Acetylation and methylation of homogalacturonans 1: optimisation of the reaction and characterisation of the products

Tetrabutylammonium (TBA) salts of homogalacturonans, obtained by acid hydrolysis of demethylated citrus pectins, were acetylated and/or methylated. Methylation took place in DMSO, using CH3I as the reagent, and was stoichiometric up to a degree of methylation (DM) of >60. The best acetylation results, with acetic anhydride as reagent, were obtained using formamide as solvent and pyridine as catalyst, giving degrees of acetylation (DAc) of up to 150% in a single step. The reaction was fast but demanded large excess of acetic anhydride. Acetylation occurred on both the O-2 and the O-3 position, with a slight preference for O-2, of galacturonic acid residues. The hydrodynamic volume of the molecules showed little change after derivatisation. Acetylation inhibited hydrolysis of homogalacturonans by endopolygalacturonase.