Selective conversion of lignin model veratryl alcohol by photosynthetic pigment via photo-generated reactive oxygen species

The selective conversion of veratryl alcohol (VA), a model compound of lignin, to produce veratraldehyde was investigated in a simple light-driven system consisting of photosynthetic pigment chlorophyllin and VA. Irradiation experiments and liquid chromatography tandem mass spectrometry analysis indicated that light in the wavelength range of 310-420 nm induced the efficient conversion of veratryl alcohol to veratraldehyde (76% in 4 h). Results from differential pulse voltammetry and linear sweep voltammetry indicated that light enhanced the two reduction peaks of chlorophyllin and boosted the photocurrent from 2.7 to 3.8 mu A/cm(2) at 0.8 V between excited chlorophyllin and electrodes, confirming the occurrence of photoelectrons transferring between excited chlorophyllin and oxygen in the presence of light. Quenching experiments further demonstrated that superoxide anion radicals, produced via one-electron reduction reaction between triplet chlorophyllin and oxygen, were responsible for the efficiently selective conversion of veratryl alcohol to veratraldehyde. Our results not only contribute to revealing the role of reactive oxygen species and excited photosynthetic pigment chlorophyllin in the photochemical conversion of lignin model compounds, but also provide ideas for applications of chemical processing in lignin utilization for valuable chemical production.