Deciphering factors which determine the Ramachandran surface of peptides.The application of isodesmic surfaces,#DELTA#E_(ID)(#psi#,#PHI#),to analyze the contribution of rotaing moieties to the shape of potential energy surfaces

摘要

A series of double rotors of the following types have been investigated via potential energy surfaces,E=E(#psi#,#PSI#),generated by ab initio Hartree-Fock molecual computations.Series of triply substituted methyne 9H-C) moieties are called upon to mimic,the conformatiosnl behaviour of peptides.In substituents Z_1 and Z_2 the "first atoms" were of sp~3 and sp~2 hyridization having Z_1=-CH_3,-CH_2F and -NHCHO,as well as the alanine model peptide.All potential energy surfaces,E=E(#psi#,#PSI#),were generated in the form of grids where points were separated by 15 deg intervals along both #psi# and #PSI# variables.This led to a total of 625 SCF points (25X25) for each surface,since both the initial (o deg) and final (360 deg) values of both periodic variables (#psi# and #PSI#),were included int he grid.The interaction between substituents introduced were monitored by isodesmic reactions computed at each of the 625 grid points leading to isodesmic energy surfaces:#DELTA#E_(ID)(#psi#,#PSI#).Comparing the influence of the different Z_1 and Z_2 substituents,undoubtedly,the introduction of two adjacent peptide bonds led to the greatest effect on the conformational energy surface.