Introduction
Phenolic acids (PAs) are secondary plant metabolites widely existed in many agricultural products such as fruits, vegetables and cereals (Crozier et al., 2009 ; Xu et al., 2018; Varga et al., 2018). In recent years, PAs have attracted considerable attention due to their biological and pharmacological activities, for example antioxidative activities, free radical scavenging, chelating metal ions, antimutation, boost immunity and neuroprotection (Balakrishna et al., 2017; Jo & Kim 2016; Kaki et al., 2015; Pei et al., 2015; Pontiki et al., 2014; Kikugawa et al., 2017). However, the unsatisfactory solubility and stability of PAs in polar/non-polar media limit their further application in water/oil systems (Sun et al., 2020; Weng et al., 2019). Therefore, the structural modifications of PAs are necessary to widen their application in food, pharmaceutical, and cosmetics industries.
Many studies have shown that the esterified derivatives of PAs could not only change their surface activity, but also significantly increase the physiological activity of the parent compound, indicating that esterification is a very effective method to improve the functional properties of PAs (Balakrishna et al., 2017; Weng et al., 2019; Yesiltas et al., 2019; Wang et al., 2020; Faggiano et al., 2022; Weng et al., 2020). As an example, phenolic acid glycerols (PAGs) such as caffeoyl glycerol (CG), feruloyl glycerol (FG) and p -hydroxycinnamoyl glycerol (p -HCG) have been prepared by PAs/ phenolic acid esters with glycerol to improve their hydrophilicity (Cumming & Marshall, 2021; Sun & Hu, 2017; Sun et al., 2017; Sun & Hou, 2019; Yao & Sun, 2020). Chemical (Molinero et al., 2014), chemo-enzymatic (Meng et al., 2018; Hollande et al., 2018) and enzymatic methods (Compton et al., 2012; Zhang et al., 2021; Sankar & Achary, 2017) in organic solvent (Cumming & Marshall, 2021), solvent-free (Sun & Hu, 2017) or ionic liquids (Sun et al., 2017) have been developed for the preparation of PAGs. Compared with chemical catalysts, enzyme was popular catalyst for the preparation of PAGs due to their mild reaction conditions (Weng et al., 2020). To improve the enzyme performance, ultrasound was used in enzymatic synthesis of PAGs (Xu et al,, 2018; Sun et al., 2020). Furthermore, high vacuum was usually indispensable to remove the byproducts including ethanol or water during the whole reaction course, which could increase the yield of product (Sun & Hu, 2017). However, until now the reported enzymatic synthesis of PAGs was rarely by the esterification of PAs with glycerol, but mostly the transesterification of phenolic acid esters, such as phenolic acid methyl (Meng et al., 2018), ethyl (Sun et al., 2020;Weng et al., 2019) or other esters (Meng et al., 2018) with glycerol in organic solvent (Cumming & Marshall, 2021; Compton et al., 2012), solvent free (Sun & Hu, 2017) or ionic liquids (Sun et al., 2017). Compared with PAs as raw material of the synthesis of PAGs, the use of phenolic acid esters synthesized by the esterification of PAs with alkanol would reduce the final reaction efficiency and increase the cost.
The present study was therefore aimed to develop a direct enzymatic esterification of PAs with glycerol for the preparation of PAGs (Scheme 1), which would have higher reaction efficiency and lower product cost than those of reported enzymatic transesterification of phenolic acid esters with glycerol. The effects of reaction variables, such as enzyme types and load, molar ratio, reaction temperature and reaction time on the esterification of PAs with glycerol were studied. High performance liquid chromatography (HPLC) was used to monitor the esterification. Three kinds of PAGs (CG, FG andp -HCG) were synthesized and identified by HPLC, MS and NMR. Subsequently, the antioxidant capacities of PAGs were evaluated by 2,2-diphenyl-1-picrylhydrazyl (DPPH).
Scheme 1. Lipase-catalyzed esterification of PAs with glycerol