Structural characterization of PAGs
CG was synthesized via lipase-catalyzed esterification of CA with glycerol in a “solvent-free” reaction (Scheme 1). Products of the reaction system were monitored and quantified by HPLC-UV at 325 nm (Fig. 1). Peak 3 in fig 1 was confirmed to be CG by the mass spectrometry (Fig. 2a) with a fragmentation cation of m/z 277.0681 representing [CG+Na]+. Similarly, FG andp -HCG were synthesized via lipase-catalyzed esterification of FA and p -HCA with glycerol and their identities were confirmed by mass spectrometry (Fig. 2b and 2c), with the fragmentation cations ofm/z 291.0829 and 261.0730 representing [FG+Na]+ and [p -HCG+Na]+, respectively. In order to further verify the structures of PAGs, 1H NMR and13C NMR analyses of isolated CG, FG and p -HCG were also performed. Their chemical shifts were consistent with the previous studies (Sun et al., 2020; Weng et al., 2019), indicating the successful synthesis of PAGs.
Hereafter, CG was used as a model to optimize reaction conditions to achieve the highest yield.
Fig. 1. HPLC chromatogram of lipase-catalyzed esterification mixture of CA with glycerol at 325 nm. Reaction conditions are as follows: CA : glycerol = 1:150 (mol : mol), Lipozyme 435 load 25% (relative to the weight of total substrates), 80 oC, 86.7 kPa, 12 h.
Fig. 2. ESI-MS analysis of PAGs. (a) CG, (b) FG, (c) p -HCG.