Abstract
The enzymatic degradation of polyethylene (PE) by oxidative enzymes remains poorly understood at the molecular level, limiting the development of effective biotechnological strategies for plastic valorization. Here, we investigate the molecular basis of PE oxidation by the laccase-like multicopper oxidase LMCO2 from Rhodococcus opacus R7 with combined DFT and molecular docking calculations. LMCO2 is a particularly relevant model system because it has been experimentally shown to oxidatively modify low-density polyethylene without the need for mediators, despite its relatively low redox potential. DFT calculations indicate that direct oxidation of aliphatic C-H bonds in pristine PE is energetically prohibitive, and that hydroxylated defects do not significantly facilitate substrate activation. In contrast, oxidation at carbon atoms adjacent to carbonyl groups is associated with substantially lower barriers, owing to stabilization of the resulting radicals through conjugation and keto-enol tautomerism. Alternative pathways involving alkoxyl radicals and β-scission were also examined and found to be energetically unfavorable. Molecular docking calculations complement this picture by showing that both pristine and pre-oxidized PE segments can access the main cavity adjacent to the T1 copper site, although binding is weak and largely non-specific. Taken together, these results support a mechanism in which LMCO2 selectively acts on pre-oxidized, carbonyl-containing regions of polyethylene, promoting further radical chemistry that may ultimately lead to chain scission and the formation of low-molecular-weight oxygenated products.