The energetics and protein dynamics of photoactive yellow protein (PYP) were studied by transient grating (TG) and photoacoustic (PA) spectroscopies. The enthalpy difference (Delta H) between the ground state (pG) and the first intermediate (pR) at 20 degrees C was determined by the To method as 160 kJ/mol, which is much larger than Delta H of the photoisomerized chromophore (p-coumaric acid) in water (50 kJ/mol). By simultaneous measurements of the TG and PA signals, the volume change (Delta V) for the pG --> pR process is determined to be -7 cm(3)/mol (contraction) at 20 degrees C. Interestingly, the volume contraction increases with decreasing temperature. At 0 degrees C the volume change becomes ca. -15 cm(3)/mol. This temperature-dependent volume change may indicate the structural fluctuation of PYP protein in the solution phase.