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 1H and 13C 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.

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► Providencia rustigianii is an opportunistic human enteric pathogen.
► We isolated a polysaccharide from a strain of P. rustigianii O11.
► The polysaccharide was studied by chemical methods, 2D NMR spectroscopy and ESI MS.
► The structure of a branched tetrasaccharide repeat of the polysaccharide was established.
► For the first time in bacterial polysaccharides, an amide of an uronic acid with a dipeptide was identified.