Structure of a polysaccharide from Providencia rustigianii O11 containing a novel amide of 2-acetamido-2-deoxygalacturonic acid with L-glutamyl-L-alanine

An acidic polysaccharide was isolated from Providencia rustigianii O11 by the phenol-water extraction. The polysaccharide was cleaved by solvolysis with triflic acid to yield disaccharides with uronic acid derivatives at the non-reducing end. The polysaccharide and the disaccharides were studied by chemical analyses, high-resolution ESI MS, and 2D (1)H and (13)C NMR spectroscopy, and the following structure of the tetrasaccharide repeating unit of the polysaccharide was established: where GalNAcA stands for 2-acetamido-2-deoxygalacturonic acid, GalNAcA6GluAla for N-(2-acetamido-2-deoxygalacturonoyl)-l-glutam-1-yl-l-alanine, QuiNAc4NAcyl for 2-acetamido-4-[(S)-3-hydroxybutanoylamino]-2,4,6-trideoxyglucose (∼75%) or 2,4-diacetamido-2,4,6-trideoxyglucose (∼25%); the d configuration of GalNA and QuiN4N was ascribed tentatively. To the best of our knowledge, this is for the first time that an amide of uronic acid with a dipeptide is found in bacterial Polysaccharides.