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 α-hydroxybenzyl-type radical, with k = (17 ± 1) s-1 at pH ≤ 5, as determined by time-resolved conductance. The α-hydroxybenzyl-type radical was also produced by reduction of veratryl aldehyde with the hydrated electron, and the pKa 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 × 109 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 × 107 M -1 s-1. With the veratryl alcohol derivative, 3,4-(MeO)2C6H3CH(OH)CMe3, the radical cation undergoes both a proton loss from the benzylic position and a Cα−Cβ fragmentation with the ratio, at pH ≤ 5, of ca. 1:2. The decay of the radical cation is strongly enhanced by OH- (k = 8.3 × 109 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.