= Photoisomerization
\b[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.<Crystal Structure of Rhodopsin: A G Protein-Coupled Receptor by Krzysztof Palczewski and others>[Introduction: retinal photoisomerization and receptor activation; dark-state retinal-binding structure.]{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 $\Phi=N_{\mathrm{product}}/N_{\mathrm{absorbed\ photons}}$. 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.
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