Abstract: To improve the water solubility and bioactivity of pachymaran, carboxymethylated pachymaran (CMP) was synthesized with monochloroacetic acid as carboxymethylation reagent. The structures of CMP with different degrees of substitution were characterized and their bacteriostatic activity were analyzed in vitro. The results showed that the optimum reaction conditions were as follows: temperature, 65 ℃; etherification time, 4 h; ethanol concentration, 80%; and molar ratio of pachymaran to sodium hydroxide to monochloroacetic acid, 1:3.5:1.75. Under these conditions, the average degree of substitution (DS) was 0.724. CMP with DS in the range of 0.350 to 0.728 could be prepared by controlling the reaction conditions. The results of Fourier transform-infrared (FT-IR) spectroscopy analysis showed that pachymaran was successfully derivatized. Gel permeation chromatography coupled online to multi-angle laser light scattering (GPC-MALLS) revealed that higher the DS, the smaller the molecular mass and the higher the absolute value of Zeta potential of CMP. In addition, the particle size increased first and then decreased, the content of neutral sugar decreased continuously, and the thermal stability firstly increased and then decreased. The results of nuclear magnetic resonance (NMR) showed that carboxymethyl derivatization mainly occurred at the hydroxyl position of C6. Scanning electron microscopy (SEM) showed that the higher the DS, the closer the spatial structure and the higher the degree of cracking. In vitro tests showed that the inhibition of CMP against foodborne pathogens was enhanced with increasing DS, and it was more active against Gram-positive bacteria than against Gram-negative bacteria.

Key words: pachymaran, chemical modification, structural characterization, antimicrobial activity