THE ROLE OF POLYSACCHARIDE SPECIFICITY IN A TWO-STEP CRYSTALLIZATION PROCESS
Coccolithophores are unicellular algae that form intricate arrays of calcium carbonate nanocrystals. The regulation of this biomineralization process is mediated by negatively-charged polysaccharides (PS). The most abundant polysaccharide consists of dimers of glucuronic acid and meso-tartrate and glyoxylate with a charge density of -4 eu. Recently, it was suggested that prior to calcite nucleation, a dense phase of Ca and PS is formed at the site of crystallization. Even though this dense Ca-rich phase has pivotal role in calcite crystallization, its chemical nature and affinity to specific locations are unclear. Here, we compare the complex biogenic macromolecules with three synthetic negatively-charged polymers, each containing a single acidic functional group per monomer. We use Dynamic Light Scattering (DLS) in addition to SEM to characterize the interactions of the polymers with calcium ions and the surface of the organic template.
The DLS measurements show that all polymers form a dense particulate phase in the presence of calcium, in agreement with phase separation theory. However, while the size of the biogenic PS – Ca particles was independent of Ca concentration, complexes of Ca and poly-acrylic acid (PAA) gradually increased with increasing concentrations of both Ca and PAA. Calcium - poly-styrene sulfonate and Ca - poly-aspartic acid complexes were an order of magnitude smaller than either Ca-PS or Ca-PAA complexes, and showed little sensitivity to reagent concentrations. In a second step, we investigated the specificity of the polymer - Ca complexes to the template surface. The results show that the localization of Ca – PS particles around the baseplate rim was mimicked only with PAA, although with different morphology. We conclude that the chemical interactions facilitating the formation of the Ca-rich dense phases prior to crystallization are complex and require more than simple electrostatics. In the future we will further explore the intriguing behavior of these dense liquid-like phases.