Theoretical principles for designing fluorescent rhodopsins: Electrostatic control of excited-state pathways

Microbial rhodopsins are widely used in optogenetics as light-driven actuators and genetically encoded voltage indicators. However, most rhodopsins exhibit extremely weak fluorescence due to rapid nonradiative relaxation of the retinal chromophore on the first excited-state potential-energy surface. Understanding the molecular origin of this behavior is therefore essential for the rational design of the brighter rhodopsin-based fluorophores. This review summarizes recent experimental and computational studies that establish electrostatic control of excited-state pathways as the central mechanism governing fluorescence in microbial rhodopsins. Following photoexcitation, the retinal chromophore relaxes from the Franck-Condon region toward a fluorescent state (FS) and subsequently toward a twisted intramolecular diradical intermediate (TIDIR) configuration located near the S-1/S-0 conical intersection. Fluorescence efficiency is therefore governed by the S-1 isomerization barrier separating the FS from the TIDIR decay region. We discuss how this barrier can be modulated through three physically well-defined parameters: protonation of Schiff-base counterions, retinal isomeric composition, and mutation-induced redistribution of electric fields within the chromophore cavity. The unusual photophysical behavior of Neorhodopsin provides direct experimental validation of this electrostatic framework, while automated quantum mechanics/molecular mechanics strategies such as the automated rhodopsin modeling protocol demonstrate how these principles can guide the discovery of new fluorescent variants. Taken together, these results identify electrostatic control of the S-1 isomerization barrier as a general design principle for engineering bright near-infrared rhodopsin fluorophores for optogenetic imaging.