Imaging an alginate polymer gel matrix using atomic force microscopy

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: α-l-guluronate (G) and β-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, H2O) 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.