Protein‐Controlled Ultrafast Photoisomerization in Rhodopsin and Bacteriorhodopsin

A light-driven sodium ion pump in marine bacteria

“…Therefore, light energy is captured by the positively charged retinal chromophore. In case of BR and PR, transient drop of the RSB pKa by retinal photoisomerization causes the proton transfer to the extracellular side, and reprotonation from the cytoplasmic side leads to the outward proton pump 3,4,22 . In case of HR, chloride binding stabilizes the protonated RSB, and retinal photoisomerization alters the interaction of the ion pair, leading to the inward chloride pump 23,24 .…”

“…All microbial rhodopsins possess an all-trans retinal as the chromophore, which binds to a lysine residue through a protonated retinal Schiff base (RSB) linkage ( Supplementary Fig. S1) 3,4,22 . Therefore, light energy is captured by the positively charged retinal chromophore.…”

Predistortion amplified in the excited state

“…2a), whereas CC bond lengths rapidly change. By contrast, if it is embedded in the protein opsin (bacteriorhodopsin in the case of all-trans-retinal, rhodopsin in the case of 11-cis-retinal), it is not planar anymore: the torsion of the double bond to isomerize is %8-13 [13,14] (and even %23-38 in its primary photoproduct bathorhodopsin [12,14], which can be photochemically reverted to rhodopsin [15]). According to Fig.…”

“…According to Fig. 2b, vertical excitation therefore causes a torsional acceleration already in the Franck-Condon region (other bonds are also twisted in the matrix: the adjacent single bonds clearly more, but other double bonds less [12,14,15]. During the ultrafast isomerization, their angles vary, too, so that the total rearrangement is relatively space-saving [13][14][15].…”

“…A recent review of the photochemistry of rhodopsins and their chromophores (protonated Schiff bases (PSB) of retinal) can be found in [12]. In solution these chromophores are planar.…”