Graphene Quantum Dot-Modified Lipase for Synthesis of L-menthyl Acetate with Improved Activity, Stability and Thermostability

Title: Pseudomonas cepacia lipase modified with Graphene Quantum Dots (GQD) offers a higher activity, stability and thermostability compared to the lipase modified with graphene oxide. Background: GQD presents extraordinary properties attracting extensive attention from scientists in chemistry, physics, materials, biology and other interdisciplinary sciences. However, no research involves the application in biocatalysts in non-aqueous media up to now. In the study, we for the first time reported a promising GQDmodified lipase for the synthesis of l-menthyl acetate. Methods and findings: Pseudomonas capaci lipase was modified with GQD and then used as a biocatalyst for the synthesis of l-menthyl acetate in 1-isobutyl-3- methylimidazolium hexafluorophosphate medium. As contrasts, the reaction was also carried out using bare lipase and graphene oxide-modified lipase as the catalyst. Besides, the modification method of enzyme, the amounts of GQD, the reaction temperature, molar ratio of the two substrates, and reuse of the enzyme were investigated. The result shows that the GQD-modified lipase as a biocatalyst was best among the three enzymes. Under optimal reaction conditions, the reaction reaches the equilibrium within 8 h with a high conversion of lmenthol (97.3%). Its initial enzyme activity and halflifetime were more than 1.08 and 2.07-fold that of the bare lipase, and 1.04 and 1.66-fold that of the graphene oxide-modified lipase, respectively. The lipase was recycled 10 times without substantial diminution in activity. In addition, the GQD-modified lipase also offers a better thermostability compared with bare lipase. These improvements could be attributed to the unique small size and edge effect of GQD. Conclusion: GQD-modified lipase showed a higher activity, stability and thermostability compared to GO- modified lipase. GQD was widely used as a promising material for enzyme modification due to its good biocompatibility and small size.