Mechanism of unfolding of a model helical peptide.

Synthetic model helical peptides, Acetyl-W(EAAAR)(5)A-amide with (13)C=O specifically labeled alanine segments in repeats n = 1,2 or 4,5 were studied in aqueous D(2)O solution as a function of temperature using Fourier transform infrared spectroscopy and two-dimensional correlation analysis. The (13)C==O provided a probe which was sensitive to the carbonyl stretch in the peptide bonds of the alanine residues at the amino terminal end in one peptide as compared to the probe in the carboxy terminal end of the other peptide during thermal perturbation. The relative stability of each terminal end was examined; the more stable terminal was determined to be the amino terminal end. Also studied were the glutamate and arginine side-chain modes involved in the salt bridging interaction. Two-dimensional correlation analysis enabled enhanced resolution in the spectral region of 1520--1700 cm(-1), and thus, the order in which these vibrational modes were perturbed as a function of increasing temperature were established.