Design and synthesis of regioisomeric triazole based peptidomimetic macrocycles and their dipole moment controlled self-assembly

Article abstract of DOI:10.1039/c2cc36566a

Two peptidomimetic macrocycles, regioisomeric in terms of the position of triazole/amide, have been synthesized. Both undergo self-assembly in a parallel manner but in solvents of opposite polarity, ascribed to (β, β) and (β-D, β-L) hydrogen bonding leading to formation of two different unique classes of organic nanostructures. The Royal Society of Chemistry.

Full text of DOI:10.1039/c2cc36566a

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COMMUNICATION
Design and synthesis of regioisomeric triazole based peptidomimetic
macrocycles and their dipole moment controlled self-assemblyw
Abhijit Ghorai,a E. Padmanaban,a Chaitali Mukhopadhyay,b Basudeb Acharia and
Partha Chattopadhyay*a
Received 10th September 2012, Accepted 25th October 2012
DOI: 10.1039/c2cc36566a
Two peptidomimetic macrocycles, regioisomeric in terms of the
position of triazole/amide, have been synthesized. Both undergo
self-assembly in a parallel manner but in solvents of opposite polarity,
ascribed to (b, b) and (b-D, b-L) hydrogen bonding leading to
formation of two different unique classes of organic nanostructures.
while retaining the backbone chirality. This class of macrocycles
displays two rotameric conformations which undergo both parallel
and antiparallel backbone to backbone H-bonding in the solution
phase. We therefore set ourselves the task of synthesising two stable
rotameric conformations of this class of macrocycles individually.
This communication describes our success in designing two pepti-
domimetic macrocycles (2a,b) regioisomeric in terms of the position
of triazole/amide bonds, where the presence of two different
rotamers could be successfully demonstrated. Of the two classes
of macrocycles one is parallel (2a) and the other is antiparallel (2b),
but both undergo self-assembly in only a parallel manner. Studies
on these isomeric macrocycles bring out that (i) unidirectionality of
functional group (i.e. amide and triazole) arrangement can be
maintained in these systems also as in cyclic b3-peptides;3a (ii)
replacement of two different amide bonds by 1,4-linked triazole
moieties can control the polarity of the peptide nanotube ensemble
due to different backbone conformation; and (iii) 1,4-linked triazole
modification in cyclic b-peptide systems can bridge between two
different classes of peptide nanotube ensembles, for example cyclic
b-peptides with unidirectional functional groups as well as
Among numerous concepts for nanotube design, self-assembly of
peptides has fascinated people most owing to the diversity in size
and shape achievable for the products which are easy to access, bio-
compatible, find application in biology, and prove to be useful in
materials science for constructing pores of transmembrane ion
channels.1,2 In particular, cyclic D,L-a-peptides as well as cyclic
b3-peptides can self-assemble into extended hollow tubular
structures due to their complementary structure and function.
Modifications of physical properties such as interior property as
well as polarity of peptide nanotubes can be accomplished by
peptide main chain backbone alteration such as insertion of an
alicyclic ring.3 For the design of cyclic peptides as well as pepti-
domimetic macrocycles with enhanced selectivity for molecular
recognition, ion transport as well as the separation process,
the direction of growth of the nanotube is an important
consideration.3,4
Triazole, an amide bond surrogate, is an attractive modification
for building up cyclic-b-tetrapeptides.3a It is a perfect mimic of the
amide bond in terms of planarity, polarity, as well as hydrogen
bond donating and accepting ability.5 But it has the capacity to
impart higher macrodipole than obtained by normal cyclic b3-tetra
peptides, as it possesses a larger dipole moment than the amide
bond. It can also influence the conductance property through
voltage gating and rectification of a transmembrane ion channel
as well as anion macrodipole interaction that responds to
membrane polarisation.6 We have additionally been interested in
constructing carbohydrate-containing triazole/amide-based pepti-
domimetic macrocycles which undergo self-assembly to form
hollow-tubular structures. Recently we reported7 a carbohydrate
based triazole/amide macrocycle which is conformationally
homologous to D,L-a-amino acids in terms of functional groups
a Chemistry Division, CSIR-Indian Institute of Chemical Biology,
Kolkata 700032, India. E-mail: partha@iicb.res.in
b Department of Chemistry, University College of Science, University
of Calcutta, Kolkata 700009, India
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c2cc36566a
Fig. 1 Compounds 2a and 2b are the two regioisomeric (based on the
location of carbohydrate and b-alanine) stable rotameric peptidomi-
metic macrocycles undergoing self-assembly in solution (carbohydrate
substituents excluded to facilitate visualisation of assembly).
c
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two macrocycles via b-sheet like hydrogen bonding.4c,7,9 ESI MS
alternative orientation of functional groups (D,L-homologous con-
formation) (Fig. 1).
contained the pseudo molecular ion peak for the dimeric species
at m/z 1143 (2M + Na)+ and for the monomeric species at 583
(M + Na)+ for both 2a and 2b in (4 : 1) CH3CN–H2O and (2 : 3)
CDCl3–CCl4 respectively.
Peptidomimetic macrocycle 2a was synthesized8 using a standard
organic protocol previously reported from our laboratory. Thus the
triazolo amino protected ester intermediate was prepared by
Cu-catalyzed azide alkyne cycloaddition reaction using azido
b-alanine methyl ester (synthesized by Cu(I) catalyzed diazo-transfer
reaction of b-alanine with triflic azide) and Cbz-protected sugar
homopropargyl amine. Triazole/amide macrocycle 2b was on the
other hand synthesised from the basic intermediate 10 via Cu(I)
catalyzed cycloaddition between 3-azido-(1,2:5,6)-diisopropylidene
glucose and homopropargyl alcohol. This intermediate was
converted to its azide derivative through the corresponding mesyl
ester. The reduction of azide to amine by stannous chloride and
subsequent protection by Cbz–Cl yielded the Cbz protected
triazolyl sugar 12. After smooth deprotection of the isopropylidene
derivative, oxidation furnished the Cbz protected sugar triazole
amino acid. This product was converted to its linear tetramer by
iterative protection and deprotection via acid treatment and then
amine coupling by the standard EDCÁHCl, HOBt protocol. The
linear tetramer was hydrolyzed and then hydrogenolysed to
yield the free amino acid which was cyclised by DPPA–Et3N to
peptidomimetic macrocycle 2b.8
1
The H NMR spectrum of macrocycle 2a in a polar solvent
mixture such as CD3CN and H2O (2%) is well resolved and shows
a high degree of C2 symmetry of the molecule. The observed
coupling constant JNH,SbH is 7.8 Hz which establishes the flatness
of the peptide backbone. The weak intensity of ROE cross-peaks
between signals of triazole and adjacent Ca protons indicates that
the conformation is analogous to that of L-a-amino acid residues in
cyclic D,L-a-peptides as well as typical cyclo-b-peptide structures.
The dramatic consequence of replacement of the triazole ring by
amide was observed in macrocycle 2b. In polar solvents like
CD3CN all the proton signals are well resolved, which reflects high
order C2 symmetry. However, the intensity of ROE correlation for
the triazole proton signal was strong with the peak for the Cb
proton and weak with that for the Ca proton, testifying to a cis-
relationship which is generally accessible by D-a-amino acids. The
observed JCaH,CbH of furanoid sugar components (around 3 Hz)
and JNH,CH2 for the b-alanine moiety (around 3, 9 Hz) indicate the
presence of a gauche conformation of the Ca–Cb unit in compound
2b. On the basis of molecular modelling studies via NMR, the
peptide backbone conformation is likely to be composed of all
D,L-a-amino acid residues as the triazole hydrogen is equivalent to
NH of amide and N2–N3 of the carbonyl group of an amide bond.
The study also reveals that the macrocyle is stable as the ‘‘clockwise
rotamer’’ keeping the direction of all triazole and amide bonds in
the same direction, whereas regioisomer 2b is stabilised as the
‘‘anticlockwise rotamer’’ by their alternative orientation. These
anticipated structures can be formed due to intramolecular hydro-
gen bonding10 between amide NH and the N2–N3 unit of the
triazole ring in 2b, which is ruled out in 2a owing to conformational
restriction of the sugar moiety. As a result macrocycle 2a is capable
of developing large dipole moment in the nanotube ensemble and is
organised as nanorods in polar solvent, whereas 2b underwent
molecular organisation in nonpolar medium (Fig. 3).
The two macrocycles were subjected to transmission electron
microscopy (TEM) to investigate the possibility of nanotube
formation. Interestingly, 2a forms nanotubes only in acetonitrile :
water (4 : 1) whereas 2b forms them in nonpolar solvents such as
(2 : 3) CDCl3 : CCl4. Besides TEM, AFM morphology was also
used to get insight into molecular association and self-assembly of
both the macrocycles 2a and 2b in solvent systems of differing
polarity, which revealed the same shape of nanorods as observed
from TEM analysis. The selected area electron diffraction patterns
of the two macrocycles also signify nanotube formation (Fig. 2).
This difference in polarity of the two nanotubes encouraged us
to study their homo-assembly property by analysing their backbone
conformation via molecular modelling coupled with multi-
dimensional NMR studies, as well as by FT-IR. The FTIR spectra
recorded for 2a and 2b as KBr pellets and in CHCl3 solution
showed, respectively, bands at 3280 and 3327 cmÀ1 for amide A,
1673 and 1668 cmÀ1 for amide I, and 1568 and 1533 cmÀ1 for
amide II, which reveals substantial evidence for self-assembly of the
Regarding the self-assembly property, investigation of concen-
1
tration dependent H NMR spectra revealed marginal downfield
Fig. 2 TEM images of rodlike assemblies of 2a (a) and 2b (b); the AFM
images are in CH3CN : H2O (4 : 1) for 2a (c) and (2 : 3) CDCl3 : CCl4
for 2b (d). The selected area electron diffraction patterns are shown as insets
in (a) and (b).
Fig. 3 Energy minimized structure of peptidomimetic macrocycles
2a and 2b using CVFF force field in insightII silicon O2 work station
98.0 in vacuo and in CHCl3 respectively: (a) resembling typical cyclo-
b-peptide structure; (b) resembling cyclic D,L-a-peptides.
c
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sugar moieties, but there was no evidence of an anti-parallel
mode of hydrogen bonding as in our previous results.7 From a
conformational point of view as discussed earlier, triazole linked
pseudo di-b-peptide and normal D,L-a-dipeptide are the same. But
due to the presence of one more carbon in b-peptide structure the
existence of (b-D, b-L) hydrogen bonding, which occurs by parallel
stacking, is more likely. This is unlike normal cyclic D,L-a-peptides
and (a,g)-cyclic peptides where only antiparallel hydrogen bonding
takes place by (a-D, a-D) or (g-L, g-L) bonding (Fig. 4 and 5).
In summary we have designed, synthesized and characterized
two different regioisomeric triazole/amide based peptidomimetic
macrocycles consisting of cis-b-furanoid sugar, b-alanine moiety
and also triazole modification, which can effectively control the
polarity of the nanotube due to different orientation of functional
groups. These macrocycles can be used as model systems for
different classes of artificial ion channels as their unidirectional
assembly pattern allows them to possess different dipole moments.
NMR, ESI-MS, FTIR, TEM, AFM, and SAED studies have been
employed to characterize the hierarchical organisation of these new
classes of peptidomimetic macrocycles. We have also established
that the two macrocycles do undergo a similar type of
parallel homo-stacking via amide NH and amide carbonyl oxygen
H-bonding although their functional group orientations are
different. The anion binding property as well as X-ray crystallo-
graphy study of their larger analogues by including one a-amino
acid will be taken up in future.
Fig. 4 Comparison between triazole/amide b- and b-D,L-conformation
and D,L-a-peptides: (a) triazole/amide b-conformation and their parallel
stacking via (b,b) H-bonding (right side), (b) parallel stacking via (b-D, b-L)
H-bonding in D,L-homologous triazole/amide macrocycle, (c) antiparallel
(a-D, a-D) H-bonding in cyclic D,L-a-peptides. Blue indicates L-amino acid
or equivalent while red indicates the D-counterpart.
The authors thank CSIR for research fellowship (to A.G.), and
Dept of Science & Technology, Govt of India, for financial
support.
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Fig. 5 Selected region of the ROESY spectrum of the self-assembled
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8 See ESIw.
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c
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