Fluorescence of the retinal chromophore in microbial and animal rhodopsins

Fluorescence of the vast majority of natural opsin-based photoactive proteins is extremely low in accordance with their functions that depend on efficient transduction of absorbed light energy. However, recently proposed several classes of engineered rhodopsins with enhanced fluorescence along with the discovery of a new natural highly fluorescent rhodopsin, NeoR, opened a way to exploit these transmembrane proteins as fluorescent sensors and draw more attention to studies on this untypical rhodopsins property. Here we review available data on the fluorescence of the retinal chromophore in microbial and animal rhodopsins and their photocycle intermediates as well as different isomers of the protonated retinal Schiff base in different solvents and the gas phase.