Reversible toggling of a natural photoswitch - application of ultrafast spectroscopy to Anabaena Sensory Rhodopsin (ASR)
Abstract:
Authors: A. Wand, R. Rozin, T. Eliash, M. Sheves, and S. Ruhman.
Photochemistry in retinal proteins is determined both by 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 lead to a 13-cis configuration. However, isolating these factors is difficult since most retinal proteins 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 photo-isomerization, as in visual pigments, is ballistic and over in a fraction of a picosecond, while the reverse is nearly ten times slower, and kinetically reminiscent of other microbial rhodopsins. Following this finding we reexamined primary events in 13-cis bacteriorhodopsin, and found that, contrary to an earlier study on this compound, the cis conformer reacts much faster than its all-trans counterpart as well. The implications of these findings with respect to the dichotomy of primary events in both families of retinal proteins will be discussed.
