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( Retinal)
63 °C The molecule that takes part in the initial step in the vision process, rhodopsin, has two components called 11-cis retinal and opsin. Retinal is a light-sensitive derivative of vitamin A, and opsin is a protein molecule. Rhodopsin is found in the rod cells of the eye. 11-cis retinal is a powerful absorber of light because it is a polyene; its 6 alternating single and double bonds make up a long conjugated electron network. When no light is present, the 11-cis retinal molecule is found in a "bent (cis) configuration" (fig A), and as such it is attached to the opsin molecule in a stable arrangement When light strikes the retina, a retinal molecule may absorb a photon, promoting it into an excited electronic state. The nature of the excited state is not well understood, but it is known that within 200 femtoseconds it returns to the ground electronic state.[1] One third of these events cause no net change, while the remaining two thirds induce a rotation in the pi bond found between the eleventh and twelfth carbon atoms. In other words, the 11-cis retinal is transformed into the all-trans retinal (fig B) in a straightened configuration.[2] The all-trans retinal configuration, subsequently, does not fit into the binding site of the opsin molecule in the same manner as the 11-cis configuration; as a result, upon isomerization, the trans isomer causes a conformational change in the protein, which triggers a G protein signaling pathway' including transducin, that results in the generation of an electrical impulse, which is transmitted through the optic nerve to the brain for processing. Trans retinal is eventually removed from opsin, and then recycled back to the 11-cis isomer for subsequent use.
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