Kinetics of Calcite Nucleation onto Sulfated Chitosan Derivatives and Implications for Water-Polysaccharide Interactions during Crystallization of Sparingly Soluble Salts.

Anionic macromolecules are found at sites of CaCO(3) biomineralization in diverse organisms, but their roles in crystallization are not well-understood. We prepared a series of sulfated chitosan derivatives with varied positions and degrees of sulfation, DS(SO(3) (-)), and measured calcite nucleation rate onto these materials. Fitting the classical nucleation theory model to the kinetic data reveals the interfacial free energy of the calcite-polysaccharide-solution system, γ(net), is lowest for nonsulfated controls and increases with DS(SO(3) (-)). The kinetic prefactor also increases with DS(SO(3) (-)). Simulations of Ca(2+)-H(2)O-chitosan systems show greater water structuring around sulfate groups compared to uncharged substituents, independent of sulfate location. Ca(2+)-SO(3) (-) interactions are solvent-separated by distances that are inversely correlated with DS(SO(3) (-)) of the polysaccharide. The simulations also predict SO(3) (-) and NH(3) (+) groups affect the solvation waters and HCO(3) (-) ions associated with Ca(2+). Integrating the experimental and computational evidence suggests sulfate groups influence nucleation by increasing the difficulty of displacing near-surface water, thereby increasing γ(net). By correlating γ(net) and net charge per monosaccharide for diverse polysaccharides, we suggest the solvent-separated interactions of functional groups with Ca(2+) influence thermodynamic and kinetic components to crystallization by similar solvent-dominated processes. The findings reiterate the importance of establishing water structure and properties at macromolecule-solution interfaces.