Retinal to Retinal Energy Transfer in a Bistable Microbial Rhodopsin Dimer

Neorhodopsin (NeoR) is a newly discovered fungal bistable rhodopsin that reversibly photoswitches between UV- and near-IR absorbing states denoted NeoR₃₆₇ and NeoR₆₉₀, respectively. NeoR₃₆₇ represents a deprotonated retinal Schiff base (RSB), while NeoR₆₉₀ represents a protonated RSB. Cryo-EM studies indicate that NeoR forms homodimers with 29 Å center-to-center distance between the retinal chromophores. UV excitation of NeoR₃₆₇ takes place to an optically allowed S3 state of 1B_u⁺ symmetry, which rapidly converts to a low-lying optically forbidden S1 state of 2A_g^- symmetry in 39 fs, followed by a multiexponential decay to the ground state on the 1-100 ps time scale. A theoretically predicted nπ* (S2) state does not get populated in any appreciable transient concentration during the excited-state relaxation cascade. We observe an intradimer retinal to retinal excitation energy transfer (EET) process from the NeoR₃₆₇ S1 state to NeoR₆₉₀, in competition with photoproduct formation. To quantitatively assess the EET mechanism and rate, we experimentally addressed and modeled the EET process under varying NeoR₃₆₇-NeoR₆₉₀ photoequilibrium conditions and determined the EET rate at (200 ps)^-1. The NeoR₃₆₇ S1 state shows a weak stimulated emission band in the near-IR around 700 nm, which may result from mixing with an intramolecular charge-transfer (ICT) state, enhancing the transition dipole moment of the S1-S0 transition and possibly facilitating the EET process. We suggest that EET may bear general relevance to the function of bistable multiwavelength rhodopsin oligomers.