Quinone is a lipid-soluble hydrogen (i.e., proton plus electron) carrier that mediates electron transfer between respiratory chain components. By definition, a quinone is a cyclohexadienedione. Those with carbonyl groups ortho to each other are called o-quinones; those with carbonyl groups para to each other are called p-quinones.
Quinones can also be obtained by oxidation of 1,2-benzenediol or 1,4-benzenediol.
Perhaps the most important chemical property of quinones is that they are readily reduced to benzenediols. For example, p-quinone is readily reduced to hydroquinone by sodium dithionote in neutral or alkaline solution.
In structure, the coenzyme Q group closely resembles the members of the vitamin K group and the tocopherylquinones, which are derived from tocopherols (vitamin E), in that they all possess a quinone ring attached to a long hydrocarbon tail.
The quinone pigments, of which there are at least 1200 structures range in color from yellow to almost black, but most are yellow, orange or red. Although they are widely, albeit sporadically, distribute in higher plants, they contribute little to plant color.
What is quinone?