Studies on the visual pigment and the indicator yellow analogues formed from 5,6-monoepoxyretinal

Abstract

A new visual pigment has been isolated from retinae of rats maintained on 5,6-mono-epoxyretinal for a period of 6 months. Indicator yellow analogues from 5,6-monoepoxyretinal, namely 5,6-monoepoxyretinylidenemethylamine and 5,6-monoepoxyretinal oxime, have been prepared in a homogenous state, the latter being obtained crystalline. 5,6-Monoepoxyretinylidene-dimethylammonium iodide has also been prepared. The spectroscopic properties, analytical data and antimony trichloride color reactions of these indicator yellow analogues confirm their Schiff base structure. The nature of chromogens formed from 5,6-monoepoxyretinylideneamino compounds with antimony trichloride and concentrated sulphuric acid is obscure although the chromogens resemble one another very closely. 5,6-Monoepoxyretinylidenemethylidenemethylamine and 5,6-monoepoxy-retinylidenedimethylammonium iodide are very labile, whereas 5,6-monoepoxyretinal oxime is quite stable. Hydrolysis of 5,6-monoepoxy-retinylidenemethylamine as a function of pH reveals that, though 5,6-monoepoxyretinyl-idenemethylammonium ions are stable, the un-ionized compound readily hydrolyses to 5,6-monoepoxyretinal. This hydrolysis can be prevented by exccess of un-ionized methylamine and hastened by formaldehyde. The bathochromic shift in the [lambda]max. of the "acid form" of 5,6-monoepoxyretinylidenemethylamine is due to the formation of its ammonium salt. The spectrum of the "acid form" is identical with that of 5,6-monoepoxyretinylidenedimethylammonium iodide in ethanol. This is also evidence for the quaternary state of the nitrogen atom during the "acid shift" of 5,6-monoepoxyretinylidenemethylamine. The significance of 5,6-monoepoxyretinal and the corresponding indicator yellow analogues in the formation and properties of a new visual pigment is discussed.