Ultrafast Measurements of Energy Flow, Geminate Recombination, and Fast Structural Rearrangements in Photoexcited Heme Proteins

Summary

Transient picosecond Raman spectroscopy is capable of differentiating vibrational relaxation from conformational changes by comparing the Stokes and anti-Stokes dynamics. We have accomplished pump-probe picosecond Raman experiments on deoxy- and oxyhemoglobin (deoxyHb and oxyHb, 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.[1–2] 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. The relative resonant enhancements for the initial oxyHb bands and the transient deoxyHb bands must be carefully considered in order to correctly interpret the picosecond dynamics data.