Visible Spectra and Optical Rotatory Properties of Cupric Ion Complexes of l-Histidine-containing Peptides

The visible absorption spectra and optical rotatory dispersion spectra of l -histidine-containing peptides and copper(II) have been investigated. A knowledge of the equilibrium constants involved has enabled the spectra to be resolved into contributions from individual species. With the particular choice of peptides it has been possible to assess the contribution of different bonded groups to the magnitude of the ligand field around the central copper(II), which is reflected in the λ max values. The order of relative effectiveness is found to be: α-amino nitrogen > peptide nitrogen > imidazole nitrogen. Decisions can thus be made among alternative structures by measurement of the absorption spectra. The optical rotatory spectra show that the anomalous dispersion around the absorption band in the visible region is the resultant of two overlapping Cotton effects. When the l -histidyl residue is carboxyl terminal, these Cotton effects are of opposite sign. In the one example in which the l -histidyl residue was penultimate from the carboxyl terminus, both Cotton effects appeared to be negative. An explanation for the Cotton effects in the visible region has been given, postulating that interaction between the carbonyl oxygen of the l -histidyl carboxyl group and the copper(II) is responsible for predominance of one of the possible configurations of ligands around the metal ion.