Optical Rotatory Dispersion of Cytochrome c

Decrease in the ionic strength of a solution of horse heart ferricytochrome c at pH 1.5 results in changes in optical rotatory dispersion similar to those observed upon helix-random coil transition in model polypeptide systems. These changes are accompanied by modification of the dispersion pattern in the Soret region and parallel alterations in the absorption spectrum indicative of transition from a partially low spin system to a predominantly high spin one. A corresponding ionic strength effect is not observed with the unmodified protein at pH 7 but can be obtained at neutral pH with a derivative in which all the carboxyl groups are esterified. Such an esterified preparation, even under conditions where the dispersion pattern in the region from 200 to 250 mµ remains essentially that of the unmodified protein, exhibits rotations which, at longer wave lengths, approximate more nearly those recorded for the nonesterified molecule in the presence of guanidine hydrochloride or urea. The possibility is considered that the propionate side chains of the porphyrin play an important role in the binding between the protein and its prosthetic group.