Lifetime, reduction potential and base-induced fragmentation of the veratryl alcohol radical cation in aqueous solution. Pulse radiolysis studies on a ligninase "mediator"

The radical cation of veratryl alcohol (3,4-dimethoxybenzyl alcohol), VA(.+), was produced in aqueous solution, mainly by oxidation with the radiation chemically generated SO4.- or Tl2+. By electron-transfer equilibration with thioanisole as a redox standard, the reduction potential of VA(.+) was determined to be 1.36 +/- 0.01 V/NHE. On the basis of product analysis results, the radical cation undergoes a C-H deprotonation reaction from the side chain, leading to an alpha-hydroxybenzyl-type radical, with k = (17 +/- 1) s(-1) at pH less than or equal to 5, as determined by time-resolved conductance. The alpha-hydroxybenzyl-type radical was also produced by reduction of veratryl aldehyde with the hydrated electron, and the pK(a) value of this radical was determined to be 10.0. The deprotonation of VA(.+) is enhanced by bases such as OH-, with the rate constant being 1.3 x 10(9) M-1 s(-1) In contrast, the corresponding rate constant for reaction of OH- with the radical cation of veratryl alcohol methyl ether; whose reduction potential is also 1.36 V/NHE, is only 2 x 10(7) M-1 s(-1) With the veratryl alcohol derivative, 3,4-(MeO)(2)C6H3CH(OH)CMe3, the radical cation undergoes both a proton loss from the benzylic position and a C-alpha-C-beta fragmentation with the ratio, at pH less than or equal to 5, of ca. 1:2. The decay of the radical cation is strongly enhanced by OH- (k = 8.3 x 10(9) M-1 s(-1)), with the base induction favoring the C-C fragmentation relative to the proton loss from the benzylic position. The possible bearing of these results with respect to the role of VA in the lignin peroxidase-catalyzed decomposition of lignin is discussed.