Rapid timescale processes and the role of electronic surface coupling in the photolysis of diatomic ligands from hemeproteins

Abstract

We have observed coherent oscillations of the heme protein myoglobin (Mb) following femtosecond laser excitation and photodissociation of the CO, O₂, and NO bound ligands. Use of a novel methodology, involving “wavelength selective modulation” of the pump and/or probe laser pulse train, allows us to discriminate between coherences created by pump fields of differing wavelength within the laser pulse versus signals that arise from the decay of either vibrational or electronic populations. The population driven signals appear when pump field interactions having the same optical frequency are allowed to contribute to the signal detection channel. One surprising result, which will be stressed in the discussion, is the observation of a distinct product state vibrational coherence (the iron–histidine stretching vibration of deoxy Mb at 220 cm⁻¹) that depends upon the presence of pump field interactions having a wavelength mismatch that is equal to the 220 cm⁻¹ vibrational frequency. This observation is surprising because the iron–histidine mode is not observed in the resonance Raman measurements on the six-coordinate reactant species. Thus, the pump-pulse laser excitation between the ground and excited state, which leads to the ligand dissociation, is evidently able to create a “field driven” vibrational coherence of a resonance Raman inactive mode that extends into non-vertical regions of the reactive excited state potential energy surface. Non-radiative electronic surface crossing, followed by the rapid development of new electronic forces on the nuclei, appears to be ruled out as a source of the coherent signals (the random phase of the optically uncoupled modes is one possible explanation for this observation). The extremely rapid timescale (≪150 fs) for the development of the (S = 2) high-spin product state of the iron atom from the initial unphotolyzed state (S = 0) is worthy of further theoretical discussion because of the spin forbidden nature of such a transition. Excited state admixtures of the iron spin states are presumably involved, and the mixing of these states, along with the unpaired electron on NO, may help to explain the ultrafast time scales and large amplitudes that characterize the NO geminate recombination in comparison to CO.