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

A reversible temperature-dependent high-spin to low-spin transition with = -60 °C has been observed in the resonance Raman spectra of the equilibrium reduced and photoreduced heme of the thermophilic heme-copper oxidoreductase. The transition is based on the frequency shifts of the spin-state marker bands (CC) and (CC) 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.