Energy-minimized conformation of gramicidin-like channels. I. Infinitely long poly-(L,D)-alanine beta 6.3-helix.

The energy-minimized conformation of an infinitely long poly-(L,D)-alanine in single-stranded beta 6.3-helix was calculated by the molecular mechanics method. When energy minimization was started from a wide range of initial geometries, six optimized conformations were obtained and identified as the right- and left-handed counterparts of the beta 4.5-, beta 6.3-, and beta 8.2-helices. It was found that their conformation energies increase in this order, the beta 4.5-helix having the lowest energy. The backbone dihedral angles of the energy-minimized beta 6.3-helix were: phi L = -116 degrees (or -131 degrees), psi L = 122 degrees (or 111 degrees), phi D = 131 degrees (or 116 degrees), psi D = -111 degrees (or -122 degrees), omega L = 173 degrees (or 173 degrees), and omega D = -173 degrees (or -173 degrees) for the right-handed (or left-handed) helix. This helix was composed of 6.30 residues/turn with a pitch of 4.97 A. All the alpha-carbons of L- and D-configurations appeared on one common circular helix. Interestingly, small deviations (approximately 7 degrees) of the peptide bonds from the planar structure caused a considerable lowering of the conformation energy, and, at the same time, they produced more favorable fitting of the hydrogen bonds; the carbonyl oxygens and the nearest-neighbor alpha-hydrogens also took more favorable relative positions.