Conformational transitions of cyclic D,L-peptides

Conformational transitions of cyclic D,L-hexapeptides have been studied by first-principles calculations. Geometry optimizations for 20 types of homoresidue cyclic D,L-hexapeptide revealed that the cyclic peptides have two types of energetically stable backbone (extended (E) and bound (B) types); and for each type, the amino acid side chains have two orientations (equatorial and axial). Among the four types of isomer [E-type equatorial (E_eq), B-type equatorial (B_eq), E-type axial (E_ax), and B-type axial (B_ax)], B_ax is the energetically most preferred by most of the 20 encoded amino acid residues, whereas Eax is the least preferred. A search for transition states indicated that six types of conformational transition are possible between the isomers of the cyclic peptide, i.e., the backbone-backbone conversions (E_eq-B_eq and E _ax-B_ax transitions), the side chain-side chain conversions (E_eq-E_ax and B_eq-B_ax transitions), and the simultaneous conversions of the backbone and the side-chain orientation (E_eq-B_ax and Eax-Beq transitions). All the six transitions proceed with the breaking of the high molecular symmetry (S₆) and go through the triangular (C₃) intermediate structure with either equatorial or axial side-chain orientation.