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Fig. 4 (a) Hydrogen-bonded dimer of compound 1 in the crystal. (b) Sheet-
like assembly of compound 1. (c) Higher order packing of compound 1 in the
crystal. (d) Schematic presentation of the packing of secondary structure
elements in natural spider silk.
self-assembled by two cooperative hydrogen bonding interactions
between amide groups to form a sheet-like structure (Fig. 4b). For
compound 1, in higher order packing, a column-like structure has
been observed (Fig. 4c). From FTIR, MS and NMR studies, the
hydrogen bond-mediated self-assembly patterns of compounds 1, 2
and 3 are very similar. Though the peptidomimetics have a larger
surface-to-core ratio than the natural biopolymers with extended or
globular domains, the molecular assembly pattern of compound 1 is
comparable with the packing pattern of hydrogen-bonded strands
and sheets in natural spider silk (Fig. 4d).14
In summary, we have shown that a peptidomimetic compound
containing leucine, tyrosine and malonic acid can mimic the
structure and function of natural spider silk and webs. The fiber
has comprehensive mechanical strength and exhibits violet-blue
birefringence under cross-polarized light, similarly to natural spider
silk. The results presented here may foster new studies to develop
Fig. 3 (a) Solid state FTIR spectra of compounds 1, 2 and 3. (b) The solvent
dependence of the NH chemical shifts of compound 2 at varying con-
centrations of (CD3)2SO in CDCl3 solution.
experiments were performed. To separate solutions of compounds 1, functionalized soft fibers with unconventional applications.
2 and 3 in CDCl3, hydrogen bond-accepting solvent DMSO-d6 was We acknowledge the CSIR, India, for financial assistance
gradually added in 10 mL amounts at room temperature, and the (Project No. 01/2507/11-EMR-II). K. Maji acknowledges the
NMR spectra were recorded. Generally, the addition of small IISER-Kolkata, India for a research fellowship. R. Sarkar and
amounts of (CD3)2SO in CDCl3 brings about monotonic downfield S. Bera thank CSIR and UGC, India for fellowships.
shifts of the solvent-exposed NH functional groups, leaving the
solvent-shielded NH groups mostly unaffected.12 The results of the
NMR titrations of compound 2 are shown in Fig. 3b. The NMR
Notes and references
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1 (a) L. Kiss and F. Fu¨lop, Chem. Rev., 2014, 114, 1116–1169; (b) A. Grauer
titration data show that the NH protons are exposed to the solvent
and there is a significant downfield shift of the NH protons for
compound 2 with gradually increasing DMSO-d6 concentration.
Similar results were obtained for compounds 1 and 3 (ESI† Fig. 1).
These results indicate that compounds 1, 2 and 3 form intermolec-
ular hydrogen-bonded structures.
The solid state conformation of compound 1 has been studied by
X-ray crystallography.13 Compounds 2 and 3 have failed to generate
crystals. The torsion angles around the leucine residues (f = ꢁ99.94
and c = 175.01) appear to play a critical role in dictating the overall
extended structure of compound 1. From the crystal structure of
compound 1, it is evident that there are two intermolecular hydro-
gen bonds (N1–H1ꢂꢂꢂO6, 2.100 Å, 2.938 Å, 1631, x, 1 + y, z and
N2–H2ꢂꢂꢂO3, 2.080 Å, 2.917 Å, 1651, x, ꢁ1 + y, 1 + z) between amide
CQO and N–H groups, leading to a dimer structure (Fig. 4a).
The dimeric assembly of compounds 1, 2 and 3 was also obtained
from mass spectrometry (see ESI†). The dimeric units are further
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