A CONFORMATIONAL STUDY OF THE DEHYDROALANINE - DIPEPTIDE AND HOMOPOLYPEPTIDE

A molecular mechanics study of polydehydroalanine [poly-(Delta Ala)] i s presented. For this purpose the AMBER 3a program has been used to pe rform the calculations. With exception of the point charges, the param eters for the terminal groups were taken from AMBER 3a libraries, wher eas those for the Delta Ala residue from Alagona et al. [J. Comp. Chem . (1991) Vol. 12, pp. 934-942]. Charges for the residue and terminal g roups have been fitted from the MNDO electrostatic potential and scale d to achieve an ab initio 6-31G quality. Calculations have been carri ed out using the continuous solvent approximation with three different dielectrics epsilon = 1, 1r, and 4r. The results show that, despite t he preferred structure for the isolated residue is an extended conform ation, a 3(10)-helix is the preferred conformation in the solid state (epsilon = 1 and 1r), whereas a peculiar structure with psi = 0 degree s is preferred with epsilon = 4r. (C) 1993 John Wiley and Sons, Inc. [ poly- ( del Ala)] is presented,For this purpose the AMBER 3a program h as been used to perform the calculations. With exception of the point charges, the parameters for the terminal groups were taken from AMBER 3a libraries, whereas those for the Delta Ala residue from Alagona et al. [J. Comp. Chem. (1991) Vol. 12, pp. 934-942]. Charges for the resi due and terminal groups have been fitted from the MNDO electrostatic p otential and scaled to achieve an ah initio 6-31G() quality. Calculat ions have been carried out using the continuous solvent approximation with three different dielectrics c = 1, 1r, and 4r. The results show t hat, despite the preferred structure for the isolated residue is an ex tended conformation, a 3(10)-helix is the preferred conformation in th e solid state (E = 1 and 1r), whereas a peculiar structure with psi = 0 degrees is preferred with c = 4r. (c) 1993 John Wiley and Sons, Inc.