Abstract
The structures of two dehydropentapeptides, Boc–Pro–ΔPhe–Val–ΔPhe–Ala–OMe (I) and Boc–Pro–ΔPhe–Gly–ΔPhe–Ala–OMe (II) (Boc: t‐butoxycarbonyl), have been determined by nuclear magnetic resonance (NMR), circular dichroism (CD), and X‐ray crystallographic studies. The peptide I assumes a S‐shaped flat β‐bend structure, characterized by two partially overlapping type II β‐bends and absence of a second 1 ← 4 (N4H · · · O1′) intramolecular hydrogen bond. This is in contrast to the generally observed 3~10~‐helical conformation in peptides with ΔPhe at alternate positions. This report describes the novel conformation assumed by peptide I and compares it with that of the conserved tip of the V3 loop of the HIV‐1 envelope glycoprotein gp120 (sequence, G:P319 to F:P324, PDB code 1ACY). The tip of the V3 loop also assumes a S‐shaped conformation with Arg:P322, making an intramolecular side‐chain–backbone interaction with the carbonyl oxygen of Gly:P319. Interestingly, in peptide I, C^γ^HVal^3^ makes a similar side‐chain–backbone CH · · · O hydrogen bond with the carbonyl oxygen of the Boc group. The observed overall similarity indicates the possible use of the peptide as a viral antagonist or synthetic antigen. Peptide II adopts a unique turn followed by a 3~10~‐helix. Both peptides I and II are classical examples of stabilization of unusual structures in oligopeptides. © 2006 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 84: 298–309, 2006
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