On the factors affecting product distribution in laccase-catalyzed oxidation of a lignin model compound vanillyl alcohol: Experimental and computational evaluation

Research output: Contribution to journalArticleScientificpeer-review

Researchers

  • Maarit Lahtinen
  • Petri Heinonen
  • Mikko Oivanen
  • Pirkko Karhunen
  • Kristiina Kruus
  • Jussi Sipilä

Research units

  • University of Helsinki
  • VTT Technical Research Centre of Finland

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.

Details

Original languageEnglish
Pages (from-to)5454-5464
Number of pages11
JournalORGANIC AND BIOMOLECULAR CHEMISTRY
Volume11
Issue number33
Publication statusPublished - 1 Jan 2013
MoE publication typeA1 Journal article-refereed

ID: 32869099