Of residual protons in incompletely

Table 7.48 Solvent Positions of Residual Protons in Incompletely Deuterated...

Both in the case of sensory rhodopsin in humans and of bacteriorhodopsin (a heptahelical membrane protein in halobacteria which is not coupled to a G protein) translocation of a Schiff-base proton is the essential step in making the protein functional (reviewed in ref 58). In rhodopsin the conversion of the inactive AH state to the AHI state that binds to the G protein is coupled to proton transfer from the Schiff base to the counterion, Glul 13, and proton uptake from the medium to the highly conserved Glul34, which serves as proton acceptor. Based on that similarity, one could consider sensory rhodopsin as an incomplete proton pump. Furthermore, a property shared by all G-protein-coupled receptors is a triplet, formed by residues 134-136 in rhodopsin, consisting of Glu-Arg-Tyr. The consequences of mutational replacement of Glul34 supports the notion that the state of protonation of this amino add is crudal for activity, and that its protonation triggers the conformational transition of the receptor from the inactive to the active state. 

A number of factors complicate the experimental measurements of CSA. These include a distribution in the values of the CSA nomandom orientation of the chains in the sample inhomogeneous interactions, such as residual line broadening due to incomplete proton decoupling and carbon-carbon dipolar coupling and molecular motion [30]. 

The potassium (Z)-enediolate obtained from racemic benzoin and Potassium Hydride when treated with (2/ ,3/ )-DPTA affords (5)-benzoin with 80% ee (optically pure after one recrystallization in methanol). In the reaction condidons, the enediolate is first O-protonated, then the resulting enediol slowly tautomerizes at low temperature into optically active benzoin with a high enandose-lecdvity. In the cases of incomplete tautomerization the residual enediol was immediately oxidized to benzil by the oxygen of the air during the workup. It was shown that the tautomerization of the enediol was hindered by the presence of an excess of (2/ ,3/ )-DPTA (eq 6). ... 

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