  Among hydrocolloids, gellan is one of the most used anionic polysaccharides,
because of its capability of forming mechanically stable gels at relatively low
concentrations. Despite its long-standing use and importance, the gellan
aggregation mechanism is still not presently understood at the microscopic
level due to the lack of atomistic information. Here we will fill this gap by
reporting molecular dynamics simulations of gellan chains at different polymer
and salt contents, being able to unveil the occurrence of the two steps in the
process, in agreement with existing hypotheses. At first, the formation of
double helices takes place, followed by the aggregation into super-structures.
For both steps, the role of bivalent cations appears to be crucial, as also
shown by rheology and atomic force microscopy measurements: they not only
facilitate the junction of the chains into double helices, but also promote
through bridging their arrangement into larger aggregates. On the other hand,
monovalent cations have a much more reduced role, making it possible to form
double helices only at very high salt content and not actively participating in
the formation of gels. Our simulations thus offer the first complete
microscopic overview of gellan aggregation and will be important for future use
of gellan-based systems for a wide variety of applications, ranging from food
science to art restoration.
