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

The comparative studies on the association of Ac-ΔAla-NMe2 and Ac-l-Ala-NMe2 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-NMe2 and Ac-l-Ala-NMe2 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-NMe2 than those of Ac-l-Ala-NMe2. While Ac-ΔAla-NMe2 mainly forms cyclic dimers, built of open conformers H/F, specific for α,β-dehydroamino acids, Ac-l-Ala-NMe2 forms cyclic and linear dimers, characteristic for the usual amino acids. Ac-ΔAla-NMe2 has a greater tendency to associate than its saturated variant, which is the result of stronger hydrogen bonds.