Electronic coulombic coupling of excitation-energy transfer in xanthorhodopsin.

Electronic coupling of excitation-energy transfer (EET) in a retinal (RET) protein, xanthorhodopsin (xR), was studied theoretically. The protein, functioning as a light driven proton pump, contains a carotenoid antenna, salinixanthin (SXN), to collect light energy for an RET chromophore through EET. The pseudo-Coulombic interaction (PCI) between the donor SXN and the acceptor RET molecules was calculated by a transition density fragment interaction (TDFI) method, which overcomes difficulty arising in the evaluation of PCI in xR by a conventional dipole-dipole (dd) method, at the ab initio TDDFT/SAC-CI level of theory. The result nicely agrees with the experimentally observed PCI. To examine the correlation between the SXN-RET alignment and the EET efficiency, we computed PCIs for SXN conformations that are virtually generated around the protein. The calculation shows that the optimal SXN alignment for the maximally tuned efficiency of EET is attained in the native xR. PCI in another retinal protein, archaerhodopsin-2, which also binds a carotenoid but lacks EET activity, was also evaluated. The computed PCI is negligibly small, well explaining the lack of EET efficiency.