Investigation of gellan networks and gels by atomic force microscopy

Methods previously developed for the atomic force microscopy (AFM) imaging of individual polysaccharides (Kirby, A. R.; Gunning, A. P.; Morris, V. J. Biopolymers 1996, 38, 355-366) have been extended in order to image networks and gels formed by the bacterial polysaccharide gellan gum. Networks were formed by air-drying solutions of K+ gellan onto freshly cleaved mica. AFM images were obtained in the direct current contact mode at constant force under butanol. Network formation can be inhibited by diluting the stock gellan solution or by converting K+ gellan into the tetramethylammonium salt form. Inhibition of network formation led to AFM images of heterogeneous populations of gellan aggregates (gel precursors). Attempts have been made to image the surface of bulk aqueous gellan gels under butanol. The quality of the images obtained improved with increasing gel modulus. For rigid acid-set gellan gels, molecular resolution was achieved, revealing a bifurcated branched fibrous network.