Evidence for formation of the veratryl alcohol cation radical by lignin peroxidase

Abstract

Lignin peroxidases (LiP) catalyze the H2O2-dependent two-electron oxidation of veratryl alcohol (VA) to veratryl aldehyde. We present here, electron spin resonance (ESR) evidence for the formation of the one-electron oxidized intermediate, the veratryl alcohol cation radical (VA.+). The ESR spectrum of VA.+ was first obtained in a fast-flow system with Ce(IV) as an oxidant and 10% HNO3 to stabilize the radical. This ESR signal was deconvoluted, and the hyperfine splitting constants were determined. The identity of the radical was confirmed by computer simulation of the ESR spectrum and calculation of spin and charge densities on the radical. An identical radical signal was observed with LiP, also in a fast-flow incubation containing 10 microM LiP, 2 mM VA, and 500 microM H2O2 at pH 3.5. The Fourier transforms of the ESR signals further confirmed that the spectra obtained with both Ce(IV) and LiP were due to the same radical species. The VA.+ had a distinct visible spectrum in 98% H2SO4 with an absorbance maximum at 529 nm. The extinction coefficient of the VA.+ spectral band at 529 nm was calculated to be 11,000 M-1 cm-1. The VA.+ was found to be a strong acid, as are other cation radicals, with the pKa at -1.0 pH. This value was determined by quantitating both the concentration of VA.+ by visual and ESR spectrometry and the g-value of the ESR signal at various pH values.