NO WAY TO SUGAR-COAT IT: SOLVENT STRUCTURING AT POLYSACCHARIDE-WATER INTERFACE INFLUENCES THE KINETICS OF CALCITE NUCLEATION

Skeletal structures, as well as microbial biominerals, often form within an organic matrix of polysaccharides, proteins, and lipids. An ongoing challenge is to decipher role(s) of the organic matrix during onset and growth of these biocomposites. Advances are stalled by a lack of appropriate materials for hypothesis testing. Studies that extract macromolecules from natural organics are challenged by obtaining & characterizing sufficient amounts & purities of materials from in vivo sites of biomineralization. Commercial hydrogels and gelatins are limited with few compositions of variable purity.

Here, we conduct a hypothesis-based study of structure-function relationships between degree of biomacromolecule sulfation DS(SO₃^-), and the kinetics of CaCO₃ nucleation. Most biomineralization studies of calcification focus on negatively charged carboxyl groups— abundant at sites of CaCO₃ formation. However, sulfate groups, also negatively charged, are likewise widely associated with biomineralization across diverse phyla.

In collaboration with biopolymer chemists, we prepared a series of sulfated chitosan derivatives with characterized DS(SO₃^-) and measured the kinetics of CaCO₃ nucleation onto these materials. Fitting the classical nucleation model to the data reveals interfacial free energy, ϒ_net, of the calcite-polysaccharide-solution system is lowest for non-sulfated controls and increases with DS(SO₃^-). The kinetic prefactor increases with DS(SO₃^-).

Simulations of Ca²⁺-H₂O-chitosan systems using polarizable molecular dynamics show Ca²⁺-SO₃^- interactions are solvent-separated by distances that inversely correlate with total DS(SO₃^-). The combined experimental and computational evidence suggests 1) sulfates regulate nucleation by increasing the difficulty of displacing near-surface water, thereby increasing ϒ_net; and 2) molecules with higher DS(SO₃^-) present more sites for sulfate-Ca²⁺ interactions to increase the kinetic prefactor. This is consistent with studies indicating disrupted water structures about Ca²⁺. The correlation between ϒ_net and multiple types of charged groups suggests solvent-separated interactions with Ca²⁺ influence crystallization similarly. The findings reiterate the role of local water structure and suggest entropic contributions during mineralization at macromolecule-solution interfaces.