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. Author manuscript; available in PMC: 2011 Sep 15.
Published in final edited form as: Free Radic Biol Med. 2010 May 21;49(6):919–962. doi: 10.1016/j.freeradbiomed.2010.05.009

Fig. 1.

Fig. 1

Shown are the basic structures of quinones, semiquinones, and corresponding hydroquinones. We show here the dominant species of each component of the triad that would be present in aqueous solution at pH 7. For example, for 1,4-hydroquinone the pKa's are 9.8 and 11.4 for the first and second protons of the phenoxyl moieties, respectively [11,12]. The pKa of the protonated 1,4-semiquinone radical (SQH•) is 4.1 [13,14]; thus, it is shown as the radical anion.