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Force spectroscopy of marine biopolymers assembled into gel network

Marine gel biopolymers were recently visualized at the molecular level using atomic force microscopy (AFM) to reveal fine fibril-forming networks with low to high degrees of cross-linking (Figure 1). Due to the inherent complexity and heterogeneity of the marine gel phase it is difficult to isolate the physical forces in the biopolymer network assemblies. In this work, we used force spectroscopy to quantify the intra- and intermolecular forces within the marine gel network. Combining force measurements with AFM imaging allowed us to identify the microscopic origins of distinct mechanical responses. At the single fibril level we uncovered force-extension curves that resemble those of individual polysaccharide fibrils. They exhibit entropic elasticity followed by extensions associated with chair-to-boat transitions specific to the type of polysaccharide at high forces. Surprisingly, a low degree of cross-linking led to sawtooth patterns that are for the first time attributed to the unraveling of polysaccharide entanglements. At a high degree of cross-linking, we observed force plateaus that arise from the unzipping of helical bundles. Finally, the complex 3D network structure gave rise to force staircases of increasing height that may correspond to the hierarchical peeling of fibrils away from the junction zones. In addition, we showed that these diverse mechanical responses also arise in reconstituted polysaccharide gels, which highlights their dominant role in the mechanical architecture of marine gels.

marine gel; polysaccharides; self-assembly; force spectroscopy; AFM

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