Direct observation of the thermal equilibria among lumirhodopsin, metarhodopsin I, and metarhodopsin II in chicken rhodopsin.

Using low-temperature time-resolved spectroscopy, we have directly observed thermal back reaction of metarhodopsin I (meta I) to lumirhodopsin (lumi) and that of metarhodopsin II (meta II) to meta I in chicken rhodopsin to demonstrate the presence of thermal equilibria among lumi, meta I, and meta II. The back reaction from meta I to lumi was observed when the rhodopsin sample irradiated at -35 degrees C was warmed to -20 degrees C, while that from meta II to meta I was observed when the sample irradiated at -10 degrees C was cooled to -20 degrees C. Thermodynamic parameters of lumi, meta I, and meta II were calculated from the equilibrium constants estimated by analyzing the spectra of the equilibrium states at temperatures ranging from -30 to -10 degrees C. The results showed that meta I has an enthalpy and an entropy considerably smaller than those of lumi and meta II, while the difference in thermodynamic parameters between lumi and meta II is not so large. These results suggest that meta I is a crucial stage of conversion of the light energy captured by the chromophore into restricted conformations of the chromophore and/or protein, from which a large conformational change of the protein starts to form meta II.