Optimization of a CALB catalyzed esterification process for the synthesis of methyl glucoside fatty acid esters: green metrics, application and antimicrobial evaluation

Abstract

Carbohydrate fatty acid esters (CFAE) are promising sustainable antimicrobials with potential therapeutic applications. Although enzymatic synthesis of CFAE offers advantages, issues such as substrate solubility, limited exploration of non-classical greener solvents, additional synthetic steps, and analytical challenges hinder applicability and scale-up. In addition, mono-protected monosaccharide/unsaturated fatty acid (uFA) based CFAE are underexplored for antimicrobial evaluation. Our study systematically investigated the Candida antarctica lipase B (CALB) catalyzed esterification of methyl α-D-glucopyranoside (MAG) and lauric acid (LA) as a model system, employing green chemistry concepts to generate a panel of potentially more sustainable antimicrobial products. Systematic reaction optimization was followed by its application to generate 18 mono-substituted CFAE, including 7 novel derivatives. Among the screened solvents, propylene carbonate (PC) and ethylene carbonate (EC) yielded 59% and 56% of monoester product, respectively. Under optimized conditions (MAG : LA – 1 : 3 equivalence), acetonitrile afforded the highest yield of 89% in 4 h at 60 °C. An enzyme reusability study showed reduction of activity when the recovered biocatalyst was dried, while direct reuse with added drying agent maintained activity over 7 cycles. In the gram-scale synthesis of MAG monolaurate (3a), recovery of unreacted LA through the non-chromatographic workup improved the reaction mass efficiency (RME) from 39.53% to 83.19%. Solvent parameter analysis using polarity, log P and Kamlet–Taft values, and green metric evaluation, through the EcoScale and CHEM21 first-pass toolkits were undertaken. These identified clear areas for further improvement. A screening cradle-to-gate Life Cycle Assessment was completed using SimaPro®, identifying auxiliary energy demand and solvent use as environmental hotspots within our study. Antimicrobial evaluation against the Gram-positive Staphylococcus aureus (ATCC 25923) identified monooleate of methyl β-D-glucopyranoside (MBG monooleate, 4e) as the most active CFAE, with MIC and MBC values of 0.0156 mM, corresponding to ∼32-fold improvement in inhibitory potency compared to oleic acid with MIC and MBC at 0.5000 mM.