Fig. 4.

Interspecies differences in the voltage-dependence of the light-induced SEIDA spectra of sensory rhodopsin II (SR II). The SEIDA spectra of SR II from H. s. show a similar voltage-induced halt of the light-induced proton transfer reaction from the protonated Schiff base to Asp-73 as observed in SR II from N. p. (Fig. 3). However, as manifested by the decreasing amplitude of the Asp-73 protonation band at 1,764 cm⁻¹, the halt occurs already at 0 V, indicating a species-dependent alteration of the energetics of this ion-transfer reaction. Experimental conditions are identical to the experiments on SR II from N. p., but spectra were recorded across a narrower voltage range (Fig. 3).