Optical rotatory properties of cupric ion complexes of simple dipeptides.

Titrations and spectral measurements of sperm whale metmyoglobin (2) and apomyoglobin (3) in the presence of cupric ion indicate that the metal ion may form chelates with imidazole groups and some other proton-bearing groups such as peptide amides. The titration and spectral properties of complexes of certain histidine-containing peptides have proved to be similar to those of the protein (4). Another technique that has proved effective in establishing parallels between complexes formed by the protein and by model peptides is measurement of optical rotatory dispersion. The formation of successive copper-peptide complexes may be observed by a combination of measurements of hydrogen ion titration, optical absorption spectra, and optical rotatory dispersion spectra. The optical rotatory dispersion spectra may be expected to be particularly sensitive to chelate formation. First, the constraint exerted by the metal ion on the peptide ligand, and any ot,her perturbing effects of the metal ion, would be expected to modify the dispersion curve characteristic of the peptide. This might be observed as a shift in the wave length of the first Cotton effect in the ultraviolet region or by changes in the values at positive and negative extrema. Second, it may be expected that the d-d transitions of the cupric ion may be rendered detectably optically active if an asymmetrical ligand is bound as a bidentate or higher chelate (5, 6). This type of effect may be observed in the visible region where the free ligand shows rather small optical activity. The results obtained with histidine-containing peptides will follow. The present report deals with the optical rotatory properties of a series of simple peptides containing no side-chain groups capable of forming coordinate bonds with cupric ion. As an adjunct to these studies and in order to form a firmer base for interpreting the equilibria with larger peptides, two separate computation methods have been programmed for determining equilibrium constants in these systems.