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

Abstract

AbstractThe 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 Nε is protonated may be more prevalent than the isomer in which Nδ is protonated.