Date Published: July 21, 2017
Publisher: Public Library of Science
Author(s): Levan Tsereteli, Andrea Grafmüller, Danilo Roccatano.
Computational models can provide detailed information about molecular conformations and interactions in solution, which is currently inaccessible by other means in many cases. Here we describe an efficient and precise coarse-grained model for long polysaccharides in aqueous solution at different physico-chemical conditions such as pH and ionic strength. The Model is carefully constructed based on all-atom simulations of small saccharides and metadynamics sampling of the dihedral angles in the glycosidic links, which represent the most flexible degrees of freedom of the polysaccharides. The model is validated against experimental data for Chitosan molecules in solution with various degree of deacetylation, and is shown to closely reproduce the available experimental data. For long polymers, subtle differences of the free energy maps of the glycosidic links are found to significantly affect the measurable polymer properties. Therefore, for titratable monomers the free energy maps of the corresponding links are updated according to the current charge of the monomers. We then characterize the microscopic and mesoscopic structural properties of large chitosan polysaccharides in solution for a wide range of solvent pH and ionic strength, and investigate the effect of polymer length and degree and pattern of deacetylation on the polymer properties.
Chitin is one of the most abundant natural biopolymers and the most abundant amino-polysaccharide on the planet . Its main derivative, Chitosan, is produced by N-deactylation of the 2-acetamide-2-deoxy-β-d-glucopyranose (GlcNac) monomers. Thus Chitosan is composed of (1-4) linked units of 2-amino-2-deoxy-β-d-glycopyranose (GlcN) as well as, because deacetylation is never complete, GlcNac monomers. Typically, the polymers are referred to as chitosan if the degree of deacetylation (DD) is larger than 50%. The amino groups of the deacetylated monomers can be protonated in mildly acidic conditions, making the polymers soluble and Chitosan one of the rare cationic polymers . Due to the weakly ionic nature of the GlcN monomers, which exist either as neutral GlcNH2 or positively charged GlcNH3+, the DD, as well as the pysico-chemical conditions of the solution strongly influence polymer solubility, size, flexibility and aggregation [3–7].
Now we systematically investigate, how different factors affect the electrostatic and structural properties of Chitosan polymers in solution.
We present an accurate CG model for equilibrium properties of Chitosan polyelectrolytes in solution, based on the free energy landscapes of the glycosidic bonds and titration of GlcNH2 monomers. We show that small variations in the free energy maps for the glycosidic bonds can have a large effect on polymer flexibility, leading to a large range of intrinsic persistence lengths, for the different monomer combinations. As a consequence, in the titration moves, we take into account the changing protonation state of the monomers, by updating the corresponding free energy map.