title: Conformational transitions of cyclic D,L-peptides.
abstract: 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 E(ax) 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 E(ax)-B(eq) transitions). All the six transitions proceed with the breaking of the high molecular symmetry (S(6)) and go through the triangular (C(3)) intermediate structure with either equatorial or axial side-chain orientation.
authors.affiliation: Department of Materials Science and Engineering, Waseda University, Tokyo 169-8555, Japan.
authors.forename: Hajime
authors.initials: H
authors.lastname: Okamoto
authors.forename: Tetsuo
authors.initials: T
authors.lastname: Yamada
authors.forename: Shuichiro
authors.initials: S
authors.lastname: Kihara
authors.forename: Kazumasa
authors.initials: K
authors.lastname: Takechi
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Takagi
authors.forename: Kyozaburo
authors.initials: K
authors.lastname: Takeda
mesh_terms: D007536:Isomerism; D008958:Models, Molecular; D015394:Molecular Structure; D010456:Peptides, Cyclic / Q000737:chemistry*; D011789:Quantum Theory
chemical_list: D010456:Peptides, Cyclic