Imaging xanthan gum by atomic force microscopy

Scanning probe microscopes (SPM) have revolutionised microscopy. Both the scanning tunnelling microscope (STM) and the atomic force microscope (AFM) have been used to image biopolymers. Scanning probe microscopes offer the prospect of superior resolution to that obtained using present electron microscopy (EM) methods. Images can be obtained under near 'native' conditions, thus avoiding the harsh and damaging preparative and imaging procedures used in EM studies. Most of the early SPM studies of biological macromolecules were made using STM. Despite early success, biological STM has now encountered a number of problems which have dampened the early enthusiasm for the technique: the imaging mechanism for large macromolecules is unknown, and certain substrate features (particularly on graphite) can mimic biopolymers giving rise to imaging artifacts. In addition, imaging is generally not reproducible, probably because of tip-molecule interactions which sweep the molecules outside the field of view. Many of these difficulties are emphasised in the small number of papers reporting STM studies of polysaccharides. The quality of the images previously presented has been disappointing, and most of the studies have not been reproduced by other workers. Three groups have reported STM studies of the bacterial polysaccharide xanthan gum. Miles and co-workers presented a single STM image of xanthan molecules deposited from aqueous solution and then air-dried onto a graphite substrate. The image contains features consistent with the molecular structure of xanthan but other researchers have been unable to reproduce such data using the same preparations and imaging conditions. Gunning and co-workers obtained STM images of xanthan after depositing the molecules from aligned, concentrated 'liquid crystalline' preparations which were then air-dried onto graphite. The images revealed aggregates containing bundles of aligned xanthan molecules. The aggregation made resolution of individual molecules difficult.