Structure characterization of exopolysaccharides from Lactobacillus casei LC2W from skim milk

LCP1, a high molecular weight fraction of the EPSs produced by strain LC2W isolated from skim milk, was studied by partial acid hydrolysis, smith degradation, methylation analysis and 1D & 2D NMR spectroscopy to elucidate its main structure feature. Monosaccharides composition analysis revealed the presence of glucose (57.8% mol%), rhamnose (27.7%, mol%) and galactose (14.5%, mol%), respectively. O-acetyl (–OAc) and N-acetyl (–NAc) groups were also evidenced by FTIR and NMR spectroscopies. The linkage patterns of sugar residues were determined by methylation analysis and partial acid hydrolysis. The main sugar residues included: T-Rhmp (18.4%), 3-Rhmp (15.7%) T-Glcp (21.7), 4-Glcp (16.3%), 3,4,6-Glcp (16.3%) and 2,6-Galp (11.9%). 1D and 2D NMR analysis revealed that both –OAc and –NAc were connected to 3-Rhmp at O-2 position. The configuration and glycosidic sequences for sugar residues were established by 2D NMR spectroscopy. A possible structure of LCP1 was proposed.