Semi-synthesis of N-alkyl-kappa-carrageenan derivatives and evaluation of their antibacterial activity

In this article, we describe the semi-synthesis of N-alkyl-kappa-carrageenan derivatives and their antibacterial activity against Staphylococcus aureus (ATCC 6538), Escherichia coli (ATCC 8739), and Pseudomonas aeruginosa (ATCC 9027). Kappa-carrageenan was submitted to partial acid hydrolysis promoting the selective cleavage of α-glycosidic bonds involving 3,6-anhydro-α-D-Galp units, giving rise to reducing low-molecular weight polysaccharide fragments, which were reacted with alkylamines of varying chain lengths by reductive amination. The carrageenan derivatives were characterized by HPSEC-MALLS-RID and 1D and 2D ¹H and ¹³C NMR spectroscopy. The antibacterial activity of N-alkyl-kappa-carrageenan derivatives was compared with N-alkyl-(1-deoxylactitol-1-yl)-amines using a microdilution test, which indicated that inhibitory activity was dependent on the degree of substitution by hydrophobic groups at the polysaccharide structure. Comparing the effect of different N-alkyl chains, those with longer chains showed higher activity.