Conformation of aminosuccinyl dipeptides Ac-Asu-X-NHMe from empirical energy calculations.

Conformational energy calculations were carried out on the N-acetyl-N'-methylamides of aminosuccinyl (Asu) peptides. Computations were performed using a procedure analogous to that of the ECEPP/2 program, on N-Ac-L-Asu-NMe, N-Ac-L-Asu-L-X-NHMe, where X = Gly, Ala, Ser, Val and N-Ac-D-Asu-L-Ala-NHMe. With the exception of N-Ac-L-Asu-L-Val-NHMe, the lowest-energy forms of all the dipeptides correspond to a beta-bend conformation of type II' or II for a L,L and D,L sequence, respectively. When X = Val, the folded conformation is destabilized, and more extended conformations are preferred. The puckering of the cyclic imide has a small but meaningful influence on the relative energies of the minima. The calculations were shown to be in agreement with available experimental data.