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

The utility of α-carbon deuterium-labeled bonds (Cα–D) as infrared reporters of local peptide conformation was investigated for two model dipeptide compounds: Cα–D labeled alanine dipeptide (Adp-d1) and Cα–D2 labeled glycine dipeptide (Gdp-d2). These model compounds adopt structures that are analogous to the motifs found in larger peptides and proteins. For both Adp-d1 and Gdp-d2, we systematically mapped the entire conformational landscape in the gas phase by optimizing the geometry of the molecule with the values of φ and ψ, 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° spacing. Since the conformations were not generally stationary states in the gas phase, we then calculated anharmonic Cα–D and Cα–D2 stretch transition frequencies for each structure. For Adp-d1 the Cα–D stretch frequency exhibited a maximum variability of 39.4 cm−1 between the six stable structures identified in the gas phase. The C...