Deciphering factors which determine the Ramachandran surface of peptides. The application of isodesmic surfaces, Delta E-ID(theta,psi), to analyze the contribution of rotating moieties to the shape of potential energy surfaces

A series of double rotors of the following types have been investigated via potential energy surfaces, E = E(phi, psi), generated by ab initio Hartree -Fock molecular computations. [GRAPHICS] Series of triply substituted methyne (H-C) moieties an called upon to mimic , the conformational behaviour of peptides. In substituents Z(1) and Z(2) t he "first atoms" were of sp(3) and sp(2) hybridization having Z(1) = -CH3, -CH2F and -NHCHO, as well as Z(2) = -CH3, -CH2F, -CHO and -CONH2. The R gro up was chosen to be -H or -F and in one case -CH3 in order to reproduce the alanine model peptide. All potential energy surfaces, E = E(phi, psi), wer e generated in the form of grids where points were separated by 15 degrees intervals along both alpha and psi variables. This led to a total of 625 SC F points (25 x 25) for each surface, since both the initial (0 degrees) and final (360 degrees) values of both periodic variables (phi and psi) were i ncluded in the grid. The interaction between substituents introduced were m onitored by isodesmic reactions computed at each of the 625 grid points lea ding to isodesmic energy surfaces: Delta E-ID(phi, psi). Comparing the infl uence of the different Z(1) and Z(2) substituents, undoubtedly, the introdu ction of two adjacent peptide bonds led to the greatest effect on the confo rmational energy surface. (C) 2000 Elsevier Science B.V. All rights reserve d.