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