Measurements of the photodissociation quantum yields of MbNO and MbO2 and the vibrational relaxation of the six-coordinate heme species

The (t similar to 0) photodissociation quantum yields (Y-0) of MbNO and MbO(2) are measured to be 50 +/- 5 and 28 +/- 6%, respectively, using MbCO (Y-0 = 100%) as a reference. When photolysis does not take place, we find that a significant portion of the photon energy contributes to heating of the residual six-coordinate heme (MbNO* and MbO(2)*). The time constant for vibrational relaxation of the six-coordinate ligand-bound heme is found to be close to 1 ps for both samples. The MbO(2) sample also shows a similar to4-ps optical response that is assigned to a rapid phase (25-30% amplitude) of O-2 geminate rebinding. We observe no additional geminate recombination in the MbO(2) sample out to 120 ps. In contrast, the MbNO sample displays significant geminate recombination over the first 120 ps, which can be adequately fit with two 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 and its effect on the geminate recombination as the system cools. Overall, the data are consistent with a hypothesis that distortions along the iron-ligand bending coordinate play a key role in the photodissociation process. The transient formation of an unphotolyzable FeO2 side-on binding geometry is suggested to be responsible for the lowered quantum yield of MbO(2) relative to MbNO.