Abstract
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.