Themodynamic and transport properties of intermediate states of the photocyclic reaction of photoactive yellow protein.

Themodynamic and transport properties of intermediate states of the photocyclic reaction of photoactive yellow protein (PYP) were studied by a combination of the pulsed laser-induced transient grating (TG), transient lens (TrL), and photoacoustic (PA) spectroscopies from tens of nanoseconds to hundreds of milliseconds. The diffusion coefficients (D) of PYP in the ground state (pG) and of the second intermediate state (pB) were determined by the TG analysis, and it was found that D of pG is about 1.2 times larger than D of pB. At the same time, D at various denatured conditions were measured using guanidine hydrochloride as the denaturant. D of completely unfolded protein is about 0.4 times that of the native form. The enthalpy of pB is estimated to be 60 kJ/mol by the TrL method with an assumption that the volume change of pB is not sensitive to the temperature. Since the enthalpy of the first intermediate state (pR) is as high as 160 kJ/mol, it implies that most of the photon energy is stored as the strain of the protein in pR, and this may be the driving force for the successive reaction to pB. From the temperature dependence of the volume change, the difference in the thermal expansion coefficients between pG and pR was calculated. All of the characteristic features of PYP, the negative volume change, the larger thermal expansion coefficient, and the slower diffusion process, indicate that the intermediate pR and pB are reasonably interpreted in terms of the unfolded (loosened) protein structure.