title: On the factors affecting product distribution in laccase-catalyzed oxidation of a lignin model compound vanillyl alcohol: experimental and computational evaluation.
abstract: Laccases (EC 1.10.3.2) are multicopper oxidases, which can oxidize phenolic substrates by the concomitant reduction of oxygen to water. The phenolic substructures of lignin are also oxidized by laccases, resulting mainly in various polymerized products. Several model compound studies indicate that variations in the reaction media, such as the pH and the enzyme dosage used, have an impact on the observed product distribution of laccase promoted oxidation, but no detailed study has been reported to explain these results. In the present study, a monomeric lignin model compound, vanillyl alcohol, was oxidized in laccase-catalyzed reactions by varying the pH, enzyme dosage and temperature. The energies of all the observed products and potential intermediates were calculated by applying density functional theory (DFT) and the polarizable continuum solvation model (PCM). The observed predominant product at pH 4.5 to 7.5 was clearly the 5-5' dimer, although the thermodynamic product according to the calculated free energies was vanillin, the difference being 5.6 kcal mol(-1). The hydrogen bonding is shown to give an additional stabilizing effect on the transition state leading to the 5-5' dimer, but also a kinetic barrier reduces the formation of vanillin. Based on the calculated pKa-values of the proposed intermediates we suggest that the rearomatization reactions of the quinones formed in the radical reactions under mildly acidic and neutral conditions would preferentially occur through deprotonation rather than through protonation.
authors.affiliation: Laboratory of Organic Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55, FIN-00014 HU, Finland. maarit.lahtinen@helsinki.fi
authors.forename: Maarit
authors.initials: M
authors.lastname: Lahtinen
authors.forename: Petri
authors.initials: P
authors.lastname: Heinonen
authors.forename: Mikko
authors.initials: M
authors.lastname: Oivanen
authors.forename: Pirkko
authors.initials: P
authors.lastname: Karhunen
authors.forename: Kristiina
authors.initials: K
authors.lastname: Kruus
authors.forename: Jussi
authors.initials: J
authors.lastname: Sipilä
mesh_terms: D001547:Benzaldehydes / Q000138:chemical synthesis / Q000737:chemistry; D001592:Benzyl Alcohols / Q000737:chemistry*; D042845:Laccase / Q000737:chemistry*; D008031:Lignin / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D010084:Oxidation-Reduction; D011789:Quantum Theory; D013816:Thermodynamics
chemical_list: D001547:Benzaldehydes; D001592:Benzyl Alcohols; D008031:Lignin; C100058:vanillin; D042845:Laccase; C024078:vanillyl alcohol