Retinal protonated Schiff base chromophore

Fig. 2 Tunability of the photochemical and photophysical properties of the retinal-protonated Schiff base chromophore in solution. Reprinted with permission from G. Bassolino, T. Sovdat, M. Liebel, C. Schnedermann, B. Odell, T. D. W. Claridge, P. Kukura and S. P. Fletcher, J. Am. Chem. Soc., 2014,136, 2650. Copyright 2014, American Chemical Society.

Fig. 6 Comparison of retinal protonated Schiff base chemical shifts in rhodopsin different photocycle states. The chemical structure of retinal chromophore in 11-cw (a) and all-trans configuration (b). Chemical shifts of the retinal atoms in ground state, and Batho-, Meta I, and Meta II intermediate states (c) and a schematic drawing of the retinal binding pocket containing all residues within 4 A to the retinal and Lys296 (d). The chemical shifts are adapted from the following references [146-148] (black) [14] (blue) [149-151] (green) and [17, 145] (red), (d) is adapted from [188] with permission from the Elsevier B.V...

The CP MAS NMR spectroscopy has been also extensively used for studies of proteins containing retinylidene chromophore like proteorhodopsin or bacteriorhodopsin. Bacteriorhodopsin is a protein component of purple membrane of Halobacterium salinarium.71 7 This protein contains 248 amino acids residues, forming a 7-helix bundle and a retinal chromophore covalently bound to Lys-216 via a Schiff base linkage. It is a light-driven proton pump that translocates protons from the inside to the outside of the cell. After photoisomerization of retinal, the reaction cycle is described by several intermediate states (J, K, L, M, N, O). Between L and M intermediate states, a proton transfer takes place from the protonated Schiff base to the anionic Asp85 at the central part of the protein. In the M and/or N intermediate states, the global conformational changes of the protein backbone take place. 

The chromophore, 11-ris-retinal, is linked to the a-amino group of lysine-296 in helix 7 via a protonated Schiff base. Protonated Schiff bases usually absorb light maximally at around 440 nm, but the of rhodopsin is near 500 nm. 

FIGURE 46. Retinal chromophore in bR is attached via a protonated Schiff base to Lys-216 on helix G and is tilted toward the extracellular side. To determine its detailed structure, retinal was selectively deuteriated on the three methyl groups on the cyclohexene ring and incorporated into bR from H. Halobium. Reprinted with permission from Reference 60. Copyright (1997) American Chemical Society... 

To begin to elucidate such issues and to create a theoretical framework for them, we have focused [4-9] on a model of a protonated Schiff base (PSB) in a nonequilibrium dielectric continuum solvent. A key feature for the Sj-Sq Cl in PSBs such as retinal which plays a key role in the chromophore s cis-trans isomerization is that a charge transfer is involved, implying a strong electrostatic coupling to a polar and polarizable environment. In particular, there is translocation of a positive charge [92], discussed further below. Charge transfer also characterizes the earliest events in the photoactive yellow protein photocycle, for example [93],... 

This was considered to affect the absorption spectra of the bound chromophore [216], This model, based on the absorption spectra of protonated Schiff bases formed from retinal and substituted anilines is, however, unlikely to have any realistic counterpart in the protein. 

Data is also shown for bovine rhodopsin, 63, for comparison. The hydroretinals 66-68 presumably assume 9-cis or 11 -cis like conformations when bound to opsin. Retinals 67 and 68 form non-bleachable pigments, i.e., no change in their A, occurs upon exposure to room light irradiation by UV light leads to decomposition products instead of separation of the chromophore from opsin. a In MeOH. In case of split chromophores the absorption maxima of the enal moieties are given. b Protonated Schiff base with n-butylamine in MeOH. ... 

In the inactive, ground state, the chromophore, 11-cis-retinal, is bound via a protonated Schiff base to a lysine (Lys296) in the centre of the seventh transmembrane helical stretch. The inactive conformation is stabilized by a salt bridge between the protonated Schiff base and the carboxyl side-chain of Glul 13 (Fig. 5.9). 

A complex multi-chromophoric system comprises the purple membrane patches from Halobacterium salinarium. These patches are composed of about 3000 bacter-iorhodopsin proteins. The hyperpolarizability of solubilized monomeric bacterio-rhodopsin was measured by HRS and found to be 2100 x 10 esu at 1064 nm. This high value is due to the presence of a chromophore in the protein, the proto-nated Schiff base of retinal. A purple membrane patch can be treated as a two-dimensional crystal of bacteriorhodopsin proteins, and its structure is known in considerable detail. The analysis of the purple membrane tensor was performed by adding the hyperpolarizabilities of the individual proteins in the purple membrane. From (depolarized) HRS measurements on purple membrane suspensions, the structure of the purple membrane patches, and an average membrane size measured by atomic force microscopy, a fi value of 2200 x 10 esu was calculated for bacteriorhodopsin [22]. The organization of the dipolar protonated Schiff base chro-mophores in the membranes was found to be predominantly octopolar. 

About half of rhodopsln s mass forms seven a-hellces, which are embedded In the lipid bllayer of rod disks. The remaining polypeptide chains extend Into the aqueous environment of the cytoplasm or the disk Interior, linking the helices. Retinal Is bound as a protonated Schiff base to a lysine amino acid residue In the carboxyl terminal helix. The chromophore is held In a pocket that is nearly parallel to the membrane surface. When light strikes rhodopsln, the 11-cis double bond of the protein-bound retinal Isomerlzes to the trans form, which leads to the separation from the protein opsin. To complete the visual cycle, the all-transretlnal slowly Isomerlzes back to the 11-cis Isomer, which recombines with opsin to reform rhodopsln. However, little Is known about how the Isomerization of retinal In rhodopsln triggers the transduction process (72,73) ... 

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