Femtosecond absorption spectroscopy of reduced and oxidized forms of cytochrome c oxidase: excited states and relaxation processes in heme $a$ and $a_3$ 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 $a_3$ (Fe(II) $a_3$) 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.