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| - Measurements of the Photodissociation Quantum Yields ofMbNO and MbO2 and the Vibrational Relaxation of theSix-Coordinate Heme Species
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| - The (t ∼ 0) photodissociation quantum yields (Y0) of MbNO and MbO2 are measured to be 50± 5 and 28 ± 6%, respectively, using MbCO (Y0 = 100%) as a reference. When photolysis does not takeplace, we find that a significant portion of the photon energy contributes to heating of the residual six-coordinate heme (MbNO* and MbO2*). The time constant for vibrational relaxation of the six-coordinateligand-bound heme is found to be close to 1 ps for both samples. The MbO2 sample also shows a ∼4-psoptical response that is assigned to a rapid phase (25−30% amplitude) of O2 geminate rebinding. Weobserve no additional geminate recombination in the MbO2 sample out to 120 ps. In contrast, the MbNOsample displays significant geminate recombination over the first 120 ps, which can be adequately fit withtwo exponentials whose amplitudes and time constants appear to depend weakly on the pump wavelength.This more complex kinetic behavior conceivably arises due to heating of the photodissociated heme andits effect on the geminate recombination as the system cools. Overall, the data are consistent with ahypothesis that distortions along the iron−ligand bending coordinate play a key role in the photodissociationprocess. The transient formation of an unphotolyzable FeO2 side-on binding geometry is suggested to beresponsible for the lowered quantum yield of MbO2 relative to MbNO.
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