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

摘要

The utility of a-carbon deuterium-labeled bonds (Ca—D) as infrared reporters of local peptideconformation was investigated for two model dipeptide compounds: Ca—D labeled alaninedipeptide (Adp-d_1) and C_α—D2labeled glycine dipeptide (Gdp-d2). These model compounds adoptstructures that are analogous to the motifs found in larger peptides and proteins. For both Adp-d,and Gdp-d2, we systematically mapped the entire conformational landscape in the gas phase byoptimizing the geometry of the molecule with the values of φ andψ ', the two dihedral angles thatare typically used to characterize the backbone structure of peptides and proteins, held fixed on auniform grid with 7.5° spacing. Since the conformations were not generally stationary states in thegas phase, we then calculated anharmonic Cα—D and Cα—D_2stretch transition frequencies for eachstructure. For Adp-d1 the Ca—D stretch frequency exhibited a maximum variability of 39.4 cm~(-1)between the six stable structures identified in the gas phase. The Ca—D2 frequencies of Gdp-d2 showan even more substantial difference between its three stable conformations: there is a 40.7 cm-1maximum difference in the symmetric Ca—D2 stretch frequencies and an 81.3 cm-1 maximumdifference in the asymmetric Cα—D_2stretch frequencies. Moreover, the splitting between thesymmetric and asymmetric Cα—D_2stretch frequencies of Gdp-d2is remarkably sensitive to itsconformation.