Effect of a range of microbial polysaccharides on the diffusion of manganese ions using spatially resolved NMR relaxometry T.D. Hart a , R.J. Hill b , P.M. Glover b , J.M. Lynch a, *, A.H.L. Chamberlain a a School of Biological Sciences, University of Surrey, Guildford, Surrey GU2 5XH, UK b School of Physics and Chemistry, University of Surrey, Guildford, Surrey GU2 5XH, UK Received 21 June 2000; received in revised form 5 October 2000; accepted 13 October 2000 Abstract In accordance with the theory of contact exchange, it is hypothesized that the presence of negative charge in microbial exopolysac- charides increases the 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 coefficient of a model cation, Mn 2+, through xanthan, de-acetylated xanthan, scleroglucan and chitosan using spatially resolved NMR relaxometry. The concentration of Mn 2+ in solution was measured by recording the change in the spin-spin (T 2 ) relaxation time of water 1 H over time in compartments either 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 polysaccharides, xanthan and de-acetylated xanthan, permitted a significantly faster rate (2–2.5) of cation transport compared to the uncharged polymer, scleroglucan. The positively-charged polysac- charide chitosan reduced the rate of Mn 2+ diffusion to around half the value obtained for scleroglucan. These results suggest that the presence and nature of fixed charges on the polysaccharide molecule affects the rate of cation transport in accordance with the theory of contact exchange. The presence of negative charge on microbial exopolysaccharides may thus improve the availability of nutrient cations at the organism cell surface. © 2001 Elsevier Science Inc. All rights reserved. Keywords: Polysaccharides; Anion exclusion; Diffusion potentials; Xanthan 1. Introduction Microbial exopolysaccharides are predominantly acidic [1] and form continuous layers around the producing organ- ism. These layers are most obvious around bacteria in the rhizosphere [2], where thickness of the gel layer may often exceed the diameter of the cell itself. In such environments, the low availability of nutrient ions may often limit growth and numbers of the resident microflora. In this paper we consider the influence of these negatively-charged exopoly- saccharides on the diffusion of cations. Microbial exopolysaccharide layers may influence the diffusion of cations on two levels. Firstly, these layers may act to immobilize toxic cations in solution before they reach the cell surface [3]. Secondly, they may affect the diffusion of nutrient cations from the immediate environment to the organism cell surface. Most workers assume that the pres- ence of these layers will restrict diffusion, due to absorption [4]. However, according to the theory of contact exchange [5], it is also possible that polysaccharides accelerate the migration of cations and this may depend on the degree of negative charge on the polymer molecule. Previously, we have shown that negatively-charged mi- crobial and soil polysaccharides selectively restrict the dif- fusion of anions [6] in accordance with the theory of perm- selectivity as first proposed by Sollner [7]. An independent theory of ion diffusion through charged membranes sug- gests that negatively-charged layers can accelerate the dif- fusion of cations by a process known as contact exchange [5]. Both theories rely on the polymer layer comprising pores of sufficiently narrow diameter such that neighboring electrostatic fields of fixed negative charges, located on the pore walls, overlap. This acts to repulse the diffusing anion, but attract the diffusing cation. As the pore contains many fields that overlap or are sufficiently near to each other, cations effectively ‘jump’ from one field to the next and thus migrate through the pore, assuming there is a concen- tration gradient for the diffusing cation. By this mechanism, * Corresponding author. Tel.: +0044-01483-879-721; fax: +0044- 01483-879-728. E-mail address: j.lynch@surrey.ac.uk (J.M. Lynch). www.elsevier.com/locate/enzmictec Enzyme and Microbial Technology 28 (2001) 370 –375 0141-0229/01/$ – see front matter © 2001 Elsevier Science Inc. All rights reserved. PII: S0141-0229(00)00331-8