Structure of Microbial Polysaccharides Using X-Ray Diffraction

Abstract

The conformation and interactions of microbial polysaccharides can be investigated using X-ray diffraction. Full three-dimensional crystallinity is not usually possible and the data is obtained from oriented samples exhibiting fibre texture. In many instances external decoration, often in the form of non-carbohydrate groups, is present in the native material which has a suppressive effect on the crystallization and allows the substances to exist in the liquid crystalline and gel states. Three examples, Klebsiella K5, gellan gum and Klebsiella K54 occur naturally with o-acetyl groups decorating their surface. These non-carbohydrate substituents are common ingredients in microbial polysaccharides and the importance of their role in regulating the biological and commercially useful functions of these polymers is becoming better appreciated and more fully understood by monitoring the effect of deacetylation using X-ray diffraction. The results from Klebsiella K5, gellan gum and Klebsiella K54 and its deacetylated counterpart XM6 are discussed as a group. In all three cases the conformation of the native molecule remains basically the same on deacetylation but the interactions between molecules are substantially altered. A further example where X-ray diffraction can be used to monitor changes as a function of environment is xanthan gum. This double-helix molecule has a cellulosic backbone decorated with relatively long side branches. The relationship of these to the backbone as a function of water content is examined and discussed.