Theoretical and experimental study on a self-assembling polysaccharide forming nanochannels: static and dynamic effects induced by a soft confinement.

It is well-known that the polysaccharide scleroglucan (Sclg) exhibits a triple-helix conformation (triplex) and it is able to form hydrogels in water solution. Furthermore, these hydrogels are influenced by the presence of borax, in terms of rheological and drug release properties. In previous works, we showed that the presence of borax stabilizes the intertriplex interactions and that the property variations, induced by borax, can be fully explained, considering that the Sclg triplexes can form nanochannel-like structures. In this paper, the stability of these aggregates has been experimentally studied by means of atomic force microscopy (AFM) and theoretically investigated by means of molecular dynamics (MD) simulations. The simulations indicate that the borax stabilizes nanochannel-like structures when seven triplexes are considered. The simultaneous presence of different Sclg triplexes in a narrow space strongly influences the properties of confined water molecules in a way similar, in many aspects, to that of water molecules located in the inner part of well-defined nanochannels (e.g., diffusion inside carbon nanotubes). As a consequence, also the conformational properties of flanking regions of Sclg triplexes are influenced. Furthermore, differential scanning calorimetry (DSC) data show that the well-known conformational transition occurring at 280 K for Sclg does not take place in the presence of borax. The MD simulations suggest that such lack of transition is a direct consequence of the presence of borax. The role of Na+ counterions in the hydrogel structure is also investigated.