Multistate Multiconfiguration Quantum Chemical Computation of the Two-Photon Absorption Spectra of Bovine Rhodopsin

Recently, progress in IR sources has led to the discovery that humans can detect infrared (IR) light. This is hypothesized to be due to the two-photon absorption (TPA) events promoting the retina dim-light rod photoreceptor rhodopsin to the same excited state populated via one-photon absorption (OPA). Here, we combine quantum mechanics/molecular mechanics and extended multiconfiguration quasi-degenerate perturbation theory calculations to simulate the TPA spectrum of bovine rhodopsin (Rh) as a model for the human photoreceptor. The results show that the TPA spectrum of Rh has an intense S-0 -> S-1 band but shows also S-0 -> S-2 and S-0 -> S-3 transitions whose intensities, relative to the S-0 -> S-1 band, are significantly increased when compared to the corresponding bands of the OPA spectrum. In conclusion, we show that IR light in the 950 nm region can be perceived by rod photoreceptors, thus supporting the two-photon origin of the IR perception. We also found that the same photoreceptor can perceive red (i.e., close to 680 nm) light provided that TPA induces population of S-2.

近期，红外光源的进展导致了人类能够探测红外线这一发现。据推测，这是由于双光子吸收（TPA）事件促使视网膜弱光视杆光感受器视紫红质达到与单光子吸收（OPA）所激发的相同激发态。在此，我们结合量子力学/分子力学以及扩展的多组态准简并微扰理论计算，模拟牛视紫红质（Rh）的双光子吸收光谱，以此作为人类光感受器的模型。结果表明，Rh的双光子吸收光谱有一个强烈的S - 0 -> S - 1带，但也显示出S - 0 -> S - 2和S - 0 -> S - 3跃迁，与单光子吸收光谱的相应谱带相比，它们相对于S - 0 -> S - 1带的强度显著增加。总之，我们表明950纳米区域的红外线能够被视杆光感受器感知，从而支持了红外线感知的双光子起源。我们还发现，如果双光子吸收诱导S - 2态的布居，同一光感受器能够感知红色（即接近680纳米）光。