Steric constraint in the primary photoproduct of an archaeal rhodopsin from regiospecific perturbation of C-D stretching vibration of the retinyl chromophore

In visual and archaeal rhodopsins, light energy is stored in the chromophore-protein interaction after retinal photoisomerization. This paper reports a novel method to monitor the steric constraint after retinal isomerization by use of enhanced C-D stretching vibrations. In the difference FTIR spectra between an archaeal light-sensor pharaonis phoborhodopsin (ppR) and the primary K intermediate at 77 K, no peaks were observed in the 2160-2330 cm^-1 region for deuterated retinals at position 7, 8, 10, 11, 12, and 15, whereas a strong peak appeared at 2244 cm^-1 for the K intermediate of ppR possessing a C₁₄-D-labeled retinal. The 2244-cm^-1 band is assigned as the C₁₄-D stretching vibration, and enhanced absorption in the K state probably originates from the local steric constraint at the C₁₄-D position (also possible electrostatic field effects) after the C₁₃=C₁₄ double bond rotation.