EFFECT OF CRYSTALLINE ENVIRONMENT UPON ROTATIONAL CONFORMATION ABOUT N-C AND C-C' BONDS (PHI AND PSI) IN AMIDES AND PEPTIDES

SCF ab initio and PCILO computations indicate the intrinsic preference of the ψ torsion angle for 60 °. In the crystal structure of simple methylamides and peptides the observed values for this torsion angle lie between 0 °-30 °. Different procedures for computing lattice energies and total crystallographic conformational energies (lattice + torsional) utilized by other authors, failed to account for this situation. We show that the procedure developed recently in our laboratory for computing lattice energies indicates that the minima of these energies for ψ in acetamide and N-methylacetamide correspond well to 0 °-30 °. Because of the low value of the barrier of the torsional potential for this angle, the total crystallographic conformational energy corresponds also to ψ = 0 °-30 ° in agreement with the experimental data. © 1978 Springer-Verlag.