Abstract
The rate constants were experimentally obtained for the oxidative reaction of three monolignols (p-coumaryl, coniferyl and sinapyl alcohol) and four similar forms of 3-mono- or 3, 5-disubstituted p-coumaryl alcohols (3-ethoxy-, 3-propoxy-, 3-methyl- and 3, 5-dimethyl-p-coumaryl alcohol) by using horseradish peroxidase (HRP) -H2O2 as the oxidant. To investigate the factor determining reaction rate of each substrate, the molecular volume and the highest occupied molecular orbital (HOMO) energy were applied. The molecular volume was obtained by using optimum conformation data with MOPAC2000. And the HOMO energy was calculated with MOPAC2000 under four conditions, i.e., when the phenolic hydroxyl form of each compound is either neutral or anion, and when each substrate is at polar or nonpolar milieu.
We infer that the reason why 3, 5-disubstituted p-coumaryl alcohol deviates from the profiles of others was due to local steric hindrance and distortion effect of the substituted groups. In assuming anion/water, the reaction rates of substrates correlated with HOMO energies excluding 3, 5-disubstituted p-coumaryl alcohols. In alternate form and milieu, no suitable relationship between HOMO energies and reaction rates was obtained. From these results, we suggest that the rate-determining step in the oxidative reaction for 3-substituted p-coumaryl alcohol is where one electron is withdrawn by oxidized HRP under anion/water condition.
