Welan gum (S-130) contains repeating units with randomly distributed L-mannosyl and L-rhamnosyl terminal groups, as determined by FABMS.

Most bacterial polysaccharides are composed of oligosaccharide repeating units. There are, however, several examples of polysaccharides in which only part of the “repeating” units contain a certain substituent, e.g., an O-ace@ group, an LX-Dglucopyranosyl group, or an acetalically linked pyruvic acid residue [l]. In a small group of polysaccharides, r_.-rhamnose in the “repeating unit” is replaced by L-mannose 121. These structural variations may be random, or regular as, for example, in a block co-polymer, or the bacterium may produce two populations of polysaccharides, each with its own repeating unit, but which are difficult to separate. This structural problem has been solved for some polysaccharides. Thus, only a fraction of the Salmonella thompson O-antigen polysaccharide which did not contain cr-b-glucopyranosyl groups was hydrolysed by the phage 414, demonstrating the presence of two populations, one without and one with such groups in the repeating unit [31. Hydrolysis of the O-antigen polysaccharide from Salmonella typhi 253T, with phage P22 on the other hand gave dimers and trimers of the repeating unit containing 0, 1, and 2 or 0, 1, 2, or 3 a-o-glucopyranosyl groups, respectively, demonstrating a random distribution of these groups [4]. A related method of studying this problem would be to subject the polysaccharide to a partial chemical degradation and characterise the fragments derived from two or more repeating units. We now report such studies of the polysaccharide welan gum elaborated by Alcaligenes ATCC 315 55. It is composed of “repeating units” with the structure 1, in which the terminal a-r_-rhamnopyranosyl groups are partially replaced by cr-r.-mannopyranosyl groups [51.