Carbon-deuterium vibrational probes of peptide conformation: alanine dipeptide and glycine dipeptide

The utility of alpha-carbon deuterium-labeled bonds (C(alpha)-D) as infrared reporters of local peptide conformation was investigated for two model dipeptide compounds: C(alpha)-D labeled alanine dipeptide (Adp-d(1)) and C(alpha)-D(2) labeled glycine dipeptide (Gdp-d(2)). These model compounds adopt structures that are analogous to the motifs found in larger peptides and proteins. For both Adp-d(1) and Gdp-d(2), we systematically mapped the entire conformational landscape in the gas phase by optimizing the geometry of the molecule with the values of phi and psi, the two dihedral angles that are typically used to characterize the backbone structure of peptides and proteins, held fixed on a uniform grid with 7.5 degrees spacing. Since the conformations were not generally stationary states in the gas phase, we then calculated anharmonic C(alpha)-D and C(alpha)-D(2) stretch transition frequencies for each structure. For Adp-d(1) the C(alpha)-D stretch frequency exhibited a maximum variability of 39.4 cm(-1) between the six stable structures identified in the gas phase. The C(alpha)-D(2) frequencies of Gdp-d(2) show an even more substantial difference between its three stable conformations: there is a 40.7 cm(-1) maximum difference in the symmetric C(alpha)-D(2) stretch frequencies and an 81.3 cm(-1) maximum difference in the asymmetric C(alpha)-D(2) stretch frequencies. Moreover, the splitting between the symmetric and asymmetric C(alpha)-D(2) stretch frequencies of Gdp-d(2) is remarkably sensitive to its conformation.