S. Raghothama et al., NMR ANALYSIS OF A CONFORMATIONAL TRANSITION IN AN ACYCLIC PEPTIDE - MODEL SYSTEM FOR STUDYING HELIX UNFOLDING, Journal of physical chemistry, 100(50), 1996, pp. 19666-19671
The stabilization of helical structures in short apolar peptides is re
adily achieved by introduction of alpha,alpha-dialkylamino acids. The
use of stereochemically constrained residues in conjunction with confo
rmationally flexible segments permits the design of peptides that are
poised to undergo structural transitions. The octapeptide Boc-Leu-Ac(8
)c-Val-Gly-Gly-Leu-Ac(8)c-Val-OMe (Ac(8)c = 1-aminocyclooctane-1-carbo
xylic acid) incorporates residues with contradictory conformational te
ndencies. NMR analysis in CDCl3, using nuclear Overhauser effects and
delineation of hydrogen-bonded NH groups establishes a 3(10)-helical c
onformation. In a polar strongly solvating medium, like DMSO, the heli
x unfolds. Studies in CDCl3/DMSO mixtures provide clear evidence for a
solvent dependent conformational transition. Amide NH chemical shifts
and temperature coefficients at varying solvent composition allow a d
etailed structural analysis of the unfolding process. The intrinsic fr
agility of the octapeptide helix provides an opportunity to examine in
vasion of the helix backbone by water molecules. Studies in DMSO solut
ion containing low concentrations of water establish that preferential
water peptide interactions may indeed be present.