First step in vision: proton transfer or isomerization?

Recent views on the photochemistry of (vertebrate) vision are examined. Problems related to the structure of the proton bridge at the Schiff base chromophore are considered, in particular the possible shapes of the section of the potential surface governing the motion of the proton in the bridge. This leads to the question as to whether proton translocation occurs in the initial step of visual transduction and if it precedes or follows cis-trans isomerisation. The related controversy could be solved through the assumption of the presence of water molecules that stabilize the ions in the proton bridge. The causes of the instability of bathorhodopsin are discussed and the importance of additional perturbations by polar groups is stressed.