Photoisomerization

Photoisomerization is a light-induced conversion between molecular isomers: absorption of a photon initiates a change in molecular arrangement without requiring a change in elemental composition. The resulting isomers can have different shapes, spectra or interactions even though they contain the same kinds and numbers of atoms.

A biological example occurs in rhodopsin. Its bound 11-cis-retinal chromophore changes toward all-trans-retinal after light absorption, initiating changes in the receptor that can activate transducin. This converts a molecular rearrangement into a signaling event.^[ref]

The change is an example of cis-trans isomerism, not fragmentation of retinal into smaller molecules. “Light-induced” also does not mean that every photon entering a sample produces one permanent isomerization. Absorption, competing excited-state processes and subsequent reactions can affect the measured outcome.

An idealized photochemical quantum yield for a specified primary product is $\Phi =N_{\mathrm{product}}/N_{\mathrm{absorbed}\mathrm{photons}}$. If 100 photons are absorbed and 30 product-forming events are counted under a defined low-conversion assay, $\Phi =0.30$. Counting incident rather than absorbed photons would answer a different question. The example is bookkeeping, not a measured rhodopsin yield.

The surrounding protein can influence the chromophore's behavior by controlling its local environment. A free molecule's light response therefore need not be identical to its behavior inside a receptor. The useful mechanism is the full chain from absorption to rearrangement to altered interaction with other molecules.