OBSERVATION OF COHERENT REACTION DYNAMICS IN HEME-PROTEINS

Femtosecond laser pulses, resonant with the Soret band of the nitric o xide complex of myoglobin (MbNO), were used to probe coherent, low-fre quency nuclear motion of the heme group after photolysis. Distinct osc illations with periods of 430 and 150 femtoseconds were observed and a re attributed to heme doming and iron-histidine motion, respectively. These results verify that the nuclear motion of the heme is strongly c oupled to the ligand binding reaction and demonstrate that such motion is not determined by overdamped (diffusive) dynamics. The relative ph ases and frequencies of the nuclear motion of the photoproduct suggest that the coherence arises from impulsive electronic forces associated with the spin-state change of the heme iron atom and the depopulation of its d(z)2 orbital during the bond-breaking event.