Lifetime, Reduction Potential and Base-Induced Fragmentation of the Veratryl Alcohol Radical Cation in Aqueous Solution. Pulse Radiolysis Studies on a Ligninase “Mediator”
1998-09-02T00:00:00Z (GMT) by
The radical cation of veratryl alcohol (3,4-dimethoxybenzyl alcohol), VA<sup>•+</sup>, was produced in aqueous solution, mainly by oxidation with the radiation chemically generated SO<sub>4</sub><sup>•-</sup> or Tl<sup>2+</sup>. By electron-transfer equilibration with thioanisole as a redox standard, the reduction potential of VA<sup>•+</sup> 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 <i>k</i> = (17 ± 1) s<sup>-1</sup> 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 p<i>K</i><sub>a</sub> value of this radical was determined to be 10.0. The deprotonation of VA<sup>•+</sup> is enhanced by bases such as OH<sup>-</sup>, with the rate constant being 1.3 × 10<sup>9</sup> M<sup>-1</sup> s<sup>-1</sup>. In contrast, the corresponding rate constant for reaction of OH<sup>-</sup> with the radical cation of veratryl alcohol <i>methy</i><i>l e</i><i>ther, </i>whose reduction potential is also 1.36 V/NHE, is only 2 × 10<sup>7</sup> M <sup>-1</sup> s<sup>-1</sup>. With the veratryl alcohol derivative, 3,4-(MeO)<sub>2</sub>C<sub>6</sub>H<sub>3</sub>CH(OH)CMe<sub>3</sub>, the radical cation undergoes both a proton loss from the benzylic position and a C<sub>α</sub>−C<sub>β</sub> fragmentation with the ratio, at pH ≤ 5, of ca. 1:2. The decay of the radical cation is strongly enhanced by OH<sup>-</sup> (<i>k</i> = 8.3 × 10<sup>9</sup> M<sup>-1</sup> s<sup>-1</sup>), 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.