Photoactivation

In 1958, George Wald and co-workers discovered
that light isomerizes 11-cis-retinal (1)
very rapidly into all-trans-retinal (2), a form
that practically does not exist in the dark
(!1 molecule/1000 years). The light-induced
structural change is so great that the resulting
atomic motion can trigger a reliable and reproducible
nerve impulse. The absorption spectrum
of rhodopsin (3) corresponds to the spectrum
of sunlight, with an optimum at a
wavelength of 500 nm. Although vertebrates,
arthropods, and mollusks have anatomically
quite different types of eyes, all three phyla use
11-cis-retinal for photoactivation.