Conformational transitions and vibronic couplings in acid ferricytochrome c: a resonance Raman study

Resonance Raman spectral changes in ferricytochrome c as a function of pH between 6.7 and 1.0 are reported and the structural implication is discussed in terms of the "core-expansion" model advanced by L. D. Spaulding et al. [(1975) J. Am. Chem. Soc. 97, 2517]. The data are interpreted as indicating the iron in high-spin ferricytochrome c (at pH 2.0) with two water molecules as axial ligands lies in the plane of the porphyrin ring. At pH 1.0 there is a different high-spin form of cytochrome c which has an estimated iron out-of-plane distance of approximately 0.46 A. The effect of a monovalent anion at pH 2.0 is to produce a thermal spin mixture with predominant low-spin species. Excitation at approximately 620 nm in acid cytochrome c (pH 2.0) enhances only three depolarized ring vibrations at 1623, 1555, and 764 cm-1. Marked enhancement of depolarized modes relative to polarized and anomalously polarized modes is attributed to the vibronic coupling between porphyrin pi leads to pi and porphyrin pi leads to iron (dpi) charge-transfer states.