Studies on beta-turn of peptides. X. Effect of the chirality of 1st and 4th amino acids of tetrapeptide sequences on their beta-turn preferences studied by conformational energy calculation.

A conformational energy study was performed upon the effect of replacement of the Gly of Ac-Gly-AA2-AA3-Gly-NHCH3 by L- or D-Ala when AA2-AA3 part forms a turn conformation. When D-Ala-L-Pro constitutes the AA2-AA3 moiety, L-Ala at the 1st and 4th positions favor a beta-turn conformation of the tetrapeptide, while D-Ala residues at these positions do not. In the case of L-Pro-L-Ala at the AA2-AA3 position, the effect of replacing the two Gly residues by L- or D-Ala was shown to be just the opposite to that calculated for the D-Ala-L-Pro sequence. Terminal Gly residues are always allowable for beta-turn conformation.