ULTRAFAST PUMP-PROBE STUDIES OF EXCITED-STATE CHARGE-TRANSFER DYNAMICS IN BLUE COPPER PROTEINS

We report the results of ultrafast pump-probe measurements on three bl ue copper proteins: spinach plastocyanin, poplar plastocyanin, and hum an ceruloplasmin. Electronic population dynamics and vibrational coher ences involving d --> d transitions of the blue copper active site are observed using both wavelength-integrated and wavelength-resolved det ection. Depending on the protein and the method of detection, the pump -induced bleaches of the electronic ground state decay with time const ants of 230-300 fs. The pump-probe signals are modulated by oscillatio ns that correspond to vibrational coherences induced by the ultrashort pulses. The frequencies of some of these oscillations can be matched with modes observed in resonance Raman studies of these proteins. For spinach plastocyanin and ceruloplasmin, wavelength-resolved signals re veal a previously unreported vibration at similar to 500 cm(-1) whose decay dynamics are consistent with the excited-state lifetime. The ori gin of this mode is argued to result from Duschinsky rotation. The rel evance of these pump-probe results to the development of a model for b iological electron transport;in these proteins is discussed.