Hyperfine Correlation Spectroscopy and Electron Spin Echo Envelope Modulation Spectroscopy Study of the Two Coexisting Forms of the Hemeprotein Cytochrome c6 from Anabaena Pcc7119

Oxidized cytochrome c6 from Anabaena PCC 7119 was studied by electron spin echo envelope modulation spectroscopy. Hyperfine couplings of the unpaired electron with several nuclei were detected, in particular those of the nitrogens bound to the iron atom. Combining the experimental information here presented and previous continuous wave-electron paramagnetic resonance and electron nuclear double resonance results, some details on the electronic structure of the heme center in the protein are obtained. These results are discussed on the basis of a molecular model that considers one unpaired electron localized mainly in the iron d orbitals and propagation of the spin density within the heme center via spin polarization of the nitrogen σ-orbitals. The coexistence of two heme forms at physiological pH values in this c-type cytochrome is also discussed taking into account the experimental evidence.