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-Rhm. p (18.4%), 3-Rhm. p (15.7%) T-Glc. p (21.7), 4-Glc. p (16.3%), 3,4,6-Glc. p (16.3%) and 2,6-Gal. p (11.9%). 1D and 2D NMR analysis revealed that both -OAc and -NAc were connected to 3-Rhm. p 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.