Photoresponse of the protonated Schiff-base retinal chromophore in the gas phase.

The fragmentation, initiated by photoexcitation as well as collisionally-induced excitation, of several retinal chromophores was studied in the gas phase. The chromophore in the protonated Schiff-base form (RPSB), essential for mammalian vision, shows a remarkably selective photoresponse. The selectivity of the gas-phase chromophore is triggered by a series of fast trans to cis isomerizations followed by a Diels-Alder cyclization with subsequent slow statistical fragmentation, leading to one specific fragment ion. The pattern of the final statistical fragmentation may be altered by chemical modifications of the chromophore. We propose that isomerizations play an important role in the photoresponse of gas-phase retinal chromophores and guide internal conversion through conical intersections. The role of protein interactions is then to control the specificity of the photoisomerization in the primary step of vision and possibly to diminish thermal noise by suppressing spontaneous isomerization by heat.