Femtosecond Absorption Spectroscopy of Reduced and Oxidized Forms of Cytochrome c Oxidase: Excited States and Relaxation Processes in Heme a and a₃ Centers

Excited electronic states and intraheme relaxation processes in the oxidized and reduced forms of mitochondrial cytochrome c oxidase extracted from a beef heart have been investigated by femtosecond absorption spectroscopy. The spectral and kinetic characteristics of short-lived intermediates have been measured from 80 fs to 20 ps after the photoexcitation. It is found that nonradiative electronic relaxation of the excitation energy in heme a, both in the oxidized (Fe(III)a) and reduced (Fe(II)a) forms, occurs successively as three processes, after the end of which heme a is in the ground state with a large store of vibrational energy. The subsequent vibrational relaxation (heme cooling) lasts for several picoseconds. It is found for reduced heme a₃ (Fe(II)a₃) that the electronic relaxation occurs as a result of two successive stages, which changes to vibrational relaxation in the ground state. The mechanism and dynamics of electronic excitation energy conversion in cytochrome c oxidase are analyzed.