Impact of the Protein Environment on Two-Photon Absorption Cross-Sections of Type I and Type Ii Retinal-Containing Proteins

Predicting and optimizing the photophysical properties of retinal containing channel proteins under two-photon absorption (TPA) conditions are required for their efficient use in optogenetics. Nonlinear two-photon excitation can also lead to photoactivation of visual rhodopsins in the IR range of 950–1000 nm. Here, by using high-level quantum chemistry calculations, we explore the factors that influence the TPA activity of the type I and type II rhodopsins upon their two photon resonant excitation to the first singlet excited state. We show that in the case of the S0 → S1 transition, the channels through permanent dipole moments, which are associated with the initial and final states only, make the largest contribution to the calculated TPA cross-sections. The fast numerical convergence of the sum-over-states formalism provides direct evidence for the applicability of the two-level model for calculating TPA cross-sections in rhodopsins, which are characterized by the large transition dipole moment and a signifi cant redistribution of the electron density upon the S0 → S1 transition. The calculated TPA cross-sections (340–610 GM) are found to be very sensitive to changes in the permanent dipole moments between the ground and excited states and highly tunable by internal electric f i eld of the protein environment. The high tunability of the nonlinear photophysical properties of the retinal protonated Schiff-base chromophore can be used for the rational design of retinal-containing proteins with optimal photoresponse.