Optical measurements of the rapid pH-change in the visual process during the metarhodopsin I-II reaction.

Fol lowing the demonstration of distinct intermediates in the thermal decay of rhodopsin, characterizable by their different spectra (Summary : WALD 2 6 ) , the main question was how to interprete these spectral shifts in chemical terms and to elucidate the molecular mechanism of visual excitation. The findings of MATTHEWS et a l . 1 8 constituted a m a j o r step in this direction. They found that in the whole scale of intermediates in rhodopsin photolysis there exists one reaction which runs down just within the latent period of the receptor and which is connected with a very remarkable spectral change. This transition f r o m metarhodopsin I to metarhodopsin II is widely accepted as being the essential step initiating visual excitation (Summaries : 1. c . 5 ) . Furthermore, this reaction is coupled with a rapid pH-change, which was first observed by WALD'S g r o u p 1 8 ' 2 3 and repeatedly conf irmed b y other groups later on 1. c . n ' 2 0 ' 2 1 .