Reversible temperature-dependent high-to low-spin transition in the heme Fe-Cu binuclear center of cytochrome: Ba ₃ oxidase

A reversible temperature-dependent high-spin to low-spin transition with T _1/2 =-60 °C has been observed in the resonance Raman spectra of the equilibrium reduced and photoreduced heme a ₃ of the thermophilic ba ₃ heme-copper oxidoreductase. The transition is based on the frequency shifts of the spin-state marker bands ν ₂ (C _b C _b ) and ν ₁₀ (C _a C _m ) 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.