Femtosecond absorption spectroscopy of cytochrome c oxidase: Excited electronic states and relaxation processes in heme a and heme a 3 centers

Cytochrome c oxidase, the key bioenergetic protein, was studied by femtosecond absorption spectroscopy. Time-resolved spectral characteristics of the difference spectra recorded in the timescale from 80 fs to 20 ps were analyzed. Electronic relaxation of the excitation energy in heme a occurs in three successive steps. After completion of these steps, heme a is in the excited vibrational state of the ground state. Vibrational relaxation, cooling of the heme, occurs for several picoseconds.