Abstract
The structural and energy dynamics and molecular diffusion after the photodissociation of sperm whale carboxymyoglobin were investigated by the transient grating (TG) method. The quantitative measurement of the TG signal provides the volume change and released heat within 10 ns (−5 cm3 mol-1 and 151 kJ mol-1, respectively) as well as with a lifetime of 700 ns at 20 °C (12 cm3 mol-1 and 8 kJ mol-1) without any assumption. From the signal intensity with a wide range of grating vector (105 to 8 × 106 m-1), the molecular origin of the 700-ns dynamics is clearly attributed to the ligand escaping process from the protein interior to the solvent. Hence, a four-state model is appropriate to describe the ligand dissociation reaction even at room temperature. Participation of the salt bridge between Arg45 and the heme 6-propionate side chain as well as the enthalpy change and the structural change of this photodissociation reaction is discussed.
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