Sensitizing pigments and their significance for vision

The primary process induced by absorption of a quantum of light in a visual pigment molecule is the isomerization of the chromophore retinaldehyde from the all-cis to the all-trans form (Wald 1968). I will show here that besides this direct interaction between light and visual pigment another process can take place: the light quantum can be absorbed by an accessory pigment which then transfers energy to the normal, Schiffbase linked chromophore of the visual pigment, which then will be isomerized. The photostable, accessory pigment therefore is acting as a sensitizing pigment. We have investigated the process of sensitization in some detail in the most common type of receptor of the fly (type Rl-6); we show, however, that sensitization of visual pigments is realized in other types of receptors of the fly and in many other insect species as well