One-electron reduction of juglone (5-hydroxy-1,4-naphthoquinone): A pulse radiolysis study

Optical absorption characteristics have been determined for juglone (pKsub(a) = 8.85) and the semiquinone radicals formed from it by one-electron reduction. The latter have pKsub(a) values of 3.65 and > 12. Second-order rate constants have been determined for the reactions of e-sub(aq), CO2- H radical and O2- with juglone, and of the semiquinone radicals with themselves (disproportionation) and with O2. Using dimethylbenzoquinone and O2 as redox standards, one-electron reduction potential values have been determined over the pH range 6-10, and shown to match with theoretical values calculated from E71 = 93 mV (vs NHE) and pKsub(a) values of the parent and its semiquinones. A comparison is made with the semiquinones of other naphthoquinone derivatives including naphthazarin. Optical absorption spectra of fully reduced juglone have been determined. (author)