Studies on rhodopsin. 9. pH and the hydrolysis of indicator yellow

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Abstract
The stability of retinylidenemethylamine, an indicator yellow analogue, was studied in aqueous solutions over a range of pH values. It was concluded that retinylidenemethylammonium ions are stable, but that uncharged retinylidenemethylamine molecules hydrolyze to retinene. This hydrolysis was prevented by the presence of excess methylamine, but not by methylammonium ions. These findings explained the influence of pH on the stability of indicator yellow solutions obtained from eyes. On adding acid to rhodopsin solutions in the dark, stable acid indicator yellow solutions could be formed, with the retinene residue attached to the same nitrogen atom as it is in rhodopsin. Essentially all the retinene residues in the rhodopsin chromophore were shown to be attached to a N atom of opsin. The status of the different types of indicator yellow formed from rhodopsin solutions is assessed.

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