Femtosecond transient absorption study on relaxation intermediates in oxymyoglobin.

We conducted femtosecond transient absorption spectroscopy of oxymyoglobin (MbO(2)) in the wavelength region of 450-750 nm to observe the charge transfer (CT) state of the heme in MbO(2). The transient absorption spectrum just after photoexcitation (t = 0-0.25 ps) was assigned to superposition of S(1)-S(n) absorption (450-750 nm) and ground-state bleaching (540 and 580 nm) of MbO(2). At t > 0.25 ps, a new absorption band appeared at around 680 nm with the decay time of approximately 5 ps. This broad absorption band was similar to that of a porphyrin cation. Hence, we concluded that this absorption band was attributed to the ligand-to-metal charge transfer (LMCT) state of the heme. The proposed photophysical pathways suggested that formation of the LMCT state was the key event in the excited state relaxation of the heme in MbO(2).