| Summary: | Extended illumination slows the rate of CO rebinding to myoglobin below 160K where CO is trapped within the protein after photolysis. The process increasing the rebinding barriers is found to be a photon-induced rather than a thermal effect. Rebinding barriers of molecules that are photolyzed do not increase unless light is incident on the sample. The effects of extended illumination in each of the three A substates of sperm whale myoglobin are studied in considerable detail and compared with the effects seen in horse myoglobin. The changes in the distribution of barriers for CO rebinding are different for the individual substates. The temperature at which the sample is illuminated is also a crucial factor determining the resulting rates for CO rebinding; higher barriers are created at higher temperatures. The effects of extended illumination on the A1 substate can be divided into three regimes. In regime I, an increase of the enthalpy barrier for rebinding by 1-2kJ/mol is observed which may be explained by a change in the azimuthal angle of the proximal histidine with respect to the heme plane. In regime II, the enthalpy barriers are increased by about 3-lOkJ/mol. Changes in spectral bands monitoring the protein conformation in this regime are similar to those in regime I. There is evidence for changes on the distal as well as the proximal side of the heme in the regime m. The increase in the enthalpy rebinding barrier is 11-22kJ/mol. The effects of illumination are compared with the conformational relaxation of MbCO that slows down rebinding at temperatures above 160K. There are some features in common with this relaxation, but also significant differences. Finally, a mechanism for the temperature dependence of the effects is suggested.
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