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.

The exothermic nonradiative relaxation processes from photoexcited nitrobenzene (NB) are studied by the picosecond time-resolved transient grating method. The decay rate constants and energy of the excited NB are determined. The lifetime of the lowest excited singlet state is found to be very short (≤10 ps) and a surprisingly short lifetime of the lowest excited triplet state (∼480 ps) is detected. From quantitative measurements of the thermal energies released from the excited states of NB, the lowest excited triplet state is determined to be located at (22−27) × 103 cm-1. The triplet formation is very efficient and its quantum yield is found to be grater than 0.80. From the temperature and solvent dependence of the triplet lifetime, it is suggested that motions of the nitro group may cause the surprisingly rapid relaxation from the lowest excited triplet state to the ground state. The photophysical properties and excited states of monosubstituted nitrobenzenes are also discussed.

Articles you may be interested inStudy of the translational diffusion of the benzophenone ketyl radical in comparison with stable molecules in room temperature ionic liquids by transient grating spectroscopy

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Diffusion coefficients ͑D͒ of various radicals created by the photoinduced hydrogen abstraction reactions from alcohols ͑ethanol and 2-propanol͒ are investigated by using the transient grating ͑TG͒ method. In all the reaction systems, D's of the transient radicals, as well as those of the parent molecules, can be measured simultaneously. The results clearly show slower diffusive motions of the radicals, at least of the radicals in the hydrogen abstraction reaction systems, compared with those of the parent molecules. D's of the parent molecules usually agree well with the calculated values based on the Spernol and Wirtz modification of the Stokes-Einstein ͑SE͒ relation. Although the measured D's of the radicals are closer to the values predicted by the simple SE equation, the agreements and the ratio of D between the radicals and its parent molecules depend on the molecular size. The ratio becomes closer to unity as the molecular size becomes large. Possible origins of this dependence are discussed.

The energetics and protein dynamics of photoactive yellow protein (PYP) were studied by transient grating (TG) and photoacoustic (PA) spectroscopies. The enthalpy difference (ΔH) between the ground state (pG) and the first intermediate (pR) at 20 °C was determined by the TG method as 160 kJ/mol, which is much larger than ΔH of the photoisomerized chromophore (p-coumaric acid) in water (50 kJ/mol). By simultaneous measurements of the TG and PA signals, the volume change (ΔV) for the pG → pR process is determined to be −7 cm3/mol (contraction) at 20 °C. Interestingly, the volume contraction increases with decreasing temperature. At 0 °C the volume change becomes ca.  −15 cm3/mol. This temperature-dependent volume change may indicate the structural fluctuation of PYP protein in the solution phase.

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