Photosensitized Retinal Isomerization in Rhodopsin Mediated by a Triplet State

Abstract

AbstractDirect 11‐cis to all‐trans retinal photoisomerization within rhodopsin is well known to be the initial chemical reaction triggering the process of vision in mammalians, such as bovine. Nevertheless, deep‐sea fish are known to use chlorophyll derivatives as photosensitizers in order to see deep‐red light, at wavelengths where retinal does not absorb. Also, some photodynamic therapy treatments were shown to enhance the vision of patients in dim light conditions. Energy transfer from the photosensitizer to rhodopsin was therefore proposed as a mechanism to populate the triplet state of the retinal chromophore. Herein, by means of hybrid quantum mechanics‐coupled‐molecular mechanics modeling techniques, we give insights into the possible energy mechanism and describe the retinal isomerization mediated by the lowest‐lying triplet state. Especially, we show how a few kcal/mol energy barrier separates a T1 minimum from a S0/T1 intersection region, hence proposing an equilibrium between phosphorescence and isomerization processes. Moreover, the eventual self‐production of singlet oxygen, constituting a potential danger for the integrity of rhodopsin, is discussed.