Effect of a range of microbial polysaccharides on the diffusion of manganese ions using spatially resolved NMR relaxometry

In accordance with the theory of contact exchange, it is hypothesized that the presence of negative charge in microbial exopolysaccharides increases t he rate of cation transport. These typically acidic materials may provide a fast-track for the diffusion of nutrient cations through the polymer layer for uptake at the organism cell surface. We have measured the diffusion co efficient of a model cation, Mn2+ through xanthan, de-acetylated xanthan, s cleroglucan and chitosan using spatially resolved NMR relaxometry. The conc entration of Mn2+ in solution was measured by recording the change in the s pin-spin (T-2) relaxation time of water H-1 over time in compartments eithe r side of a polymer layer. This approach provides a sensitive, in situ, non -invasive method of measuring the rate of diffusion of paramagnetic cations through hydrophilic polysaccharides. The negatively-charged polysaccharide s, xanthan and de-acetylated xanthan, permitted a significantly faster rate (2-2.5X) of cation transport compared to the uncharged polymer, sclerogluc an. The positively-charged polysaccharide chitosan reduced the rate of Mn2 diffusion to around half the value obtained for scleroglucan. These result s suggest that the presence and nature of fixed charges on the polysacchari de molecule affects the rate of cation transport in accordance with the the ory of contact exchange. The presence of negative charge on microbial exopo lysaccharides may thus improve the availability of nutrient cations at the organism cell surface. (C) 2001 Elsevier Science Inc. All rights reserved.