Schiff base nitrogen

Proton translocation to the Schiff base nitrogen was proposed to occur by concerted double proton transfer (as shown in Fig. 15) leading to a retro retinal structure in the batho intermediate [127, 196], However, this model can be eliminated as it is inconsistent with the formation of batho intermediates from pigment analogs based on 5-desmethylretinal [145] and y-retroretinal [146,147], It also disagrees with the resonance Raman results. 

A mechanism involving deprotonation of the Schiff base nitrogen [197] as the primary event is now considered unlikely because of resonance Raman experiments and also because deprotonation would lead to blue shift rather than the observed red shift in bathorhodopsin. 

Fig. 20. Model for the primary event in vision. Isomerization of the 11.12 bond leads to charge separation at Ihe Schiff base site. This process, as shown, can possibly be followed by proton transfer, the latter resulting from the charge separation. In rhodopsin, the second negative charge responsible for wavelength regulation is shown close to the 11,12 bond of the polyene chain. This model assumes that hypsorhodopsin is the unprotonated form of the Schiff base, and that it is formed possibly by proton transfer from the Schiff base nitrogen in some pigments. From Honig ct al. [207].

Blatz et al. [211,220] proposed that the absorption maxima of pigments could be regulated by varying the distance between the protonated Schiff base nitrogen and... 

Proton translocation to the Schiff base nitrogen (214-216,301,308,309). Fransen et al. (308) and Van der Meer et al. (309) have suggested that the formation of BAT involves a concerted double proton transfer from the methyl group at position 5 to a protein heteroatom, and from a second heteroatom to the Schiff base nitrogen. 

The process entails shifting of double bonds along the polyene chain, with the formation of a "retro-retinal" structure. Peters et al. (301) interpreted their observations by identifying PBAT with an excited state of rhodopsin, where single proton transfer toward the Schiff base nitrogen leads to the formation of bathorhodopsin. This approach has been supported by the theoretical interpretation of the spectrum of rhodopsin in terms of a nonprotonated Schiff base (214-216). A mechanism involving deprotonation of the Schiff base has also been suggested (310). All these models do not require cis-trans isomerization as a primary event in the chromo-phore. 

Figure 32.23. Atomic Motion in Retinal. The Schiff-base nitrogen atom moves 5 A as a consequence of the light-induced isomerization of 11-cw-retinal to all-tra 5-retinal by rotation about the bond shown in red. 

The mechanism by which these enzymes facilitate decarboxylation and avoid transamination was originally proposed by Dunathan 107,108), who suggested that reactivity is controlled by the conformation of the single bond between the a-carbon of the amino acid and the Schiff base nitrogen (Scheme IX). Decarboxylation requires the conformation shown in Scheme IX, in which the carboxyl group is out of the plane of the aromatic system and the a-hydrogen and the alkyl group are on the other side of the plane. When decarboxylation occurs. 

There are several other results that also support the proton-transfer mechanism. The absorption spectrum of bathorhodopsin is red shifted with respect to the spectrum of rhodopsin itself, therefore, bathorhodopsin may be a more tightly protonated Schiff base than rhodopsin. Model studies by Sandorfy " show explicitly that translocating the proton toward the Schiff base nitrogen could account for the spectral red shift. 

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