Stretching a single sugar chain

Single molecule-stretching experiments have been extensively performed to evaluate the mechanical properties of biomacromolecules including nucleic acids and proteins but very few on polysaccharides. Polygalacturonic acid (PGA), a major pectin component in plant cell walls, plays key roles in plant physiology. In this study, an assay was developed to stretch PGA molecules by single-molecule experiments. The aldehyde of the reduced end of PGA was attached to an aminosilane-coated glass while the other end was biotin-functionalized and attached to a streptavidin-coated microbead. The elasticity of PGA was investigated under wide solution conditions. The persistence length (P) of PGA decreased from 5.8 to 2.8 nm as the concentration of monovalent cations was increased to 4 M. Notably, increasing the concentration of divalent cations first decreased P from 5.8 to 0.97 nm then increased P to 2.1 nm, with a reversal around 200 mM, matching zeta-potential evidence of charge neutralization then charge reversal. Simulations results revealed that Ca²⁺ formed bidentate coordination and induced significant cross-linking of PGA, whereas Mg²⁺, due to its strong hydration shell, bound in a monodentate manner and induced softening but not cross-linking of PGA.