Is proton transfer the initial photochemical process in vision?

Abstract

THERE has been renewed effort¹ to test the so-called proton transfer hypothesis (PTH)² or hydrogen shift¹ formulation for the initial photochemical step in vision. This hypothesis proposes an alternative to photochemical cis to trans isomerisation of rhodopsin, while attempting to account for some of the spectral properties of bathorhodopsin (previously called prelumirhodopsin), particularly certain low frequency lines at 920, 877 and 856 cm⁻¹. Spectral lines in this region are not unique to the resonance Raman spectrum of bathorhodopsin³, however, for similar spectral absorption has been found with 11-cis retinal crystals⁴. The discovery that the batho- intermediate formed by irradiating chicken iodopsin at −195 °C spontaneously reverted to iodopsin at −140 °C by a dark, thermal process⁵, rather than decaying to lumiiodopsin, also seemed to require such an alternative hypothesis² (Fig. 1). Although an attempt to demonstrate proton transfer by means of tritium incorporation into the retinylidene chromophore failed², Fransen et al. apparently now confirm the PTH by their report¹ of a small, though significant incorporation of deuterium into retinal. Their newer experiments are similar in design to the earlier ones, differing mainly in their use of deuterium rather than tritium and their method of analysis of labelled retinal. The experiments described here bear directly on the proposed PTH and were performed with an analogue of retinal, 5-desmethyl (dm) retinal: They show that the initial compound produced by irradiating a visual pigment, the batho-intermediate, cannot result from the transfer of a proton from the chromophore to opsin.