Lipase-Catalyzed Synthesis of Phloretic Acid Esters with Geraniol and Myrtenol: Reaction-Level Sustainability Indicators and Preliminary In Vitro Antimicrobial Evaluation

Abstract

The development of antimicrobial compounds using biocatalytic synthesis routes is of interest in sustainability-oriented chemical and food-related research. In this study, phloretic acid was enzymatically esterified with two naturally occurring terpenoid alcohols, geraniol and myrtenol, using Candida antarctica lipase B as a biocatalyst under mild conditions. The study combined terpenoid and phenolic molecular building blocks with a biocatalytic synthesis route and evaluated the resulting esters as preliminary in vitro antimicrobial candidates. The resulting esters were isolated in moderate yields, reaching 65.51% for geranyl 3-(4-hydroxyphenyl)propanoate and 54.50% for myrtenyl 3-(4-hydroxyphenyl)propanoate. Their antimicrobial activity, together with that of the parent compounds, was evaluated against a limited panel comprising two Gram-positive bacteria, two Gram-negative bacteria, and two yeast species, by determining the minimum inhibitory concentration (MIC) and minimum microbicidal concentration (MMC). The tested terpenoid alcohols showed notable antifungal activity, whereas esterification was associated with a marked increase in activity against the tested Staphylococcus aureus PCM 2054 strain. Both phloretic acid terpenyl esters showed the lowest measured MIC and MMC values against S. aureus, at 0.227 and 0.455 mM, respectively. These MIC values represented 16- and 64-fold decreases relative to the corresponding parent terpenoid alcohols and phloretic acid, respectively. Geranyl 3-(4-hydroxyphenyl)propanoate was selected for time-kill analysis because both esters showed the same measured MIC and MMC values against S. aureus PCM 2054, whereas the geranyl ester provided a higher isolated yield, more favorable calculated reaction-stage process descriptors, and a slightly lower geometric mean MIC across the tested microbial panel. In the time-kill experiment, G4HPP at 0.25 and 0.50 mM, corresponding to approximately 75.6 and 151.2 mg/L, respectively, produced an estimated 3-log reduction in viable S. aureus PCM 2054 counts after approximately 4.7 and 4.3 h. A small increase in viable counts was observed between 8 and 24 h. Supporting in silico analysis indicated that esterification increased predicted lipophilicity and decreased predicted aqueous solubility, accompanying the microorganism-dependent changes observed in the antimicrobial activity profile. Overall, the synthesized esters should be regarded as preliminary in vitro antimicrobial candidates displaying greater in vitro activity against the tested S. aureus PCM 2054 strain than the corresponding parent compounds. Broader strain-panel studies and evaluation in food-relevant matrices are required before any potential food-preservation application can be considered.