Photochemistry of polyenes. 23. Application of the H.T.-n mechanism of photoisomerization to the photocycles of bacteriorhodopsin. A model study

Are C14-C15 single bond isomerizations of the retinal chromophore involved in the proton-pumping mechanism of bacteriorhodopsin?

“…One appealing feature of this model is that thermal 14-s-cis --14-s-trans isomerization could occur after deprotonation of the Schiff base and function as a "proton switch." An analogous model has been presented recently by Liu and coworkers (6). These suggestions have remained untested due to the lack of a method for determining C-C single bond conformation in retinal pigments.…”

Synthetic retinals as probes for the binding site and photoreactions in rhodopsins

“…Of special interest is the observation that K is formed even with the synthetic chromophore derived from aldehyde 10, which is constituted by a single C~C bond (analogous to Cj3=C~4). [Evidence showing that neither C~4-C~5 rotation (Schulten & Tavan, 1978;Tavan & Schulten, 1986;Liu, Mead & Asato, 1985) nor C~5=N isomerization participate in the primary event in bR has been provided by resonance-Raman studies. (For a review, see .)]…”

Bacteriorhodopsin's L550 intermediate contains a C14-C15 s-trans-retinal chromophore.

“…A number of models have been proposed to explain how isomerization and Schiff-base deprotonation are coupled to proton transport. Isomerization-driven charge separation is one widely accepted idea (2-4), and models involving a cis conformation at the single bond between C14 and C15 (14-s-cis) for the K and L550 intermediates have been proposed by Schulten and Tavan (5) and by Liu et al (6).…”

“…A number of models have been proposed to explain how isomerization and Schiff-base deprotonation are coupled to proton transport. Isomerization-driven charge separation is one widely accepted idea (2-4), and models involving a cis conformation at the single bond between C14 and C15 (14-s-cis) for the K and L550 intermediates have been proposed by Schulten and Tavan (5) and by Liu et al (6).Vibrational spectroscopy has been the primary tool for determining the structure of the retinal chromophore in BR's early photointermediates (7). In a recent study, Smith et al (8) examined the conformation of the C14-C15 bond in K and L550 by recognizing that one characteristic of an s-cis bond is the -100 cm -1 lowering of the C14-C15 stretching mode.…”

“…Isomerization-driven charge separation is one widely accepted idea (2)(3)(4), and models involving a cis conformation at the single bond between C14 and C15 (14-s-cis) for the K and L550 intermediates have been proposed by Schulten and Tavan (5) and by Liu et al. (6).…”