Visualization of the algal polysaccharide carrageenan by scanning tunnelling microscopy

Scanning tunnelling microscopy has been used to obtain images in the constant-current mode in air and moist conditions at molecular resolution for the kappa- and iota-carrageenan algal polysaccharides. The molecules were deposited from an aqueous solution onto a graphite substrate. The samples formed aligned nematic-like arrays and were also found as individual molecules. The molecular dimensions of width, height and repeat distance along the molecule were found to be close to the values previously determined by X-ray diffraction. The results support a molecular model based on a double-helix structure for carrageenan.