Probing the reaction coordinate for ligand binding in hemoglobin using ultrafast transient Raman spectroscopy

Transient picosecond Raman spectroscopy is capable of differentiating vibrational relaxation from conformational changes by comparing the Stokes and anti-Stokes dynamics. 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 geminate recombination of photodeligated O2 must occur in either less than two picoseconds or longer than a nanosecond.