One-Electron Reactions in Biochemical Systems As Studied by Pulse Radiolysis

• Published in: The Journal of biological chemistry 1970-04, Vol.245 (8), p.1890-1894
• Main Authors: Land, E.J., Swallow, A.J.
• Format: Article
• Language: English
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1016/S0021-9258(18)63182-5

The maximum possible bimolecular rate of electron transfer to ubiquinone (coenzyme Q with six isoprenoid units in the side chain) is likely to be that from solvated electrons, now determined by pulse radiolysis to be 1.7 x 10 10 m -1 sec -1 . ·CH 2 O - radicals also reduce ubiquinone, with k = 2.0 x 10 9 m -1 sec -1 . The reduction product in both cases is the ubisemiquinone free radical anion, ε 445 nm = 7200 m -1 cm -1 . In acid solutions reduction is produced only by ·CH 2 OH radicals with k = 1.4 x 10 9 m -1 sec -1 . The reduction product is the neutral ubisemiquinone free radical, ε 420 nm = 3000 m -1 cm -1 . Neutral ubisemiquinone free radicals disproportionate with k = 4.8 x 10 7 m -1 sec -1 . In neutral solution ubiquinone is reduced both by solvated electrons and by ·CH 2 OH radicals: formation of the neutral ubisemiquinone is followed by deprotonation to yield the anion, with k = 1.0 x 10 4 sec -1 . The pK of the semiquinone free radical is 6.45 ± 0.15.