Abstract

Ab initio multiconfigurational quantum chemistry is employed to investigate the photochemical isomerization of distant retinal proteins with light-sensory functions. These are the microbial sensory rhodopsin from the cyanobacterium Anabaena sp. PCC 7120, the dim light vertebrate (bovine) and invertebrate (squid) visual photoreceptors and the human non-visual photoreceptor melanopsin, It is found that, in spite of sequence diversity and distinct double-bond selectivities, the isomerization invariably occurs via different implementations of the same space-saving mechanism first proposed by Warshel. In contrast, trajectory computations and transition state optimizations indicate, consistently with the observations, photoisomerization dynamics spanning one order of magnitude (i.e. in the 50-500 fs range) and thermal isomerization barriers decreasing proportionally with the absorption wavelength. Analysis of the excited state electronic structure progression shows that the microbial rhodopsin is “trapped” in a region of electronic degeneracy, which is avoided in the vertebrate rhodopsin.

© 2016 Optical Society of America