Optical Rotatory Dispersion of Cytochrome c: II. COMPARATIVE DATA FOR A HEME OCTAPEPTIDE

Abstract

Abstract The optical rotatory dispersion of horse heart cytochrome c has been compared with that of model heme octapeptide systems. When one of the coordination positions about the heme iron of the peptide systems is occupied by the imidazole side chain of the single histidine residue, dispersion curves are obtained which, in a number of respects, are similar to those of the intact protein. There are also, however, major differences. The most striking of these occur in the Soret region, where the patterns recorded for the peptide complexes resemble more nearly those for myoglobin and hemoglobin than those for horse heart cytochrome c. Only with the heme peptide in aggregated form were curves obtained which approach the pattern for the parent ferricytochrome molecule in the Soret region. Complexes of the oxidized heme peptide with extrinsic imidazole and with methionine derivatives yield curves which exhibit, in addition to the Cotton effects in the Soret region, a broad positive extremum in the range 500 to 600 mµ, and multiple Cotton effects in the ultraviolet. Reduction of these complexes results in shifts of the curves to longer wave lengths in the Soret region, the appearance of well defined Cotton effects associated with the hemochrome bands near 520 and 550 mµ, and extensive modification of the complicated pattern in the ultraviolet region. The curves obtained differ for the two oxidation states throughout the wave length range studied (220 to 620 mµ). Rotations recorded at wave lengths near 230 mµ are greater than those calculated for 8 residues in random coil form on the basis of relationships for simple polypeptides. This excess rotation is not diminished by the addition of 8 m urea, but is reduced greatly upon protonation or photooxidation of the histidine residue of the peptide. Such treatment leads also to loss of the large Cotton effect in the Soret region.