Energetics and Volume Changes of the Intermediates in the Photolysis of Octopus Rhodopsin at a Physiological Temperature

Abstract

Enthalpy changes (Δ H) of the photointermediates that appear in the photolysis of octopus rhodopsin were measured at physiological temperatures by the laser-induced transient grating method. The enthalpy from the initial state, rhodopsin, to bathorhodopsin, lumirhodopsin, mesorhodopsin, transient acid metarhodopsin, and acid metarhodopsin were 146 ± 15 kJ/mol, 122 ± 17 kJ/mol, 38 ± 8 kJ/mol, 12 ± 5 kJ/mol, and 12 ± 5 kJ/mol, respectively. These values, except for lumirhodopsin, are similar to those obtained for the cryogenically trapped intermediate species by direct calorimetric measurements. However, the Δ H of lumirhodopsin at physiological temperatures is quite different from that at low temperature. The reaction volume changes of these processes were determined by the pulsed laser-induced photoacoustic method along with the above Δ H values. Initially, in the transformation between rhodopsin and bathorhodopsin, a large volume expansion of +32 ± 3 ml/mol was obtained. The volume changes of the subsequent reaction steps were rather small. These results are compared with the structural changes of the chromophore, peptide backbone, and water molecules within the membrane helixes reported previously.