PICOSECOND RAMAN-STUDY OF ENERGY-FLOW IN A PHOTOEXCITED HEME PROTEIN

We have directly observed the ultrafast vibrational dynamics in photoexcited deoxyhemoglobin. These results indicate that excess porphyrin vibrational energy completely dissipates within 15 ps in photoexcited deoxyhemoglobin. The Stokes and anti-Stokes Raman spectra were probed with 8-ps 355-nm pulses following photoexcitation at 532 nm. Stokes Raman bands show negative transients, while anti-Stokes bands at the same Raman shifts show positive transients with the identical decay time constants. This combination of spectral features can be assigned only to vibrationally hot molecules. In contrast, conformational changes of the heme structure would be indicated by Stokes and anti-Stokes band transients that both go positive or else both go negative. These data cannot be explained as conformational changes in the heme. Dynamics spectra show the rise and fall of Raman scattering at a fixed frequency as a function of delay time between pump and probe pulses. Positive transients at time zero are detected in the anti-Stokes spectra for all heme bands observed; all Stokes Raman bands observed are bleached at time zero. All transients show a l/e time constant of almost-equal-to 2-5 ps. These findings unambiguously determine vibrational cooling dynamics. The data quantitatively indicate a vibrational temperature of 36 K above room temperature, averaged over our 8-ps pulse width at early time. Results are interpreted in terms of models of vibrational cooling in large molecules and compared to a previous molecular dynamics simulation of cooling in a heme protein.