Dark-adapted bacteriorhodopsin

The 13-c/j retinal-chromophore in dark-adapted bacteriorhodopsin exhibits a very different photocycle, whose predominant intermediate has an absorption maximum at 610 nm [199], and which contains no intermediate [202,238] analogous to M. The 610 nm intermediate will decay to either the 13-c/s chromophore or the dW-trans form, the latter pathway being responsible for the phenomenon of light-adaptation [199]. This pathway does not explain, however, why monomeric bacteriorhodopsin shows poor light-adaptation [168,239]. The chromophore in the 13-c/s configuration is not associated with proton translocation [240]. Indeed, reconstitution of bacterio-opsin with 13-demethyl retinal, which traps the retinal moiety in the 13-c/s configuration, results [241] in a non-transporting photocycle. 

The purple membrane fragments that contained dark-adapted bacteriorhodopsin were used to form reconstituted vesicles with a mixture of phospholipids that contained 80% egg phosphatidyl choline and 20% bovine phosphatidylserine (Lipid Products, Nuttfield, England). The lipids in chloroform and methanol solutions were mixed to the desired composition, dried in a stream of nitrogen, placed in a 0.1-torr... 

Figure 2. Absorption spectra of retinal isomers and rhodopsins. [Retinal spectra (in hexane at room temperature) are reproduced from refs. 52, 168, and 174.] The spectra of rhodopsin and iso-rhodopsin (A > 350 nm) are for digitonin-solubilized preparations in aqueous glycerol mixtures at 4°K (ref. 287), and room temperature (A < 350 nm. ref. 6). (Those of light-adapted (BrML) and dark-adapted (BRjjgg) bacteriorhodopsin, both for aqueous7membrane suspensions at room temperature, are reproduced from refs. 259 and 377.

After a few minutes illumination ( light-adaptation ) bacteriorhodopsin immobilized in the purple membrane lattice contains 100% aW-trans retinal [73-78]. Light-adapted solubilized bacteriorhodopsin [79,80] and halorhodopsin [81-83], on the other hand, contain a mixture of about % aW-trans and /3 n-cii retinal. Dark-adaptation which takes minutes or hours, depending on conditions, results in thermally stable mixtures of /3 a -tmns and % 13-c/s chromophores in all cases. The dark-adapted 13-cis chromophores are stable because the overall shape of retinal is not very different from that of the a -trans chain, the C=N bond having assumed the syn rather than the anti configuration [84,85]. 

One point that has been used to argue against the isomerization model in bacteriorhodopsin is that in rhodopsin the isomerization is cis->trans while in BrJ it would have to be trans->cis (presumably 13-cis). Since both lead to a red shifted photoproduct and since the thermal trans->13-cis process associated with converting light adapted (BR ) to dark adapted (50% BR cis bacteriorhodop leads to a blue shift, an apparent contradiction is involved. Ho ever, we have recently shown that photochemical isomerization. 

Logunov, I., Schulten, K. (1996). Quantum chemistry Molecular dynamics study of the dark-adaptation process in bacteriorhodopsin. Journal of the American Chemical Society, 118(40), 9727-9735. 

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