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Semiquinone (or ubisemiquinone) is a free radical resulting from the removal of one hydrogen atom with its electron during the process of dehydrogenation of a hydroquinone to quinone or alternatively the addition of a single H atom to a quinone.[1] It is highly unstable.

It is the first of two stages in reducing the supplementary form of CoQ10 ubiquinone to the active form ubiquinol.


  1. ^ Song, Y; Buettner, GR (Sep 15, 2010). "Thermodynamic and kinetic considerations for the reaction of semiquinone radicals to form superoxide and hydrogen peroxide.". Free radical biology & medicine 49 (6): 919–62. doi:10.1016/j.freeradbiomed.2010.05.009. PMC 2936108. PMID 20493944.