Association of model peptides and dehydropeptides: N-acetyl- l-alanine- and dehydroalanine N′, N′-dimethylamides

Abstract

The comparative studies on the association of Ac-ΔAla-NMe 2 and Ac- l-Ala-NMe 2 in carbon tetrachloride were performed by the analysis of their average molecular weight, dipole moments and FTIR spectra. To aid spectroscopic interpretation and gain some deeper insight into the nature of associates, the geometries of the minimum energy of the dimers of Ac-ΔAla-NMe 2 and Ac- l-Ala-NMe 2 were calculated by the B3LYP/6-31+G** method. The average molecular weight in the studied concentration range, for the ΔAla and l-Ala peptide, as determined by the osmometric method, did not exceed 1.5 and 1.2 of the monomeric mass, respectively. Accordingly, the percentage of the monomeric form (α) decreased as concentration was increased more significantly for the ΔAla analogue than for its saturated counterpart. In the studied concentrations, the dipole moment of the unsaturated compound decreases and that of its counterpart is almost constant. We identified a wider range of dimeric forms of Ac-ΔAla-NMe 2 than those of Ac- l-Ala-NMe 2. While Ac-ΔAla-NMe 2 mainly forms cyclic dimers, built of open conformers H/F, specific for α,β-dehydroamino acids, Ac- l-Ala-NMe 2 forms cyclic and linear dimers, characteristic for the usual amino acids. Ac-ΔAla-NMe 2 has a greater tendency to associate than its saturated variant, which is the result of stronger hydrogen bonds.