Conformation of histidine model peptides. III. Chiroptical properties of cyclo-L-alanyl-L-histidine and cyclo-L-histidinyl-L-histidine

The chiroptical properties of the cyclic dipeptides cyclo-L -alanyl-L -histidine and cyclo-L -histidinyl-L -histidine have been investigated as a function of molecular conformation. The rotatory strengths of the n-π* transitions of the peptide chromophores and the lowest energy π-π* transitions of the imidazole chromophores have been calculated as a function of the angle of fold of the cyclic dipeptide group and the dihedral angles χ¹ and χ² of the amino acid side chains. The results of this investigation are consistent with the preferred position of the dihedral angle χ¹ occurring near 60° in the free base form of cyclo-L -alanyl-L -histidine, and near 180° when the imidazole side chain is protonated. Furthermore, in the case of the free base form of the imidazole group, it is possible that the tautomeric isomer in which N^ε is protonated may be more prevalent than the isomer in which N^δ is protonated.