Rational construction of triazole/urea based peptidomimetic macrocycles as pseudocyclo-β-peptides and studies on their chirality controlled self-assembly

Article abstract of DOI:10.1021/ol501172d

A tandem macro-dimerization reaction via a Cu(I) catalyzed azide/alkyne cycloaddition reaction has been employed to construct triazole/urea based peptidomimetic macrocycles considered as pseudocyclo-β-peptides. Introduction of one particular chirality in

Full text of DOI:10.1021/ol501172d

Letter
pubs.acs.org/OrgLett
Rational Construction of Triazole/Urea Based Peptidomimetic
Macrocycles as Pseudocyclo-β-peptides and Studies on Their
Chirality Controlled Self-Assembly
Abhijit Ghorai, Samanth Reddy K, Basudeb Achari, and Partha Chattopadhyay*
Chemistry Division, CSIR-Indian Institute of Chemical Biology, Kolkata, 700032, India
S
* Supporting Information
ABSTRACT: A tandem macro-dimerization reaction via a Cu(I)
catalyzed azide/alkyne cycloaddition reaction has been employed to
construct triazole/urea based peptidomimetic macrocycles considered
as pseudocyclo-β-peptides. Introduction of one particular chirality in
the peptide backbone can alter the conformation as well as nature of
self-assembly from cyclic D-,L-,α-peptide to cyclo-β-peptide. One of
them (16a) forms antiparallel dimers while the other (16b) undergoes
higher order aggregation to form a nanorod structure.
eptides are versatile units for the construction of H-
bonded tubular assemblies and other biomimetic materials
to display a good anion binding property, we expected to find
new anion binding peptidomimetic macrocycles during this
investigation (Scheme 1).2,4
P
with potentially useful applications.1 The design of unnatural
amino acid based peptidomimetic macrocycles with predictable
self-assembly patterns (i.e., organic nanotube forming) and
function has, in particular, attracted considerable attention over
the past two decades owing to the diversity in size and shape,
easy access, and biocompatibility of such macrocycles. Among
these, urea/amide based macrocyclic hybrids proved to be
anion selective transporters in biological membranes, with
numerous anticipated applications in catalysis, biotechnology,
and material science.2−4 The attractive features of this class of
molecules prompted us to design specific kinds of macrocycles
(parallel and antiparallel units in terms of functional groups) for
future exploration of the importance of macrodipoles in anion
binding, anion transport, and voltage gating.2
Scheme 1. Compounds 16a and 16b are Peptidomimetic
Macrocycles of which 3a and 3b are the Pseudo-β-amino
Acid Residues
Several variations have been made in cyclic D-,L-,α-peptides
as well as β-peptides to produce selective ion channels.3b,4a,5
Cyclic β-peptides are conformationally more stable and possess
an additional CH2 which imparts flexibility to the ring toward
self-assembly. Recently we succeeded in designing various
regioisomers of cyclic-β-peptides incorporating 1,4-linked 1,2,3-
triazole moieties to generate different (parallel and antiparallel)
peptidomimetic macrocycles from carbohydrate precursors
which undergo self-assembly to form organic nanotubes, but
in solvents of varying polarity.6 We envisioned that, by retaining
the (1,4)-linked triazole in the backbone, urea should be a
suitable replacement for the other amide groups to create
greater divergence in macro-dipoles of these classes of
compounds. The dipole moments of ureas exceed those of
amides (4.8−4.9 D for simple symmetrical aliphatic ureas
versus 3.7−3.9 D for simple amides),2 while urea shares a
number of features with the amide linkage, namely, rigidity,
planarity, polarity, and hydrogen bonding capacity. As urea/
amide based peptidomimetic macrocycles have been reported
Our long-standing interest in exploiting carbohydrate
derivatives as chiral synthons propelled us to synthesize
macrocyclic triazole/urea oligomers using carbohydrate pre-
cursors. Considering one NH of urea as equivalent to a
methylene group, we chose the achiral N-methyl propargyl
amine and two of its enantiomeric derivatives7 (based on an α-
amino acid) as the nonsugar component to construct
macrocyclic triazole/urea oligomers.
Although the strategy of modifying a partial peptide
backbone by N-methylation has already been used to study
the thermodynamics of nanotube formation via truncated stacks
(i.e., H-bonded dimers), our initial approach was to promote
cyclo-oligomerization of linear N-methylated ureido azido
Received: April 23, 2014
© XXXX American Chemical Society
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Letter
alkyne precursors to access C2 symmetric triazole/urea based
peptidomimetic macrocycles. The stability of rotameric
conformations of the cis-β-furanoid sugar moiety prompted
the use of N-methyl propargyl amine as the achiral unit and N-
methylated D-benzyl propargyl amine as its chiral analogue.
Methylation was done only on the amino alkyne specifically to
investigate the conformational correlation of the remaining
ureido-NH with the triazole N2/N3 atoms (equivalent to the
carbonyl group of an amide bond) and also to analyze the
nature of self-assembly of the peptidomimetic macrocycle
resulting from that conformation. Our study highlights the
design and successful synthesis of two novel N-methylated
triazole/ureido based peptidomimetic macrocycles via Cu(I)
catalyzed tandem dimerization of linear N-methylated-(ureido)-
azido-alkyne precursors and the observation of their different
self-assembly properties in solution phase. To our delight, the
study revealed that the chirality element of a peptidomimetic
macrocycle can bridge two homologous conformations of cyclic
peptides and also encourage self-assembly reminiscent either of
cyclo-β-peptides or of cyclic N-methylated D-,L-α-peptides/
cyclic α,γ-peptides which form truncated dimers in solution
phase as well as in the solid phase.
Scheme 3. Synthesis of Peptidomimetic Macrocycles 16a and
16b
The linear ureido-azido alkynes were prepared by coupling
the common intermediate cis-β-furanoid sugar azido succini-
midyl ester 5 with two different N-methylated amino alkynes
such as the chiral D-phenyl alanine derived N-methylated amino
alkyne 10 and N-methylated propargyl amine 13 (Schemes
2,3). The Boc-D-phenyl alanine 6 based propargyl amine
Conformational analysis of the two peptidomimetic macro-
cycles was carried out by molecular modeling combined with
findings from proton as well as multidimensional NMR
spectroscopy such as DQF-COSY and ROESY. 1H NMR
spectra of 16a in polar and nonpolar solvents (DMSO-d6,
CCl4−CDCl3, or MeOH-d4) are well-defined, reflecting a high
degree of C2 symmetry. The observed coupling constant
Scheme 2. Synthesis of Activated cis-Furanoid-β-azido
Succinimidyl Carbamate
J
NH,CαH(S) is approximately 4.6 Hz, implying a pseudo positive φ
angle (i.e., 60°) which is generally accessible with D-α-amino
acid residues.2,6 Strong ROESY cross peaks between N-Me and
N-H indicate the symmetrical (trans, trans) nature of the
molecule like those of aliphatic ureas.2 Molecular modeling and
ROESY spectroscopic studies revealed that the peptidomimetic
macrocycle tends to minimize nonbonded intramolecular side
chain−side chain and side chain−backbone interactions by
adopting a flat ring-shaped conformation (Figure 1). The urea
and triazole backbones are perpendicular to the mean plane of
the peptide ring, where N2/N3 and urea carbonyl (or urea-NH
derivative was synthesized by the Ohira−Bestmann reaction8a
of the corresponding aldehyde 8, which was readily accessible
by reduction of the Weinreb8b amide 7 of Boc-D-phenyl alanine
with LiAlH4. The N-methylated protected alkyne 10 was
prepared by the reaction of methyl iodide and sodium hydride
with the Boc protected amino alkyne 9. Intermediate 5 was
synthesized from acetonide protected D-glucose 1 to give
(Scheme 2), via the azide-diacetonide 2, the cis-β-furanoid
sugar azido acid 3, which was treated with DPPA/Et3N to get 4
and then subjected to Curtius rearrangement to get the
isocyanate. Activation of the latter by N-hydroxy succinimide
afforded the desired azido succinimidyl carbamate derivative 5
smoothly.
Two different linear ureido-azido-alkyne precursors 14 and
15 were obtained by coupling the two different N-methylated
amino alkynes separately with intermediate 5 under basic
conditions. These two azido-alkyne derivatives were then
treated with CuI/DIEA in the presence of a catalytic amount of
TBTA to provide the desired peptidomimetic macrocycles 16a
and 16b. Only dimeric products (ESI-MS) were isolated by
HPLC though evidence for the formation of higher oligomers
in traces was obtained.
Figure 1. Energy minimized structures of peptidomimetic macrocycles
16a and 16b: (a) Functional group mimics of cyclic D-,L-α-peptides.
(b) Mimics of cyclo-β-peptides.11
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Letter
and triazole CH) groups9 are alternatively oriented along
opposite faces of the pseudo peptide backbone (Figure 2a).
Figure 4. A typical side view of H-bonded dimer 16a by molecular
modeling. The right part is shown for Roesy interpretation in (2:3)
CDCl3:CCl4 (600 MHz, 298 K).
Figure 2. Triazole and amide equivalency: (a) −NH and (N2−N3) in
same direction like L-α-amino acid residue in cyclic D-,L-α-peptides;
(b) −NH and (N2−N3) in opposite directions like β-amino acids in
the cyclo-β-peptide conformation.11
(from 4.43 to 4.20 ppm at 223 K) as the concentration
increased from 1 to 50 mM.
The retention of the unique pattern of proton NMR is likely
due to fast equilibrium on the NMR time scale.2,6,10 Definitive
evidence in favor of antiparallel dimerization came from the
observed ROESY cross-peaks between the N-Me and α-CH
proton as well as β-CH proton signals of the sugar moiety in
(2:3) CDCl3:CCl4 (Figure 5).
In analogy with partially N-methylated D-,L-α-peptides and
α,γ-peptides, the cyclic urea/triazole oligomer 16a was thought
to self-assemble in an antiparallel manner to form H-bonded
dimers maintained by a set of four strong H-bonds involving
urea NH groups and N2−N3 atoms (equivalent to amide
carbonyl groups)8 of the triazole ring of identical pseudo-β-
amino acid residues (homostacking) (Figure 3). Another
Figure 3. Possibility of self-organization of 16a in solution phase: (4a)
parallel stacking via (β-D,β-L) mode of H-bonding to form higher order
aggregation; (4b) antiparallel mode of hydrogen bonding leading to
truncated-dimer formation; (4c) parallel stacking of 16b via (β,β)-H
bonding similar to cyclo-β-peptides.10
Figure 5. Selected region of the ROESY spectrum of self-assembled
macrocycle 16a, showing dimer formation through cross-peaks of N-
Me with S-CHα as well as S-CHβ (600 MHz, 298 K, 2:3 CDCl3:CCl4).
possibility of self-organization to form organic nanotubes via
heterostacking can involve parallel (β-D,β-L) H-bonding of NH
to carbonyl CO and of CH to (N2−N3) of (1,4)-linked
triazole due to the identity of chirality throughout the peptide
backbone as noted by us earlier.6
To support this conclusion, the study of AFM morphology
on a mica foil was employed. For this, compound 16a was
dissolved to a concentration of 50 μM in a nonpolar solvent
such as (2:3) CDCl3:CCl4. The nonpolar solvent was chosen
because the dipole moment of the ensemble is not expected to
be large, as the triazole moiety and the urea linkage are oriented
in opposite directions. Interestingly this showed several ball-like
composites (reflecting several numbers of dimers in solution
phase) stacking together instead of any nanotube formation
(via β-L and pseudo β-D H-bonding). The cyclic urea/triazole
oligomer 16a was therefore concluded to self-assemble in an
antiparallel manner to form H-bonded dimers as discussed
above, by analogy with partially N-methylated cyclic D-,L-α-
peptides and cyclic-(α,γ)-peptides.
The β-sheet-like arrangement was confirmed by the FTIR
spectrum which shows amide A, I, and II bands at 3304, 1645,
and 1520 cm−1 in CHCl3 (15 mM) as observed with previously
reported cyclic peptides as well peptidomimetic macrocycles.2,6
To gain insight into oligomer formation, we performed 1H as
well as multidimensional NMR experiments. A substantial
downfield shift (>0.1 ppm) of the CHα proton of the sugar
moiety reflects a dimeric β-structure formation by 16a in a
nonpolar solvent such as CDCl3 or (2:3) CDCl3:CCl4 (Figure
4). Concentration-dependent NMR experiments provided
further evidence for the formation of H-bonded dimers in
solution in CDCl3. In agreement with the proposed antiparallel
dimerization scheme, the proton resonance of ureido NH but
not of triazole CH experienced a significant downfield shift
Substituting the chiral N-methylated α-propargyl amine in
urea/triazole cyclodimers with an achiral unit as in 16b had
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Organic Letters
Letter
dramatic consequences on both ring geometry and self-
assembly properties of the resulting cyclodimer.
ACKNOWLEDGMENTS
■
The authors thank CSIR-India for research fellowship (to A.G.)
and Dept of Science and Technology, Govt. of West Bengal,
India for financial support.
1H NMR spectra of this macrocycle in nonpolar (CDCl3) as
well as polar (CD3CN) solvents are also well-defined and
predictive of a C2 symmetric nature. But in contrast to 16a, the
ureido NH proton chemical shift of 6.12 in CD3CN with a large
coupling constant of 10.2 Hz testified to an antiperiplanar
arrangement of the ureido-amide proton with the neighboring
αCH of the sugar moiety. Therefore, the urea-triazole backbone
should be perpendicular to the mean plane of the peptide ring
where N2/N3 (equivalent to a carbonyl group) and a urea
carbonyl (or urea-NH and triazole CH) are oriented along the
same face of the pseudo peptide backbone which is similar to
any self-assembling cyclic-β-peptide conformation (Figures 1
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peptides (peptidomimetic macrocycles) by applying Cu(I)
catalyzed tandem dimerization−click chemistry on linear N-
methylated ureido-(azido/alkyne) precursors. One of them
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backbone chirality.
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In solution phase self-assembly behavior, it exactly resembles
cyclic D-,L-α-peptides as well as cyclic α,γ-peptides. The other
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organization conducive for nanotube formation which has been
confirmed by NMR, FT-IR, TEM, and AFM study. The anion
binding properties of these novel macrocycles will be the
subject of future studies.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Detailed experimental procedure, spectral data, and TEM, AFM
images. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: partha@iicb.res.in.
Notes
The authors declare no competing financial interest.
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