Polymethionyl,-valyl and-glycyl derivatives of equine heart cytochromec heme octapeptide

Polymethionyl ((Met)_5·3-H8PT), polyvalyl ((Val)_4·4-H8PT) and polyglycyl ((Gly)_3·2-H8PT) derivatives of the heme octapeptide of equine heart cytochrome c were prepared with the aid of the N-carboxyl anhydrides of their respective amino acids. Only for the (Met)_5·3-H8PT did the γ-peak in the absorption spectrum of the reduced form shift from 412.5 nm, the value for the unsubstituted octapeptide, to 414 nm, the value close to the γ-peak of intact ferrocytochrome c. The γ-peak of the oxidized form of the octapeptide did not shift significantly from 397 nm. Amino acids attached to the N-terminal cysteine of the octapeptide, with methionine excepted, have little or no effect on the spectrum, and apparently cannot serve as either an intramolecular or intermolecular ligand for the iron. This result is in marked contrast to those previously obtained with the substituted cytochrome c heme undecapeptide (ref. 8) where all three derivatives had altered spectra. For the octapeptide, the ratio, absorbance of the γ-peak of the oxidized form to absorbance of the γ-peak of the reduced form, did not change appreciably for any of the derivatives, although when the reaction products from excess methionine anhydride were present (before centrifugation and dialysis) the ratio was lower and approached the ratio for cytochrome c.