Retinal chromophore

Very recently first SERRS results about bacteriorhodopsin have been communicated by Nabiev et al. Bacteriorhodopsin is a membrane protein found in bacteria which functions as a light driven proton pump. Using the short-range mechanism of SERS (Chapt. 4.1) the active site (retinal chromophore) position of the protein in the membrane has been estimated with high accuracy It is interesting to note, that adsorption of bacteriorhodopsin on silver colloids seems to fix light-induced cyclic transformations in the protein active sites. 

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The first study in which a full CASSCE treatment was used for the non-adiabatic dynamics of a polyatomic system was a study on a model of the retinal chromophore [86]. The cis-trans photoisomerization of retinal is the primary event in vision, but despite much study the mechanism for this process is still unclear. The minimal model for retinal is l-cis-CjH NHj, which had been studied in an earlier quantum chemisti7 study [230]. There, it had been established that a conical intersection exists between the Si and So states with the cis-trans defining torsion angle at approximately a = 80° (cis is at 0°). Two... 

Natural Sensitizing Dyes and Photodynamic Therapy. The chlorophylls are, of course, among the natural sensitizers for photosynthesis. Considerable iaterest exists ia chlorophyll and related pigments as photosensitizers ia biology and medicine (75), isomeric retinal chromophores as visual pigments (76,77), and the use of synthetic photosensitizers ia neurobiology (9), hematology (78), and photodynamic therapy (79). 

FIGURE 10.22 The reaction cycle of bacteriorhodopsin. The intermediate states are indicated by letters, with subscripts to indicate the absorption maxima of the states. Also indicated for each state is the configuration of the retinal chromophore (all-tram or 13-cas) and the protonation state of the Schiff base (C=N or C=N H). 

FIGURE 10.21 The Schiff base linkage between the retinal chromophore and Lys" ... 

FIGURE 10.23 The folding of halorhodopsin with the transmembrane segments indicated. The only lysine residue in the protein is Lys , to which the retinal chromophore is covalently linked. 

The values of the 15N CP MAS chemical shift of Lys296 nitrogen bonded to retinal via the —C=N bond ( Schiff base) was equal to 155.4 ppm for rhodopsin and 282.8 ppm for metarhodopsin (relative to 5.6 M aqueous NH4C1).70 The results proved the imine bond polarisation, which facilitates Schiff base hydrolysis. The comparison between chemical shifts for metarhodopsin and model compounds suggested that Schiff base linkage of the all-frans retinal chromophore in Metall is in a polar environment. 

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. 

Significantly, the bio-inorganic and polymer-containing PM nanocomposites showed no significant shift in the protein amide I and II vibration bands, or in the characteristic 567 nm optical absorption band of the retinal chromophore of BR, indicating that the structural and dynamical properties of the membrane-bound... 

The initial act in the process of vision involves the photochemical cis-trans isomerisation of the 11-cis C=C bond of the retinal chromophore... 

The general strategy for establishing the sites of protein-chromophore interactions in rhodopsin involves introduction of selective 13 C labels at each position along the length of the retinal chromophore. 

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... 

The study of the mechanism of vision in vertebrates 23>24) has progressed to the point where the first consequence of photon absorption has been described as an activation of the isomerization of the 11 -cis retinal chromophore of rhodopsin to all-trans. That triggers a complex sequence of reactions leading to the mysterious inside of the brain. Brrr, I had better get back — it looks dark in there. But the brain can generate sensations of light. Maybe, one day, we will be able to see enough to understand, but we ll go back just the same to a safer subject. 

Kometani etal.(93) used a theory for energy transfer from a donor to acceptors in a plane to determine the location of the retinal chromophore relative to the membrane surface. Another similar study on the location of the active site of chloroplast ATPase relative to the membrane surface has also been carried out.(94)... 

Light striking the retina changes the color of rhodopsin from red to yellow. The primary photochemical event in this process was established by G. Wald (Nobel Laureate in Physiology and Medicine, 1967), who showed that light absorption led to a change of configuration about the C11-C12 double bond of the retinal chromophore from cis to trans ... 

M. Garavelli, T. Vreven, P. Celani, F. Bernardi, M. A. Robb, and M. Olivucci,/. Am. Chem. Soc., 120, 1285 (1998). Photoisomerization Path for a Realistic Retinal Chromophore Model The Nonatetraeniminium Cation. 

Zhukovsky, E. Robinson, P. Oprian, D. (1991) Transducin activation by rhodopsin without a covalent bond to the 11-cis-retinal chromophore Science, vol. 251, pp 558-559... 

Bacteriorhodopsin (bR) is a transmembrane protein located in the cell membrane of purple bacteria and contains in its ground state an all-trans retinal chromophore that absorbs at 570 nm. After illumination, the chromophore isomerizes, and a proton is pumped in five consecutive steps from the cytoplasm to the extracellular side of the membrane. The resulting pH gradient is then used to synthesize ATP. In the first proton-transfer step, the proton located at the retinal chromophore Schiff base is transferred to a nearby aspartate residue (Figure 14-6). Our studies on this first proton-transfer step in bacteriorhodopsin (bR) after photoisomerization [90,91,92]... 

Figure 14-6. The bR retinal binding pocket showing the NH group at die retinal chromophore, die two aspartates 85 and 212 and the three water molecules forming a strong hydrogen-bonded network...

Nielsen IB, Lammich L, Andersen LH (2006) SI and S2 excited states of gas-phase schiff-base retinal chromophores. Physical Review Letters 96 018304/1-018304/4. 

Andersen LH, Nielsen IB, Kristensen MB, El Ghazaly MOA, Haacke S, Nielsen MB, Petersen MA (2005) Absorption of schiff-base retinal chromophores in vacuo. Journal of the American Chemical Society 127 12347-12350. 

Post-ingestion precursor for RA-R agonist RA 11-cis-Retinal chromophore linked to protein opsin, in visual excitation isomerizes to all-fram-Retinal -George Wald (USA, retinal isomerization), Ragnar Granit (Finland/ Sweden) Haldan Hartline (USA) (Nobel Prize, Physiology/Medicine, 1967, vision)... 

The biochemistry that mediates photon absorption in the disk membranes and closing of the ionic channels in the plasma membrane is summarized in Fig. 2 and discussed in detail in reviews that provide access to the original literature (Helmreich and Hofmann, 1996 Menon et al, 2001 Hamm 1998, 2001 Bunemann and Hosey, 1999 Krupnick and Benovic, 1998). In the dark (inactive) state, rhodopsin (R) contains a covalently bound 11 -cis retinal chromophore buried within the protein core. In rhodopsin, the chromophore absorbs maximally around 500 nm, and absorption of a photon isomerizes the retinal to the all-trans form within about 200 fs (Peteanu et al, 1993) with a quantum efficiency of 0.67 (Dartnall, 1972). The change in chromophore shape creates strain, and subsequent thermal relaxation of both chromophore... 

The thermal factors reveal that the extracellular domain, consisting of the N-terminal sequence and the extracellular links E1-E3, is the most highly ordered region of the structure in the crystal (Fig. 4). Together with the extracellular ends of the TM helices, the extracellular domain forms the binding pocket for the 11-ds retinal chromophore, shown as a... 

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