Rhodopsins contain a retinal molecule, and convert light into chemical energy or signal. The chromophore of visual or microbial rhodopsins is a retinal Schiff base of the 11-cis or all-trans form, respectively, where specific chromophore-protein interaction determines their colors. Upon light absorption, ultrafast photoisomerization initiates protein structural changes, leading to each functional expression. By use of spectroscopic methods, we have been studying how rhodopsins respond to light. Ultrafast spectroscopy of visual rhodopsin revealed that cis-trans isomerization is the primary event in our vision, which is optimized in protein environment. Fourier-transform infrared (FTIR) spectroscopy of visual and microbial rhodopsins provides various important vibrational bands related to structural changes of these proteins. Detection of protein-bound water molecules is one of the research highlights, and the comprehensive FTIR study has shown that a strongly hydrogen-bonded water molecule is the functional determinant of light-driven proton pump proteins. Here I review our spectroscopic challenge for > 25 years, particularly focusing our recent findings.
