Investigation of the reaction coordinate for ligand rebinding in photoexcited hemeproteins using transient Raman spectroscopy

We report pump-probe picosecond Raman experiments on oxy- and deoxyhemoglobin (oxyHb and deoxyHb, respectively) using 8 ps 532 nm pump pulses and 8 ps 355 nm probe pulses. Heme-to-protein vibrational cooling has been directly observed in deoxyHb for the first time, and the deconvolved cooling time constant is measured to be 2-5 ps. By applying our mode-specific Stokes and anti-Stokes technique to oxyHb, we find that any germinate recombination of photodeligated O2 must occur in either less than two picoseconds or longer than a nanosecond