The energetics and structural volume changes after photodissociation of carboxymyoglobin are quantitatively investigated by laser-induced transient grating (TG) and photoacoustic calorimetric techniques. Various origins of the TG signal are distinguished: the phase grating signals due to temperature change, due to absorption spectrum change, and due to volume change. We found a new kinetics of approximately 700 ns (at room temperature), which was not observed by the flash photolysis technique. This kinetics should be attributed to the intermediate between the geminate pair and the fully dissociated state. The enthalpy of an intermediate species is determined to be 61 +/- 10 kJ/mol, which is smaller than the expected Fe-CO bond energy. The volume of MbCO slightly contracts (5 +/- 3 cm(3)/mol) during this process. CO is fully released from the protein by an exponential kinetics from 25 to -2 degrees C. During this escaping process, the volume expands by 14.7 +/- 2 cm(3)/mol at room temperature and 14 +/- 10 kJ/mol is released, which should represent the protein relaxation and the solvation of the CO (the enthalpy of this final state is 47 +/- 10 kJ/mol). A potential barrier between the intermediate and the fully dissociated state is DeltaH(*) = 41.3 kJ/mol and DeltaS(*) = 13.6 J mol(-1) K(-1). The TG experiment under a high wavenumber reveals that the volume expansion depends on the temperature from 25 to -2 degrees C. The volume changes and the energies of the intermediate species are discussed.

A simple method to measure a rate of the translational temperature increase after the nonradiative transition with a few ps time resolution is presented. This method uses an acoustic peak shift of the transient grating signal, which can be determined accurately, and is very sensitive to rates and the relative amount of the thermal energy from the nonradiative transition. Using this method, the thermalization rate after the photoexcitation of trans-azobenzene to the S2(ππ*) state is measured. The acoustic peak shift indicates that the temperature of the solvents rises within less than ∼3 ps after the nonradiative transition of the S2→S1 process. The thermalization after the S1→S0 transition completes faster than the decay of the S1 state (16 ps in acetonitrile and 18 ps in ethanol). The fast energy transfer from the solute to the solvent is discussed.

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