Single molecule investigation of the onset and minimum size of the calcium-mediated junction zone in alginate

•Ca2+-mediated crosslinking of oligoguluronic acids (“oligoGs”) with dp 6–20 was studied by single molecule force spectroscopy.•The minimum length of an oligoG required to form a stable eggbox junction zone is 8 monomers.•Rapidly forming weak Ca2+-mediated bonds initially outcompete and hinder the slower formation of stable egg box junctions.•This competition between crosslinking modes accounts for the long relaxation times seen during the formation of alginate gels. One of the principal roles of alginate, both natively and in commercial applications, is gelation via Ca2+-mediated crosslinks between blocks of guluronic acid. In this work, single molecule measurements were carried out between well-characterised series of nearly monodisperse guluronic acid blocks (‘oligoGs’) using dynamic force spectroscopy. The measurements provide evidence that for interaction times on the order of tens of milliseconds the maximum crosslink strength is achieved by pairs of oligoGs long enough to allow the coordination of 4Ca2+ ions, with both shorter and longer oligomers forming weaker links. Extending the interaction time from tens to hundreds of milliseconds allows longer oligoGs to achieve much stronger crosslinks but does not change the strength of individual links between shorter oligoGs. These results are considered in light of extant models for the onset of cooperative crosslinking in polyelectrolytes and an anisotropic distribution of oligoGs on interacting surfaces and provide a timescale for the formation and relaxation of alginate gels at the single crosslink level.