Carbohydrate Research 315 (1999) 330 – 333 Note Imaging an alginate polymer gel matrix using atomic force microscopy Alan W. Decho * Department of Enironmental Health Sciences, School of Public Health, Uniersity of South Carolina, Columbia, SC 29208, USA Received 16 September 1998; accepted 6 December 1998 Abstract The polysaccharide structure of alginic acid was examined as individual molecules and as dense gels using tapping-mode atomic force microscopy (AFM). Dilute (picomolar) non-ionic solutions of polymer molecules were sorbed onto mica surfaces. Under these conditions, the molecules exhibited frequent ‘kinks’ or abrupt right-angle changes in orientation. It is proposed that the kinks may correspond to chair backbone configurations that are predicted from the molecular structure of alginate, and occur at linkages between the monomers: -L-guluronate (G) and -D-mannuronate (M). Dense alginate gels (2% concentration) generated under strong ionic conditions (30 parts per thousand seawater) assumed a repeatable steric arrangement, and exhibited a relative regular spacing of solvent cavities (namely, H 2 O) within the gel. This suggests that cation bridges are formed at regular intervals along adjacent polymers under these conditions. This work demonstrates the utility of tapping-mode AFM for examining the structure and gel conformation of a pliant polymeric matrix. © 1999 Elsevier Science Ltd. All rights reserved. Keywords: Extracellular polymers; Alginate; Polysaccharide; AFM; Structure Extracellular polysaccharides (EPS) pro- duced by bacteria, microalgae, and macroal- gae are molecules of applied and environmental importance. EPS form the an- choring mechanism for biofilm formation [1], and contribute to microbial-induced metal corrosion, dental plaque formation, and a va- riety of human disease processes [2]. The physical properties of EPS are largely determined by interactions of their chemical structure with the surrounding ionic environ- ment. The large molecular weight (generally  100 kDa) and anionic nature of EPS allow these molecules to exist in a continuum of physical states, ranging from dense gels to dilute solutions. Imaging and quantifying the interactions between adjacent molecules of EPS are important to understanding how physical transitions occur between gel and so- lution states, and more generally, in mediating biofilm formation. Atomic force microscopy (AFM) [3] is a potential tool for observing individual poly- mer molecules, and further, how they interact in forming a gel matrix. We imaged alginate molecules in both gel and solution states using AFM. Alginate used in this study is derived from the seaweed Macrocystis pyrifera. Algi- nate is a heteropolymer, generally greater than 130 kDa in size, and is composed of two acidic monomers: (1  4)-linked -L-guluron- * Fax: +1-803-777-3391. E -mail address: adecho@sph.sc.edu (A.W. Decho) 0008-6215/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved. PII:S0008-6215(99)00006-3