"Retinal is an interesting molecule because it is the reason we are able to see. Or at least one of them. Retinal, or more correctly, 11-cis-retinal, is a small molecule which fits into the binding site of a large protein called opsin. Together they make up rhodopsin (also known as 'visual purple', the structure of which is shown below. This is where the terms 'rods' (think Rhodopsin) and 'cones' come from, referring to cells in the retina of our eyes which contain rhodosin and isodoposin pigments, respectively."
"Retinal comes in two forms, 11-cis- and all-trans. The -cis prefix comes from the fact that one of the double bonds (at the 11th carbon) has the two largest substituents (that is, the largest chains coming off it) on the same side. The other double bonds are all -trans, or with the bulky substituents positioned on opposite sides. A more modern nomenclature uses the letters E (from the German, entgegen; apart) and Z (from zusammen; together). The trans- (E) version is long and straight, whereas the cis- (Z) version is bent in two."
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