Investigation of Gellan Networks and Gels by Atomic Force Microscopy

Abstract

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.

Unassociated Molecular Chains in Physically Crosslinked Gellan Gels

“…Adding KCl, cesium chloride (CsCl) or CaCl 2 facilitate lateral association of gellan double helices. [23][24][25]36,37 The region of lateral association of double helices plays a role of crosslinking domain in gel network structure. Therefore, it is considered that following diffusion of K þ or Ca 2þ into the gels, the double helical gellan chains, which might have been released out in the absence of K þ or Ca 2þ , are associated with the crosslinking domains.…”

“…The gels are completely dissolved when C TMAC ¼ 50 mmol kg À1 at 10 C. It has been revealed from atomic force microscopy measurements that TMA þ prevents lateral association of double helicies. 24,25,34 It is considered that a fraction of the unreleased gellan chains is converted into released gellan chains by disruption of the lateral association of double helical gellan chains following diffusion of TMA þ . It is clear from the results shown in Figures 7 and 8 that the double helical gellan chains, which can be released, are unassociated with the crosslinking domains.…”

“…It has been known that lateral association of gellan double helices is inhibited by transforming counter ions into tetramethylammonium ions (TMA þ ). 7,[23][24][25] Irrespective of the type of crosslinking, some fraction of polymer remains uncrosslinked to gel network. In terms of the gellan physical gel, it seems reasonable to posit that not all of the gellan chains contribute to formation of the gel network.…”

High Acyl Gellan Networks Probed by Rheology and Atomic Force Microscopy

“…It has been established for deacylated gellan that the coil-helix transition leads to the formation of local networks, even without involving inter-helical aggregation, but the mechanical strength of such networks is relatively weak, exhibiting liquid-like characteristics (G′ < G″ ) in the entire frequency range examined (Gunning et al, 1996;Ikeda et al, 2004). In the presence of cations such as potassium, lateral aggregation between helices is enhanced, resulting in the formation of thicker networks exhibiting characteristics typical of strong gels (Gunning et al, 1996). It is thus difficult to explain the existence of an effective yield stress of high acyl gellan network revealed in Fig.…”

Tissue engineering with gellan gum

“…Multiple GG helices then aggregate into junction zones that act as cross-linking sites for the network. This structure was initially resolved by Gunning and Morris using light scattering 55 , and later observed directly using atomic force microscopy [56][57][58] . Importantly, the junction zones of GG networks are strongly stabilised by multivalent cations, commonly Ca 2+ or Mg…”