Imaging an alginate polymer gel matrix using atomic force microscopy

The polysaccharide structure of alginic acid was examined as individual mol ecules and as dense gels using tapping-mode atomic force microscopy (AFM). Dilute (picomolar) non-ionic solutions of polymer molecules were sorbed ont o mica surfaces. Under these conditions, the molecules exhibited frequent ' kinks' or abrupt right-angle changes in orientation. It is proposed that th e kinks may correspond to chair backbone configurations that are predicted from the molecular structure of alginate, and occur at linkages between the monomers: alpha-1-guluronate (G) and beta-D-mannuronate (M). Dense alginat e gels (2% concentration) generated under strong ionic conditions (30 parts per thousand seawater) assumed a repeatable steric arrangement, and exhibi ted a relative regular spacing of solvent cavities (namely, H2O) within the gel. This suggests that cation bridges are formed at regular intervals alo ng adjacent polymers under these conditions. This work demonstrates the uti lity of tapping-mode AFM for examining the structure and gel conformation o f a pliant polymeric matrix. (C) 1999 Elsevier Science Ltd. All rights rese rved.