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 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 ), 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, is lowest for nonsulfated controls and increases with DS(SO ). The kinetic prefactor also increases with DS(SO ). Simulations of Ca-HO-chitosan systems show greater water structuring around sulfate groups compared to uncharged substituents, independent of sulfate location. Ca-SO interactions are solvent-separated by distances that are inversely correlated with DS(SO ) of the polysaccharide. The simulations also predict SO and NH groups affect the solvation waters and HCO ions associated with Ca. Integrating the experimental and computational evidence suggests sulfate groups influence nucleation by increasing the difficulty of displacing near-surface water, thereby increasing γ. By correlating γ and net charge per monosaccharide for diverse polysaccharides, we suggest the solvent-separated interactions of functional groups with Ca 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.