Dynamics of Structure and Energy of Horse Carboxymyoglobin after Photodissociation of Carbon Monoxide

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Abstract
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 ∼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 cm3/mol) during this process. CO is fully released from the protein by an exponential kinetics from 25 to −2 °C. During this escaping process, the volume expands by 14.7 ± 2 cm3/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 ΔH⧧ = 41.3 kJ/mol and ΔS⧧ = 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 °C. The volume changes and the energies of the intermediate species are discussed.