Reversible temperature-dependent high-to low-spin transition in the heme Fe-Cu binuclear center of cytochrome ba3 oxidase

A reversible temperature-dependent high-spin to low-spin transition with T1/2¼ 60 C has been observed in the resonance Raman spectra of the equilibrium reduced and photoreduced heme a3 of the thermophilic ba3 heme–copper oxidoreductase. The transition is based on the frequency shifts of the spin-state marker bands n2 (CbCb) and n10 (CaCm) and is attributed to the displacement of the heme iron along the heme normal as a consequence of the Fe–Np repulsion at temperature below 40 C which will increase the ligand field strength forcing the pairing of d electrons into the lower energy orbitals.