Modelling retinal chromophores photoisomerization: from minimal models in vacuo to ultimate bidimensional spectroscopy in rhodopsins

Retinal chromophores are the photoactive molecular units of visual and archaeal rhodopsins, an important class of light-activated biological photoreceptors. Extensive computational studies aimed to reveal the intrinsic photophysical and photochemical behavior of retinals in vacuo and the environmental effects that tune their properties in proteins and in solution are reviewed. Multiconfigurational and multireference perturbative ab initio methods have been used to study retinal models with increasing size, from minimal to unreduced models. The hybrid quantum mechanics/molecular mechanics (QM/MM) approach has been employed for modeling retinals in solution and in proteins. QM/MM studies of the retinal photoisomerization in rhodopsin, a prototype opsin protein responsible for the peripheral vision, have provided fundamental understanding of the electrostatic effects regulating the spectral tuning in proteins and have elucidated the photoisomerization mechanism with atomistic details, consistently with ultrafast optical spectroscopy experiments with sub-20 fs resolution. Different photochemical behaviors are observed for retinals in proteins and in solution. A molecular mechanism involving the interplay between ionic and covalent states during photoisomerization has been hypothesized but it remains uncertain and direct experimental evidences are lacking. Here, we propose transient bidimensional electronic spectroscopy as a conceivable tool for obtaining key information on the retinal photoisomerization in different environments. Combination of computational techniques and ultimate ultrafast spectroscopy experiments could provide fundamental insights on retinals photochemistry and basic understanding for the design of biomimetic photochromic devices.