Conformation of histidine model peptides. I. Conformational energy calculations for L ‐alanyl‐ L ‐histidine diketopiperazine

Semiempirical conformational energy calculations were carried out for the cyclic dipeptide L‐alanyl‐L‐histidine diketopiperazine. The results indicate that electrostatic effects are probably significant in determining the conformation assumed by this molecule. When the imidazole group is in its uncharged state the most stable conformations of the molecule are those with the imidazole ring folded over the diketopiperazine ring (χ1 = 60°). Upon protonation of the imidazole group the folded conformation may be destabilized relative to conformations characterized by χ1 positions near 180°.