Retinal Schiff base

Using two-dimensional NMR spectroscopy, the spatial location of various carboxylate anions relative to the polyene chain of the protonated Schiff base of all-fraws-retinal was determined. The observed intermolecular NOE cross-peaks between a proton on the counterion and a proton near the nitrogen atom indicate the existence of ion-pair formation between the protonated retinal Schiff base and various counterions in chloroform. The results suggest that the most likely site of the carboxylate group of the counterion is in the immediate vicinity of the positively charged nitrogen atom of the retinal Schiff base. 

The retinal Schiff base chromophore is embedded in rhodopsin with its transition dipole moment parallel to the plane of the discs, i.e., perpendicular to the direction of travel of the incoming photons. Absorption of a photon leads to a sequence of readily detectable spectral changes.37,46113,499,500 The relaxation times indicated in Eq. 23-37 are for 20°C. 

The reaction sequence of Eq. 23-37 can be slowed by lowering the temperature. Thus, at 70K illumination of rhodopsin leads to a photostationary state in which only rhodopsin, bathorhodopsin, and a third form, isorhodopsin, are present in a constant ratio.510 Isorhodopsin (maximum absorption at 483 nm)513 contains 9-ds-retinal and is not on the pathway of Eq. 23-37. Resonance Raman spectroscopy at low temperature supports a distorted all-frans structure for the retinal Schiff base in bathorhodopsin.510 The same technique suggests the trans geometry of the C = N bond shown in Eqs. 23-38 and 23-39. Simple Schiff bases of 11-cz s-retinal undergo isomerization just as rapidly as does rhodopsin.514... 

Some aspects of a possible mechanism for pumping the single proton bound to the retinal Schiff base are included in Fig. 23-46. In bR568 the Schiff base... 

Figure 23-45 (A) Some aspects of the structure of bacteriorhodopsin. Ribbon diagram with the retinal Schiff base in ball-and-stick representation. At the top the helices are labeled as in Fig. 23-41. The locations of aspartate, glutamate, and arginine residues that might carry protons during the proton pumping action are indicated. Retinal is shown attached to lysine 216.

The chromophores of rhodopsin and bacteriorhodopsin are 11 -cis- and all-trans-retinal Schiff bases, respectively. Upon binding to the proteins, their unsaturated carbons show anomalous 13C chemical shifts compared with those of corresponding model compounds. This indicates the occurrence of interactions between the chromophore and its surrounding protein matrix. Ab inito shielding calculation reveals that the major part of such anomalous shifts originates in the conformational change of the chromophore. 

At present, we know the 13C chemical shift values for all the unsaturated carbons of the chromophores of Rh and bR before illumination. Figure 2 shows the chemical shift differences between the chromophores and their model compounds (free protonated retinal Schiff bases, hereafter abbreviated as PRSB). As for Rh, the chemical shifts of the carbons from C8 to C13 show significant downfield displacements relative to those for the model. As for bR, the chemical shifts of C5 and C8 are displaced significantly to downfield and upfield, respectively. These anomalous displacements suggest the occurrence of interactions between the chromophore and the surrounding protein matrix, including the resultant conformational change of the chromophore. 

Fig. 15. Structure of the prosthetic group in (a) rhodopsin, where the retinal Schiff base is in an 1 l-co,6-j-cir-l2-s-ew conformation (b) delocalized excited singlet state (c) bathorhodopsin. where the chromophore is a hexaene amine-imidazole complex. From van der Meer et al. [127],...

Figure 7. Correlation between the ethylenic (C=C) stretching frequency and the main absorption maximum of the retinyl moiety in a variety of pigments and in free retinal Schiff bases in solution. (Data, based on resonance-Raman experiments at room temperature, from refs. 221, 225, 226, and 323.)...

Retinal Schiff bases and visual pigments have also been studied by i.r. spectroscopy/ ... 

Schobert B, Brown LS, Lanyi JK. Crystallographic structures of the M and N intermediates of bacteriorhodopsin assembly of a hydrogen-bonded chain of water molecules between Asp96 and the retinal Schiff base. J. Mol. Biol. 2003 330 553-570. 

The isomerization of the retinal Schiff base takes place within a few picoseconds of a photon being absorbed. The initial product, termed bathorhodopsin, contains a strained all-tran5 -retinal group. Within approximately 1 millisecond, this intermediate is converted through several additional intermediates into metarhodopsin II. In metarhodopsin II, the Schiff base is deprotonated and the opsin protein has undergone significant reorganization. 

In the case of the retinal Schiff base (6) the efficiency of cis-trans isomerization of the double bond between C-II and C-12 is considerably enhanced by polar solvents on the one hand and by protonation of the Schiff base on the other hand (Becker and Freedman, 1985). This is rather important because 6 is the chromophore of rhodopsin and this isomerization represents one of the primary steps in vision. 

Figure 14 also shows the amino acid residues playing a key role in the proton transport Asp 85, Asp 212, and Tyr 185 with negatively charged terminal groups that are counterions with respect to the positively charged protonated Schiff base Asp 85 and Asp 96 are inner proton acceptor and donor, respectively Arg 82 favors the deprotonation of the retinal Schiff base. The data... 

The light-induced isomerization of the retinal shifts the protonated Schiff base into a new environment. A subsequent charge separation triggers the rearrangement of the active site, resulting in determination of the connection of the proton with the retinal Schiff base down to the critical value when the Schiff s base proton is transferred to Asp 85. 

Hendrickx et al. [41 2] have reported the first hyperpolarizabilities of retinal, retinal Schiff base and retinal protonated Schiff base at 1064 nm excitation wavelength. Retinal protonated Schiff base is responsible for the linear and NLO properties of bacteriorhodopsin protein. Their measured hyperpolarizabilities are 3600 X 10 ° esu for retinal protonated Schiff base and 470 x IO" esu for retinal Schiff base. They also investigated theoretical understanding of the first hyperpolarizabilities of retinal derivatives. Results are shown in Table 5. 

Schmalzlin et al. [90] have used the HRS technique to determine the molecular first hyperpolarizabilities (3 values of retinal Schiff base in its protonated and unprotonated form. Results of their HRS measurements performed at 1064,1300 and ISOOnm were reported. The derived hyperpolarizabilities are self-consistent with the two-state model for all three wavelengths, but they are an order of magnitude lower than those reported by Hendrickx et al. 

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