Asymmetric toggling of a natural photoswitch: ultrafast spectroscopy of Anabaena sensory rhodopsin.

Photochemistry in retinal proteins (RPs) is determined both by the properties of the retinal chromophore and by its interactions with the surrounding protein. The initial retinal configuration, and the isomerization coordinates active in any specific protein, must be important factors influencing the course of photochemistry. This is illustrated by the vast differences between the photoisomerization dynamics in visual pigments which start 11-cis and end all-trans, and those observed in microbial ion pumps and sensory rhodopsins which start all-trans and end in a 13-cis configuration. However, isolating these factors is difficult since most RPs accommodate only one active stable ground-state configuration. Anabaena sensory rhodopsin, allegedly functioning in cyanobacteria as a wavelength sensor, exists in two stable photoswitchable forms, containing all-trans and 13-cis retinal isomers, at a wavelength-dependent ratio. Using femtosecond spectroscopy, and aided by extraction of coherent vibrational signatures, we show that cis-to-trans photoisomerization, as in visual pigments, is ballistic and over in a fraction of a picosecond, while the reverse is nearly 10 times slower and kinetically reminiscent of other microbial rhodopsins. This provides a new test case for appreciating medium effects on primary events in RPs.