Suppression mechanism of model humic constituents on laccase-enabled 17β-estradiol oxidation and oligomerization
Humic constituents (HCs) are ubiquitous in the aquatic ecosystems , and contain various functional groups that seriously impact the conversion of 17 β -estradiol (17 β -E2) by fungal laccase. The purpose of this study was to explore the influencing mechanism of HCs on Trametes versicolor laccase-enabled 17 β -E2 oxidation and oligomerization. Herein, T. versicolor -secreted laccase could rapidly convert 99.2% of 17 β -E2 (rate constant = 3.7 × 10 −2 min −1 , half-life = 18.7 min) into multifarious oligomers at 25 °C and pH 5.0, by phenolic radical-caused C–C and/or C–O self-linking routes, whereas HCs with O -phenolic hydroxyl groups ( O -p-OH, i.e. , catechol, pyrogallol, gallic acid, and caffeic acid) dramatically suppressed 17 β -E2 oligomerization. Compared with HC-free, 17 β -E2 rate constants weakened 6.3–15.8 fold in the presence of HCs containing O -p-OH. It is largely because the O -p-OH was preferentially oxidized by T. versicolor laccase to create the electrophilic O -quinone monomers/oligomers. These unstable reactive O -quinone intermediates strongly reversed 17 β -E2 phenolic radicals to their monomeric molecules via two proton-transfer versus two electron-transfer channels, thus intercepting 17 β -E2 oxidation and oligomerization. These findings highlight new insights into the effect of HCs containing O -p-OH on T. versicolor laccase-started 17 β -E2 conversion, which is beneficial to re-understanding the fate and geochemical behavior of 17 β -E2 in water.
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