Investigation of enzymatic oxidation mechanism of model lignins using LC-ESI-MS/MS

Abstract

There are growing demands for the valorization of lignin into depolymerized aromatic chemicals. In our previous work, enzymatic oxidation using laccase, a multicopper oxidase, was reported to produce valuable benzaldehyde chemicals from plant lignin, but the mechanism of transformation remains unknown. To understand the mechanism of laccase-mediated oxidation of lignin, the oxidation products of lignin model dimers with three different linkage types (G(8-O-4)G, G(8-5)G, and G(8-8)G) were characterized using LC-ESI-MS/MS coupled with UV detector for selective detection of aromatic chemicals. Under laccase treatment for 24 h, 69%, 61%, and 89% of G(8-O-4)G, G(8-5)G, and G(8-8)G model dimers were transformed into oxidation products, respectively. The abundance of an aldehyde-functionalized oxidation product (m/z 713.2595, 2(G(8-5)G+CH2O)) of G(8-5)G was increased six times compared with the control sample without enzyme treatment, whereas the abundances of aldehyde-functionalized products of G(8-O-4)G and G(8-8)G were unchanged. MS/MS spectrum of the aldehyde-functionalized oxidation product (m/z 713.2595, 2(G(8-5)G+CH2O)) revealed the neutral losses of two aldehyde groups (CH2O and 2CH2O, m/z 30.01 and 60.02). Collectively, these results suggest that the enzymatic transformation of lignin into benzaldehyde chemicals could be associated with the oxidation of (8-5)-type lignin linkage.