Retinal chromophore charge delocalization and confinement explain the extreme photophysics of Neorhodopsin

Fluorescent proteins that self-assemble and localize in the neuron membrane are vital in neurosciences, particularly in optogenetics applications. Here the authors present a quantum-mechanics/molecular mechanics model for the photoisomerization of the natural highly fluorescent Neorhodopsin, explaining the highly fluorescent quantum yield that could lead to effective visualization of neural signals.The understanding of how the rhodopsin sequence can be modified to exactly modulate the spectroscopic properties of its retinal chromophore, is a prerequisite for the rational design of more effective optogenetic tools. One key problem is that of establishing the rules to be satisfied for achieving highly fluorescent rhodopsins with a near infrared absorption. In the present paper we use multi-configurational quantum chemistry to construct a computer model of a recently discovered natural rhodopsin, Neorhodopsin, displaying exactly such properties. We show that the model, that successfully replicates the relevant experimental observables, unveils a geometrical and electronic structure of the chromophore featuring a highly diffuse charge distribution along its conjugated chain. The same model reveals that a charge confinement process occurring along the chromophore excited state isomerization coordinate, is the primary cause of the observed fluorescence enhancement.