborty et al. / Tetrahedron 58 )2002) 2853±2859
2855
quently annealed, energy-minimized and superimposed
aligning the hydrogen bonded parts clearly reveal cyclic
conformations of 3 involving hydrogen bonds between
LeuNH and sugarC3±OH. In the dimmer 4, the same pattern
is observed in both Gaa-Phe-Leu units.
Fleet et al. have shown that oligomers of some of the sugar
amino acids take a b-bend ribbon spiral structure in CDCl3
consisting of successive b-turns.3b±d In view of this, we also
undertook NMR studies of peptide 3 and 4 in CDCl3, the
data for which are listed in Tables 3 and 4, respectively. In
peptide 4, all the amide resonances in CDCl3, with the
exception of Gaa;I)NH, appeared at very low ®eld
;.7 ppm) implying their participation in hydrogen bonding.
Solvent titration studies ;Table 5) showed that addition of
DMSO-d6, a hydrogen bonding solvent, did not change
these amide chemical shifts by large amounts con®rming
further their participation in hydrogen bonding.6 In the
second Gaa-Phe-Leu fragment in peptide 4, we also
observed the proximity of Leu;II)NH to Gaa;II)C3±OH,
which indicated a NH!C3±OH hydrogen bond. The large
temperature coef®cients of 212 and 210 ppb/K for
Gaa;II)NH and Phe;II)NH, respectively, further supported
the preponderance of structures with these amides partici-
pating in hydrogen bonding.6b These hydrogen bonds imply
the presence of b-turns as observed by Fleet3b±d and
illustrated schematically in the Fig. 3. Due to the broadening
of spectral lines, we were unable to obtain other details of
hydrogen bonding of amides from the ROESY spectra.
Figure 1. Schematic representation of the hydrogen bonded structures of 3
and 4 in DMSO-d6 with the long range rOes seen in its ROESY spectrum.
Leu;I)NH-Gaa;I)C3±OH, Leu;I)bH;pro-R)-Gaa;I)C3±OH,
Phe;I)NH-Leu;I)NH, Phe;II)NH-Leu;II)NH, Leu;II)NH-
Phe;II)aH, Gaa;II)NH-Leu;I)aH, Leu;I)NH-Phe;I)aH,
Leu;II)NH-Phe;II)bH;pro-S), Leu;I)dCH3;pro-R)-Gaa;I)C6-
H;pro-R) were observed supporting the presence of unusual
turn like structures in both the fragments, as illustrated in
Fig. 1. Due to spectral overlap relevant couplings of only
one Leu side-chain were obtained, with any degree of
accuracy. It showed that the Leu;I) side-chain was rigid.
We prefer not to over interpret these couplings.
In peptide 3 also the participation of PheNH and LeuNH in
hydrogen bonding in CDCl3, as shown in Fig. 3, were
supported by both their low ®eld chemical shifts and
moderate variation of these shifts in solvent titration as
shown in Table 5.
The cross-peak intensities in the ROESY spectra of 3 and 4
in DMSO-d6, shown schematically in Fig. 1, were used for
obtaining the restraints in the MD calculations. Based on the
cross-peak intensities, the nOes between various protons
were classi®ed as strong, medium, weak and very weak.
All the above NMR spectroscopic studies in CDCl3 were
carried out with very low concentrations of the peptides,
,0.6 mM for 3 and ,0.3 mM for 4. With increasing
concentrations of the peptides, all the amide resonances
showed monotonic down®eld shifts indicating their aggre-
gation in CDCl3. As the concentrations were gradually
increased, at nearly 5±6 mM both compounds started form-
ing gels.
Ê
These were translated into distance restraints as 2.0±2.8 A
Ê
for strong, 2.0±3.5 A for medium, 2.0±4.0 A for weak and
Ê
5
2.0±4.5 A for very weak nOes. Several long-range distance
Ê
constraints, torsional restraints and an H-bonding restraint
were used in the energy calculations. The result derived
from the 120 ps MD run for compound 3 is shown in Fig.
2. The superimposed display of the 20 structures sampled at
regular intervals of 6 ps during a 120 ps MD run, subse-
Solutions of 3 and 4 in CHCl3 ;,1 wt% gelators) turned into
gels upon standing at ambient temperature. To examine the
Figure 2. Stereoview of the 20 superimposed structures of 3 in DMSO-d6, sampled at 6 ps intervals during 120 ps MD simulations, subsequently annealed,
energy-minimized and superimposed aligning the hydrogen bonded parts, displaying folded conformations with intramolecular hydrogen bonds between
LeuNH and sugarC3±OH.

2856
T. K. Chakraborty et al. / Tetrahedron 58 )2002) 2853±2859
Table 3. 1H Chemical shifts ;d) in ppm and coupling constants ;J) in Hz of 3 in CDCl3 ;500 MHz)
Protons
Phe
Leu
NH
CaH
CbH ;down)
CgH
CdH
7.83 ;d, 3JNH±aHˆ8.4 Hz)
7.08 ;d, 3JNH±aHˆ8.5 Hz)
3
3
4.77 ;dt, 3Ja±b;pro-S)ˆ6.60 Hz, Ja±b;pro-R)ˆ8.4 Hz)
4.65 ;ddd, 3Ja±b;pro-S)ˆ4.9 Hz, Ja±b;pro-R)ˆ9.4 Hz)
3.25 ;m)
1.61 ;m)
1.47 ;m)
0.90 ;d, Jg±d;pro-R)ˆ6.5 Hz), 0.86 ;d, Jg±d;pro-S)ˆ6.5 Hz)
3.69 ;CO2CH3)
Others
7.23±7.33 ;m, Ar-H)
Gaa: NHˆ4.81 ;bt), 4.24 ;d, C3±OH, JH3±OHˆ3.3 Hz), 2.15 ;d, C4±OH), 4.62 ;d, C2±H, JH2±H3ˆ4.0 Hz), 4.45 ;C3±H, JH3±H4ˆ3.7 Hz), 4.01 ;C5±H,
0
JH4±H5ˆ2.7 Hz, JH5±H6ꢀpro-R†ˆ9.1 Hz, JH5±H6ꢀpro-S†ˆ4.4 Hz), 4.05 ;C4±H), 3.52 ;C6±H;pro-R), JH6±NHˆ6.7 Hz), 3.19 ;C6±H;pro-S), JH6 ±NHˆ6.7 Hz)
Due to broadening of lines some of the couplings could not be obtained.
1
Table 4. H Chemical shifts ;d) in ppm, coupling constants ;J) in Hz and temperature coef®cients of the amide protons ;Dd/DT) in ppb/K of 4 in CDCl3
;500 MHz)
Protons
Phe;I)
Leu;I)
Phe;II)
Leu;II)
NH
CaH
CbH
CgH
7.69 ;JNH±aHˆ7.8 Hz)
7.37 ;JNH±aHˆ8.2 Hz)
8.14 ;JNH±aHˆ8.4 Hz)
7.54
4.42
1.4
1.6
0.83;d)
4.74
3.18
4.45
1.48
4.62
3.24
CdH
Others
2Dd/DT ;ppb/K) for NH
0.83 ;d)
3.68 ;s, CO2CH3)
8.0
7.14±7.28 ;m, ArH)
2.0
7.14±7.28 ;m, Ar-H)
10.0
3.69 ;s, CO2CH3)
4.5
Gaa;I): NHˆ5.02 ;bt), C6±H;pro-R)ˆ3.52, C6±H;pro-S)ˆ3.10, C5±Hˆ3.96, C4±Hˆ3.93, C3±Hˆ4.31, C2±Hˆ4.51, C3±OHˆ4.84, C4±OHˆ4.84
Gaa;II): NHˆ7.28 ;bt), C6±H;pro-R)ˆ3.48, C6±H;pro-S)ˆ3.20, C5±Hˆ3.93, C4±Hˆ3.95, C3±Hˆ4.30, C2±Hˆ4.50, C3±OHˆ5.47, C4±OHˆ4.55.12;
2Dd/DT ;ppb/K) for NHˆ12.0
Due to broadening of lines some of the couplings could not be obtained.
Table 5. Changes in the chemical shifts of the amide protons ;Dd in ppm)
on addition of DMSO-d6 ;200 mL) to solutions of 3 and 4 in CDCl3
morphology of the gels, scanning electron microscopic
;SEM) analysis of the xerogels was carried out. Wet gels
were placed on aluminium stubs, air dried, gold coated in
HUS-5GB vacuum evaporator and observed in Hitachi
S-520 Scanning Electron Microscope at an acceleration
voltage of 10 kV.
;0.6 mL)
Compound
3
NH
Gaa
Dd ;ppm)
1.56
0.49
0.61
1.48
0.05
0.58
0.32
0.14
0.69
Phe
Leu
4
Gaa;I)
Phe;I)
Leu;I)
Gaa;II)
Phe;II)
Leu;II)
There was no signi®cant size reduction in the gel obtained
from 3 after drying. The gel was porous and the 3D structure
was well preserved, which was evident by the clear-cut open
cell structure with each cell having at least two pores
beneath ;Fig. 4a). The pore size ranged from ca. 1 to
4.5 mm. Smaller pores ;,0.3 mm) were regular ;spherical)
in shape. The cell walls comprised of ®bres of ca. 0.3 mm
diameter, which formed a dense network. At few places,
these ®bres formed macro®bres of ca. 2 mm diameter
;Fig. 4b).
In case of 4, a drastic size reduction in the gel was observed
on drying. The SEM image of the gel from 4 ;Fig. 4c)
revealed a compact three-dimensional ®brous structure
with an average ®bre diameter of ca. 1.5 mm. While the
®bres were straight where they were dense, they were
wavy and tending to form helices where they were loose
;Fig. 4d).
The aggregation phenomena observed here that led to the
formation of excellent organogels could possibly be
attributed to the amphipathic nature of these sugar amino
acid based peptides that contain the polar hydroxyl groups
on sugar rings along with the Phe and Leu hydrophobic side-
chains. The intermolecular hydrogen bondings in nonpolar
organic solvent in addition with other supramolecular
Figure 3. Schematic representation of the possible hydrogen bonded
structures of 3 and 4 in CDCl3.

T. K. Chakraborty et al. / Tetrahedron 58 )2002) 2853±2859
2857
using dry, freshly distilled solvents, unless otherwise
noted. Reactions were monitored by thin layer chromato-
graphy ;TLC) carried out on 0.25 mm silica gel plates with
UV light, I2, 7% ethanolic phosphomolybdic acid-heat and
2.5% ethanolic anisaldehyde ;with 1% AcOH and 3.3%
conc. H2SO4)-heat as developing agents. Silica gel ®ner
than 200 mesh was used for ¯ash column chromatography.
Yields refer to chromatographically and spectroscopically
homogeneous materials unless otherwise stated. Melting
points are uncorrected. IR spectra were recorded as neat
liquids or KBr pellets. Mass spectra were obtained under
liquid secondary ion mass spectrometric ;LSIMS)
technique.
3.2. NMR Spectroscopy
NMR spectra were recorded on 500 MHz spectrometers at
308C with 7±10 mM solutions in appropriate solvents using
tetramethylsilane as internal standard or the solvent signals
as secondary standards and the chemical shifts are shown in
d scales. Multiplicities of NMR signals are designated as s
;singlet), d ;doublet), t ;triplet), q ;quartet), br ;broad), m
;multiplet, for unresolved lines), etc. 13C NMR spectra were
recorded with complete proton decoupling. The assign-
ments were carried out with the help of two-dimensional
total correlation spectroscopy ;TOCSY).8 For some cases
the rotating frame nuclear Overhauser effect spectroscopy
;ROESY) experiments,8 which provide the information on
the proximity of protons, were additionally used to con®rm
the assignments made. All the experiments were carried out
in the phase sensitive mode using the procedure of States et
al.9 The spectra were acquired with 2£256 or 2£192 free
induction decays ;FID) containing 8±16 transients with
relaxation delays of 1.5±2.0 s. The ROESY experiments
were performed with mixing time of 0.3 s. For ROESY
experiments a spin-locking ®eld of about 2 kHz was used.
The TOCSY experiments were performed with the spin
locking ®elds of about 10 kHz and a mixing time of
0.08 s. The two-dimensional data were processed with gaus-
sian apodization in both the dimensions. The chemical
shifts, coupling constants and temperature coef®cients
;Dd/DT) of amide proton chemical shifts of 3 and 4 in
DMSO-d6 and CDCl3 are given in Tables 1±4. To obtain
the temperature coef®cients of NH-chemical shifts in
DMSO-d6, the spectra were recorded between 30 and 708C.
Figure 4. SEM pictures of xerogels from 3 ;a and b) and 4 ;c and d) in
CHCl3.
noncovalent forces, like possible aromatic stackings and
electrostatic interactions, might have resulted in the forma-
tion of aggregates that entrapped solvent molecules to
create gelatinous structure. Sugar-based gelators have an
advantage over others as one can choose from a large
repertoire of carbohydrate library to design an appropriate
gelator that is most suitable for a particular solvent.
In conclusion, this paper demonstrates further the abilities
of sugar amino acids to introduce interesting secondary
structures in peptides that can have many potential applica-
tions in designing novel peptidomimetics. Also noteworthy
is their tendency to form organogels in nonpolar solvents,
observed for the ®rst time in any sugar amino acid based
molecular designs. Development of new low molecular
weight gelators, especially, of organic solvents is of intense
current interest as the diverse nanostructures displayed by
these organogels have many potential practical utilities in
the development of new functional materials.7 The gels
formed by sugar amino acid based peptidomimetics are
also expected to ®nd many useful applications.
3.3. Molecular dynamics
All molecular mechanics/dynamics calculations were
carried out using Sybyl 6.7 program on a Silicon Graphics
O2 workstation. The Tripos force ®eld with default para-
meters was used throughout the simulations. A dielectric
constant of 47 Debye was used in all minimizations as
well as MD runs. Minimizations were done ®rst with
steepest decent, followed by conjugate gradient methods
for a maximum of 2000 iterations each or RMS deviation
of 0.005 kcal/mol, whichever was earlier. The energy-
minimized structures were then subjected to MD simula-
tions. A number of inter-atomic distances and torsional
angle constraints10 obtained from NMR data in DMSO-d6
including the v dihedrals, all of which were ®xed as trans,
were used as restraints in the minimization as well as MD
3. Experimental
3.1. General procedures
All reactions were carried out in oven or ¯ame-dried glass-
ware with magnetic stirring under nitrogen atmosphere
Ê
runs. Distance constraints with a force constant of 15 kcal/A

2858
T. K. Chakraborty et al. / Tetrahedron 58 )2002) 2853±2859
were applied in the form of a ¯at-bottom potential well with
Ê
a common lower bound of 2.0 A and an upper bound of 2.8,
by column chromatography ;SiO2, 60% EtOAc in petroleum
ether eluant) afforded the Bn-protected dimer ;200 mg,
73%), which was used directly in the next step for Bn-
deprotection without characterization.
Ê
3.5, 4.0 and 4.5 A, in accordance with the nOe intensities.
Ê
Force constants of 30 kcal/A and 5 kcal/rad were employed
for H-bond distance and dihedral angle constraints, respec-
tively. The energy-minimized structure was subjected to
constrained MD simulations for duration of 120 ps ;20
cycles, each of 6 ps period, of the Simulated Annealing
protocol). The atomic velocities were applied following
Boltzmann distribution about the center of mass to obtain
a starting temperature of 1000 K. After simulating for 1 ps
at this high temperature, the system temperature was
reduced stepwise over a 5 ps period to reach a ®nal tempera-
ture of 300 K. Resulting structures were sampled after every
6 ps ;one cycle), leading to an ensemble of total 20 struc-
tures. The samples were minimized using the above
mentioned energy minimization protocol, compared and
superimposed as shown in Fig. 2.
To a solution of the above Bn-protected dimer ;200 mg,
0.15 mmol) in MeOH ;3 mL), Pd;OH)2 on C ;20%,
100 mg) was added and the mixture was hydrogenated
under atmospheric pressure using a H2-®lled balloon for
12 h. It was then ®ltered through a short pad of Celite and
the ®lter cake was washed with MeOH. The ®ltrate and the
washings were combined and concentrated in vacuo. Puri-
®cation by column chromatography ;SiO2, 10% MeOH in
CHCl3 eluant) afforded 4 ;123 mg, 85%) as a white solid.
20
Rfˆ0.5 ;silica, 12% MeOH in CHCl3); [a]D ˆ17.0 ;c 0.45
in MeOH); mp 125±1288C; IR ;KBr): nmax 1725, 1650,
1
1600 cm21; H NMR ;500 MHz, DMSO-d6): see Table 2;
13C NMR ;125 MHz, DMSO-d6): d 172.43, 170.83, 170.26,
169.56, 169.01, 156.42, 138.36, 138.10, 128.88, 128.75,
128.04, 128.01, 126.04, 87.18, 82.30, 79.08, 77.92, 77.71,
53.16, 51.73, 51.22, 50.42, 42.84, 41.52, 40.72, 36.17,
28.17, 23.78, 22.85, 22.59, 21.34, 21.13; MS ;LSIMS):
m/z ;%) 871 ;36) [M11H22Boc], 971 ;10) [M11H], 993
;26) [M11Na].
3.3.1. Synthesis of 3. A solution of Boc-Gaa;Bn2)-Phe-Leu-
CO2Me4 ;100 mg, 0.136 mmol) in MeOH±EtOAc ;1:1,
2 mL) was hydrogenated in the presence of Pd;OH)2 on C
;20%, 50 mg) using a H2-®lled balloon for 5 h. It was then
®ltered through a short pad of Celite and the ®lter cake was
washed with MeOH. The ®ltrate and the washings were
combined and concentrated in vacuo. Puri®cation by
column chromatography ;SiO2, 8% MeOH in CHCl3 eluant)
afforded 3 ;70 mg, 92%) as a white solid. Rfˆ0.3 ;silica, 8%
Acknowledgements
20
MeOH in CHCl3); [a]D ˆ29.7 ;c 0.3 in MeOH); mp 186±
Authors wish to thank Dr M. Vairamani for mass spectro-
scopic assistance, CSIR, New Delhi for research fellowships
;S. J., P. S. and A. R. S.) and DST, New Delhi for ®nancial
support ;T. K. C.).
1
1888C; IR ;KBr): nmax 1725, 1675, 1650 cm21; H N MR
;500 MHz, DMSO-d6): see Table 1; 13C NMR ;125 MHz,
DMSO-d6): d 172.38, 170.53, 169.03, 156.37, 138.25,
128.68, 128.00, 126.03, 88.12, 82.29, 77.87, 77.73, 77.88,
52.42, 51.74, 50.20, 42.81, 35.96, 28.14, 23.58, 22.65,
20.88; MS ;LSIMS): m/z ;%) 551 ;30) [M1]; HRMS
;LSIMS): calcd for C27H42N3O9 [M11H]: 552.2921,
found: 552.2946.
References
1. ;a) Kirshenbaum, K.; Zuckermann, R. N.; Dill, K. A. Curr.
Opin. Struct. Biol. 1999, 9, 530±535. ;b) Barron, A. E.;
Zuckermann, R. N. Curr. Opin. Chem. Biol. 1999, 3, 681±
687. ;c) Soth, M. J.; Nowick, J. S. Curr. Opin. Chem. Biol.
1997, 1, 120±129. ;d) Gellman, S. H. Acc. Chem. Res. 1997,
31, 173±180.
3.3.2. Synthesis of 4. To a solution of Boc-Gaa;Bn2)-Phe-
Leu-CO2Me4 ;150 mg, 0.205 mmol) in THF/MeOH/H2O
;3:1:1, 2.5 mL) at 08C, LiOH/H2O ;25 mg, 0.615 mmol)
was added and stirred at the same temperature for 2 h.
The reaction mixture was then acidi®ed to pH 2 with 1N
HCl. It was diluted with CH2Cl2, washed with brine, dried
;Na2SO4), ®ltered and concentrated in vacuo to get the acid.
2. For some leading references see: ;a) Lohof, E.; Planker, E.;
Mang, C.; Burkhart, F.; Dechantsreiter, M. A.; Haubner, R.;
Wester, H.-J.; Schwaiger, M.; Holzemann, G.; Goodman,
S. L.; Kessler, H. Angew. Chem., Int. Ed. Engl. 2000, 39,
2761±2764 and references therein. ;b) van Well, R. M.;
Overkleeft, H. S.; Overhand, M.; Carstenen, E. V.; van der
Marel, G. A.; van Boom, H. Tetrahedron Lett. 2000, 41,
9331±9335. ;c) Schrey, A.; Vescovi, A.; Knoll, A.; Rickert,
C.; Koert, U. Angew. Chem., Int. Ed. Engl. 2000, 39, 900±902.
;d) Chakraborty, T. K.; Jayaprakash, S.; Diwan, P. V.;
Nagaraj, R.; Jampani, S. R. B.; Kunwar, A. C. J. Am. Chem.
Soc. 1998, 120, 12962±12963.
In another round bottom ¯ask a solution of Boc-Gaa;Bn2)-
Phe-Leu-CO2Me ;150 mg, 0.205 mmol) in CH2Cl2 ;2 mL)
was taken. To this tri¯uoroacetic acid ;0.5 mL) was added
and stirred at 08C for 1 h. The reaction mixture was then
concentrated in vacuo to give TFA-H2N-Gaa;Bn2)-Phe-
Leu-CO2Me.
The above prepared crude acid was dissolved in CH2Cl2
;2 mL) and cooled to 08C. Then it was sequentially treated
with HOBt ;41 mg, 0.307 mmol) and EDCI ;59 mg,
0.307 mmol). After 15 min, TFA-H2N-Gaa;Bn2)-Phe-Leu-
CO2Me, prepared above and dissolved in CH2Cl2 ;2 mL),
was added to the reaction mixture followed by DIPEA
;0.07 mL, 0.41 mmol). After stirring for 8 h at room
temperature, the reaction mixture was diluted with EtOAc,
washed with saturated NH4Cl solution, water, brine, dried
;Na2SO4), ®ltered and concentrated in vacuo. Puri®cation
3. For some leading references see: ;a) Chakraborty, T. K.;
Jayapraksh, S.; Srinivasu, P.; Chary, M. G.; Diwan, P. V.;
Nagaraj, R.; Ravi Sankar, A.; Kunwar, A. C. Tetrahedron
Lett. 2000, 41, 8167±8171 and references therein. ;b) Brittain,
D. E. A.; Watterson, M. P.; Claridge, T. D. W.; Smith, M. D.;
Fleet, G. W. J. J. Chem. Soc., Perkin Trans. 1 2000, 3655±
3665. ;c) Hungerford, N. L.; Claridge, T. D. W.; Watterson,
M. P.; Aplin, R. T.; Moreno, A.; Fleet, G. W. J. J. Chem. Soc.,
Perkin Trans. 1 2000, 3666±3679. ;d) Hungerford, N. L.;

T. K. Chakraborty et al. / Tetrahedron 58 )2002) 2853±2859
2859
Fleet, G. W. J. J. Chem. Soc., Perkin Trans. 1 2000, 3680±
3685.
D. G. J. Am. Chem. Soc. 2000, 122, 2399±2400. ;c) Jung, J. H.;
Amaike, M.; Shinkai, S. Chem. Commun. 2000, 2343±2344.
;d) van Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed.
Engl. 2000, 39, 2263±2266. ;e) Maitra, U.; Kumar, P. V.;
Chandra, N.; D'Souza, L. J.; Prasanna, M. D.; Raju, A. R.
Chem. Commun. 1999, 595±596. ;f) Terech, P.; Weiss, R. G.
Chem. Rev. 1997, 97, 3133±3159. ;g) Bhattacharya, S.;
Acharya, S. N. G.; Raju, A. R. Chem. Commun. 1996,
2101±2102.
4. Chakraborty, T. K.; Ghosh, S.; Jayaprakash, S.; Sarma,
J. A. R. P.; Ravikanth, V.; Diwan, P. V.; Nagaraj, R.; Kunwar,
A. C. J. Org. Chem. 2000, 65, 6441±6457.
5. Lian, L. Y.; Barsukov, I. L.; Sutcliffe, M. J.; Sze, K. H.;
Roberts, G. C. K. Methods Enzymol. 1994, 239, 657±700.
6. For leading references on intramolecular hydrogen bonding in
small peptides see: ;a) Fisk, J. D.; Powell, D. R.; Gellman,
S. H. J. Am. Chem. Soc. 2000, 122, 5443±5447.
;b) Chitnumsub, P.; Fiori, W. R.; Lashuel, H. A.; Diaz, H.;
Kelly, J. W. Bioorg. Med. Chem. 1999, 7, 39±59. ;c) Gung,
B. W.; Zhu, Z. J. Org. Chem. 1996, 61, 6482±6483.
;d) Nowick, J. S.; Abdi, M.; Bellamo, K. A.; Love, J. A.;
Martinez, E. J.; Noronha, G.; Smith, E. M.; Ziller, J. W.
J. Am. Chem. Soc. 1995, 117, 89±99.
8. ;a) Cavanagh, J.; Fairbrother, W. J.; Palmer, III, A. G.;
Skelton, N. J. Protein NMR Spectroscopy; Academic: San
Diego, 1996. ;b) WuÈthrich, K. NMR of Proteins and Nucleic
Acids; Wiley: New York, 1996.
9. States, D. J.; Haberkorn, R. A.; Ruben, D. J. J. Magn. Reson.
1982, 48, 286±292.
10. Kessler, H.; Griesinger, C.; Lautz, J.; MuÈller, A.; van
Gunsteren, W. F.; Berendsen, H. J. C. J. Am. Chem. Soc.
1988, 110, 3393±3396.
7. For some recent work and reviews see: ;a) Menger, F. M.;
Caran, K. L. J. Am. Chem. Soc. 2000, 122, 11679±11691.
;b) Wang, R.; Geiger, C.; Chan, L.; Swanson, B.; Whitten,