Simultaneous parallel and antiparallel self-assembly in a triazole/amide macrocycle conformationally homologous to d-, L-α-amino acid based cyclic peptides: NMR and molecular modeling study

Article abstract of DOI:10.1021/ol2022356

A 1,4-linked triazole/amide based peptidomimetic macrocycle, synthesized from a triazole amide oligomer of cis-furanoid sugar triazole amino acids, possesses a conformation resembling the d-,l-α-amino acid based cyclic peptides despite having uniform back

Full text of DOI:10.1021/ol2022356

ORGANIC
LETTERS
2011
Vol. 13, No. 20
5512–5515
Simultaneous Parallel and Antiparallel
Self-Assembly in a Triazole/Amide
Macrocycle Conformationally
Homologous to D-,L-r-Amino Acid
Based Cyclic Peptides: NMR and
Molecular Modeling Study
Abhijit Ghorai,† Anindita Gayen,‡ Goutam Kulsi,† E. Padmanaban,† Aparna Laskar,†
Basudeb Achari,† Chaitali Mukhopadhyay,‡ and Partha Chattopadhyay*,†
Chemistry Division, Indian Institute of Chemical Biology (CSIR), Kolkata, 700032,
India, and Department of Chemistry, University College of Science, University of
Calcutta, Kolkata, 700009, India
partha@iicb.res.in
Received August 18, 2011
ABSTRACT
A 1,4-linked triazole/amide based peptidomimetic macrocycle, synthesized from a triazole amide oligomer of cis-furanoid sugar triazole amino
acids, possesses a conformation resembling the D-,L-R-amino acid based cyclic peptides despite having uniform backbone chirality. It undergoes
a unique mode of self-assembly through an antiparallel backbone to backbone intermolecular H-bonding involving amide NH and triazole N2/N3 as
well as parallel stacking via amide NH and carbonyl oxygen H-bonding, leading to the formation of a tubular nanostructure.
In recent years, the construction of H-bondedorganic
tubular assemblies of macrocyclic peptides has become
an important area of research for biomimetic materials
with potentially useful applications.1 Cyclic D,L-R-
peptides,2,3 cyclic β- and δ-peptides,4 peptide hybrids
(e.g., R,β-,5a R,γ-,5b and R,ε-peptides5c), and oligoureas6
constitute the range of all known flat macrocyclic mole-
cules forming β-sheet-like assemblies. However, β-peptide
based novel peptidomimetics represent a significant mod-
ification of the R-peptide analogues toward biomedical
(4) (a) Seebach, D.; Matthews, J. L.; Meden, A.; Wessels, T.;
Baerlocher, C.; McCusker, L. B. Helv. Chim. Acta 1997, 80, 173.
(b) Gauthier, D.; Baillargeon, P.; Drouin, M.; Dory, Y. L. Angew. Chem.
2001, 113, 4771. (c) Baillargeon, P.; Bernard, S; Gauthier, D.; Skouta,
R.; Dory, Y. L. Chem.;Eur. J. 2007, 13, 9223.
(5) (a) Karle, I.; Handa, B. K.; Hassall, C. H. Acta Crystallogr., Sect.
B 1975, 31, 555. (b) Amorin, M.; Castedo, L.; Granja, J. R. J. Am. Chem.
Soc. 2003, 125, 2844. (c) Horne, W. S.; Stout, C. D.; Ghadiri, M. R.
J. Am. Chem. Soc. 2003, 125, 9372 and references therein.
(6) (a)Fischer, L.; Decossas, M.; Briand, Jean-Paul.; Didierjean, C.;
Guichard., G. Angew. Chem., Int. Ed. 2009, 48, 1625 and references
therein.
† Indian Institute of Chemical Biology (CSIR).
‡ University of Calcutta.
(1) (a) Gao, X.; Matsui, H. Adv. Mater. 2005, 17, 2037. (b) Ulijn,
R. V.; Smith, A. M. Chem. Soc. Rev. 2008, 37, 664.
(2) Bong, D. T.; Clark, T. D.; Granja, J. R.; Ghadiri, M. R. Angew.
Chem. 2001, 113, 1016. Angew. Chem., Int. Ed. 2001, 40, 988.
(3) (a) Clark, T. D; Buehler, L. K.; Ghadiri, M. R. J. Am. Chem. Soc.
1998, 120, 651. (b) Ghadiri, M. R.; Kobayashi, K.; Granja, J. R.;
Chadha, R. K. Angew. Chem. 1995, 107, 76. (c) Clark, T. D.; Kobayashi,
J. M.; Isler, M. P.; McRee, D. E.; Ghadiri, M. R. J. Am. Chem. Soc. 1998,
120, 8949.
r
10.1021/ol2022356
Published on Web 09/22/2011
2011 American Chemical Society

applications because they show higher stability in vivo and
do not bind to the active site of proteolytic enzymes. We
were particularly attracted to heterocyclic backbone mod-
ified macrocyclic sugar based β-peptides because they
possess greater conformational flexibility to form flat ring
structures, an important condition for intermolecular
H-bond directed self-assembly, and also because the pep-
tide main chain can provide an opportunity to alter and
fine-tune the chemical properties of peptide nanotubes.
Synthesis of cyclo-tri-β-peptides having a 14-helix archi-
tecture from the amide oligomer of a cis-furanoid sugar
amino acid7 encouraged us to synthesize a 1,4-linked
triazole backbone containing a hybrid triazol/amide
macrocycle from triazole-amide oligomers of cis-furanoid
sugar triazole amino acids basically for two reasons: (a) the
1,4-triazole linkage serves as a trans peptide bond isostere
in terms of planarity, polarity, and hydrogen bond donat-
ing as well as accepting capacities;8aꢀc (b) separation of the
triazole and amide by a two carbon unit permits it to adopt
a chair-like conformation effectively so that the triazole
ring can also participate in noncovalent interaction during
the nanotube formation. Our study highlights that repla-
cing the amide bond in a cyclo-β-peptide witha 1,4-triazole
linkage leads to a conformation resembling the D-,L-R-
amino acid based cyclic peptides5b in the orientation of the
amide groups but the mode of self-assembly is distinctly
different. Both parallel and antiparallel β-sheet intermole-
cular hydrogen bondingsexist inthe triazole/amidemacro-
cycle with identical chirality throughout the backbone
unlike the alternatingly chiral cyclic D-,L-R-peptides (only
antiparallel β-sheet intermolecular hydrogen bonding).
The basic intermediate for the solution phase synthesis9
of the peptide macrocycle 9 is the 1,2,3-triazole di-β-
peptide isostere 5, readily synthesized from N-Cbz pro-
tected cis-furanoid homopropargyl sugar amine 49,10 from
protected sugar amine 2 and cis-furanoid azido ester 37a
derived from diacetone glucose 1 via Cu(I) catalyzed azide
alkyne cycloaddition.11a,b The intermediate dimeric Cbz
protected triazole amino ester was converted to two dif-
ferent intermediates: the free amino ester 7 by hydrogena-
tion and the Cbz protected amino acid 6 by LiOH H2O
treatment. After coupling the two intermediates by stan-
3
dard protocol (using EDC HCl and HOBt), the ester
3
group was hydrolyzed again by LiOH H2O to obtain the
3
tetramer Cbz protected amino acid 8. Activation of the
linear tetramer acid 8 by pentafluorophenol and subse-
quent hydrogenation via in situ cyclization gave the final
triazole/amide macrocyclecompound 9 which waspurified
by preparative HPLC (Scheme 1).
Scheme 1. Preparation of Compound 9
1H NMR spectra of 9 in polar and nonpolar solvents
(DMSO, CCl4ꢀCDCl3, MeOH) are well-defined, reflect-
ing a high degree of C2 symmetry. The observed coupling
constant JNH,CβH(S2) is approximately 3.6 Hz, implying a
pseudo positive j angle (i.e., ∼þ60), which is generally
accessible with the D-R-amino acid residues;6 the value
observed in typical cyclo-β-peptides is nearly 8.5 Hz.7a,b
The low intensity of ROE cross peaks of the triazole ring
proton with the C(β) proton of sugar S1 and C(R) proton
of sugar S2 (Figure 1) definitely indicates a pseudo trans
configuration, which is normally observed with L-R-amino
Figure 1. (a and b) Two rotamers of the pseudo cyclic β-peptide
9; (c and d) two sugar derived components of that peptide.
(7) (a) Jagannadh, B.; Reddy, M. S.; Lohitha Rao, C.; Prabhakar,
A.; Jagadeesh, B.; Chandrasekhar, S. Chem. Commun. 2006, 4847.
(b) Fujimura, F.; Fukuda, M.; Sugiyama, J.; Morita, T.; Kimura, S.
Org. Biomol. Chem. 2006, 4, 1896.
(8) (a) Angell, Yu L.; Burgess, K. Chem. Soc. Rev. 2007, 36, 1674.
(b) Angelo, N.; Arora, P. S. J. Am. Chem. Soc. 2005, 127, 17134. (c) Brik,
A.; Alexandratos, J.; Lin, Y. C.; Elder, J. H.; Olson, A. J.; Wlodawer, A.;
Goodsell, D. S.; Wong, C. H. ChemBioChem 2005, 6, 1167.
(9) See Supporting Information.
(10) (a) Hauske, J. R.; Dorff, P.; Julin, S.; Martinelli, G.; Bussolari, J.
Tetrahedron Lett. 1992, 33, 3715. (b) Ohira, S. Synth. Commun. 1989, 19,
561.
(11) (a) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless,
K. B. Angew. Chem., Int. Ed. 2002, 41, 2596. (b) Tornøe, C. W.;
Christensen, C.; Meldal, M. J. Org. Chem. 2002, 67, 3057.
Org. Lett., Vol. 13, No. 20, 2011
5513

acid residues,8aꢀc suggesting that the peptide backbone
conformation is similar to that of cyclic peptides composed
of alternative D-,L-R-amino acids (Figure 2). Furthermore,
dipoleꢀdipole interaction between furanoid CꢀO and an
adjacent triazole ring of one pseudo cyclic peptide back-
bone unit can also provide an additional explanation for
this conformation (see Supporting Information).8b Mole-
cular modeling and ROESY spectroscopic studies reveal
that the cyclic peptide structure tends to minimize non-
bonded intramolecular side chainꢀside chain and side
chainꢀbackbone interactions by adopting a flat ring-
shaped conformation. The amide and triazole backbone
was perpendicular to the mean plane of the peptide ring
where N2ꢀN3 (equivalent to a carbonyl group8aꢀc) and
amide carbonyl, as well as amide NH and triazole CH
(equivalent to NH8aꢀc), groups are alternatively oriented
along opposite faces of the peptide backbone.
on dilution. Additionally it also provided information on
dimeric β-structure formation as a substantial downfield
shift (>0.1 ppm) was noted for signals of a CR proton of
sugar S1 in going from a polar (DMSO) to nonpolar
solvent (2:3 CDCl3/CCl4) (see Supporting Information).
The presence of a unique set of signals suggested fast
equilibrium between the monomer and dimer (or higher
order) on the NMR time scale.6
ESI-MS of the cyclic peptide 9 recorded in CDCl3/CCl4
(2:3) showed the pseudo molecular ion peaks of the
monomeric species at 811 (MþNa)þ and the dimeric
species at 1599 (2MþNa)þ. The molecular association
and self-assembly of the cyclic peptide 9 into nanorods,
whosediameter wasinthe range of 120ꢀ400nmand length
several micrometers, was evident from SEM and TEM
images obtained in CDCl3/CCl4.9
In analogy with the cyclic D-,L-R-peptides, the cyclic
triazole/amide oligomer can self-assemble in two modes of
stacking: (a) basically antiparallel (LꢀL) and (b) parallel
(LꢀD). Four strong H-bonds between amide NH and
N2ꢀN3 atoms of the triazole ring lead to the formation
of the (LꢀL) mode of stacking (Figure 3). In the parallel
(LꢀD) mode of stacking two H-bonds form between amide
NH and amide carbonyl groups and two between CꢀH of
a triazole ring and N2ꢀN3 atoms of another half.
The FT-IR spectrum for compound 9 recorded in
CHCl3 solution, which showed the amide A, I, and II
bands at 3304, 1677, and 1537 cmꢀ1, suggested the formation
The ROESY spectrum in CDCl3 provides evidence for
interaction between S1(R)ꢀS1(δ) and S2(R)ꢀS2(δ) hydro-
gen atoms of the sugar moiety which can only occur from
parallel mode of stacking (LꢀD) as reported previously
for furanoid sugar amino acid based cyclic β-peptides
(Figure 4).7a Interestingly, in a more nonpolar solvent such
as (2:3) CDCl3/CCl4 not only did the chemical shifts change
(>0.1 ppm for S1(R) and S1(β) protons) but also an addi-
tional interaction between S1(R) and S2(β) hydrogen atoms
of the furanoid sugar moiety was observed; evidence for
the parallel interaction was however retained. This obser-
vation leads to the conclusion that an antiparallel mode
of stacking (LꢀL) is maintained by four strong H-bonds
between NH and N2ꢀN3 atoms of the triazole ring.
Literature reports on molecular modeling and other
studies on cyclic D-,L-R-peptides and also R,γ-peptides
show that the R-peptides always favor antiparallel β-sheet
hydrogen bonding over the parallel one where steric inter-
action occurs between the side chain and backbone. In our
system with identical backbone chirality, the steric inter-
action is minimal9 for both antiparallel and parallel ar-
rangements. Therefore there is a possibility for both types
of β-sheet hydrogen bonding to exist.
Figure 2. Two possible lowest energy conformations of pseudo
peptidomimetic triazole/amide macrocycle 9: (a) structure re-
sembling the D-,L-amino acid based cyclic peptides, (b) structure
resembling typical cyclo-β-peptides, (c) structure of typical
cyclo-β-peptides7a containing cis-furanoid sugar amino acids.
ROESY and molecular modeling (modeling using biopolymer
module, minimization using CVFF force field on Insight II
Silicon Graphics O2 workstation 98.0 via in vacuo simulation)
study shows that structure (a) is energetically more stable (by
about 16.9 kcal/mol) than (b). For clarity isopropylidene ring
residues are not shown.
To establish whether the two types of hydrogen bond-
ings exist in different tubular sheets or are mixed up in one
tubular sheet structure, we carried out a low temperature
NMR study where the unique set of signals was retained.
From this result we conclude that two β-sheet hydrogen
bonding arrangements exist in one tubular sheet because
the existence of two tubular sheets with different modes of
of a β-sheet-like structure as previously reported for cyclic
peptides.8c,9 Investigation of concentration dependent 1H
NMR atꢀ20°C in a nonpolarsolvent suchasCDCl3/CCl4
(2:3) also provided evidence for the formation of inter-
molecular hydrogen bonding between the two peptide
rings as the NH chemical shift changed from 8.93 to 8.80
(12) (a) Adhikary, N. D.; Chattopadhyay, P. Eur. J. Org. Chem. 2010,
1754. (b) Majhi, T. P.; Neogi, A.; Mukhopadhyay, R.; Chattopadhyay,
P. Synthesis 2005, 2307.
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Org. Lett., Vol. 13, No. 20, 2011

Figure 3. Schematic representation of solution self-assembly by
2-fold symmetrical cyclo-β-peptide. (a) Antiparallel hydrogen
bonding formation where N2 and N3 atoms of the triazole ring
serve as a hydrogen bond acceptor; (b) cartoon representation
for parallel stacking. Only peptide backbone is shown in (a), and
one sugar moiety and isopropylidene ring residue are omitted in
(b) for the sake of clarity.
stacking should have given rise to a double set of signals.
But in case of one tubular sheet with higher order aggre-
gation, a unique set of signals can be retained. This is in
consonance with previous quantum mechanical energy
calculations13 on cyclic D-,L-R-peptides where the existence
of two β-sheet hydrogen bondings in one tubular sheet
structure has been proposed.
Figure 4. Selected region of ROESY spectrum of the self-
assembled macrocycle: (a) antiparallel heterostacking mode
revealed by cross-peaks between S1CRH and S2CβH signals
and (b) parallel homostacking mode by S1CδH and S2CδH
cross-peaks (600 MHz, 298K, 2:3 CDCl3/CCl4).
The present study therefore reports the preparation
of a chiral pseudo dipeptide derived building block
from furanoid sugar molecules and its use to generate
a hybrid triazole/amide macrocycle by cyclooligomer-
ization. We have also established, by NMR and mole-
cular modeling studies, the novel conformation of the
triazole/amide macrocycle, which resembles the D-,L-
amino acid based cyclic peptides with identical back-
bone chirality. The formation of a well-defined tubular
nanostructure was suggested to have occurred by cross-
strand antiparallel backbone to backbone intermole-
cular hydrogen bonding involving amide NH and tria-
zole N2/N3 as well as by parallel stacking via amide
NH and amide carbonyl oxygen H-bonding. Although
several literature reports suggest that two types of β-sheet
hydrogen bondings can coexist, this is the first experimen-
tal demonstration of this unique mode of solution
phase self-assembly. NMR, ESI-MS, FT-IR, TEM, and
SEM studies revealed the formation of hierarchical
organization. The X-ray crystallographic study of appro-
priate derivatives and anion binding properties of this
type of macrocycle as well as of its larger derivatives
derived by functionalizing the sugar moiety will be taken
up in future.
Acknowledgment. Authors thank CSIR for research
fellowship (to A.G.), Dept of Science & Technology, Govt
of India, for financial support; Dr. Rupak Bhadra, IICB
for SEM analysis; and Dr. Anup Bhattacharjya, IICB for
helpful suggestions.
Supporting Information Available. Detailed experimen-
tal procedure, spectral data, and TEM, SEM images. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(13) Qu, W; Tan, H; Chen, G; Liu, R. Int. J. Quantum Chem. 2010,
110, 1648.
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