First-principle conformational analysis of glycine residues in the alphabeta-tubulin dimer

Amino acids, especially glycine, have been extensively studied whereas their conformational behaviors in proteins are rather limited. In this work, all the polypeptides containing glycine residues are truncated from the alphabeta-tubulin dimer and refined with the partial optimization technique, where the backbone atoms of the previous and posterior residues to the glycine residues are fixed at the experimental Cartesian positions whereas the others fully relaxed. The combination of the polypeptide models and partial optimization technique is validated by twolayer ONIOM calculations, being effective to retain the local structures of proteins and meanwhile optimizing the concerned glycine residues towards energy minima. Owing to the lack of side chains, various types of hydrogen bonding interactions are detected in the glycine residues. The conformational analyses of proteins are mainly based on the dihedrals. Theta(1)( angleH(5)C(2)C(3)O(4)), Theta(2)( angleH(6)C(2)C(3)O(4)) and Theta(3)( angleN(1)C(2)C(3)O(4)) are the three key dihedrals to determine the glycine conformations, which were found to change synchronously and correlate with each other by four linear equations. It thus provided the first evidence of the correlation of intra-dihedrals for the amino acid residues in proteins. In addition, the three dihedrals of the gas-phase glycine conformations were also found to suit well with these linear equations, elucidating the rationality of using amino acids as the computational models of proteins.