Femtosecond multicolor pump-probe spectroscopy of ferrous cytochrome c

Ligand photolysis and subsequent electronic and structural relaxation, foll owed by ligand recombination, in ferrous cytochrome c (cyt c) have been stu died using ultrafast laser spectroscopy. A broad-band white-light continuum , generated by amplified pulses from a Ti:sapphire laser, was used to monit or the transient absorption spectra of cyt c in the Soret and Q bands follo wing 50-fs pulsed photoexcitation at 400 nm. The.reconstructed photoproduct absorption spectrum is found to closely resemble that of a model pentacoor dinate histidine-ligated complex, microperoxidase (MP-8), suggesting methio nine photolysis, Vibrational modes at similar to 40, similar to 80, and sim ilar to 220 cm(-1) are observed in femtosecond coherence spectroscopy (FCS) measurements, which also indicates photodissociation of the methionine lig and. The quantum yield of ligand photolysis is found to be greater than or equal to 80%, which is consistent with the ultrafast photolysis time consta nt (less than or equal to 40 fs) needed to induce coherent oscillations in the FCS measurements. The combination of high quantum yield and short time constant helps to resolve the longstanding question of the origin of the sh ort lifetime (tau (e)) and large Soret-state electronic damping factor (T ( e),) previously found in cytochrome c. We propose a simple multilevel model to describe the observed experimental data. The global analysis of the mea sured kinetics leads to a characterization of the major kinetic rates, incl uding the 6.2-ps geminate rebinding of methionine to the heme iron.