The interplay between calcite crystal nucleation on polydiacetylene template and its amorphization by phosphoserine

Organisms use a diverse range of organic-inorganic hybrid materials for a variety of purposes, including mechanical support, navigation and protection. These materials are mostly crystalline and are characterized by unique composition, polymorph, crystallite size, shape and crystallographic orientation. The crystalline biominerals are generally formed through amorphous, hydrated transient minerals, but in some, the amorphous phases are stable and persist. Using a biomimetic approach, we address aspects of biological mineralization in vitro and gain insight into the processes and interactions that play roles in the natural systems, in-vivo. In this work, we demonstrate two essential but conflicting methods that are likely to act simultaneously in many mineralizing systems. These are directed crystal nucleation on organic templates, and on the other hand, crystal inhibition to produce the transient amorphous phase. The experimental method in this project mimics aspects of biomineralization processes of calcium carbonate (CaCO3) nucleation. Polydiacetylene (PDA) – a robust, linear conjugated polymer, made from amphiphilic long-chain diacetylene monomers, which upon surface compression, followed by UV polymerization form an ultrathin, stable monolayer structure. PDA simulates the organic template for the CaCO3 crystallization in our experimental system in that it exposed a dense array of acidic groups in well-defined orientation and being a semi-rigid template surface. On PDA templates, calcite crystals nucleate from a (01.2) face and in every single domain of the PDA film they are all coaligned with the crystals' a-axes oriented parallel to the polymer backbone. Supersaturated solutions for CaCO3were prepared either by mixing CaCl2 and Na2CO3, or by bubbling CO2 into CaCO3 suspension, or by slow diffusion of ammonium carbonate into CaCl2 solution in a desiccator. Phosphoserine (P-ser) was added to CaCO3 deposition systems as a crystallization inhibitor, which results in amorphous calcium carbonate (ACC) deposition. The phosphate groups substitute a part of the carbonate groups during the deposition and this way, inhibit the crystallization process. Various concentrations of P-ser in deposition system on PDA templates result in different morphologies and degrees of crystallinity of CaCO3. In this biomimetic system, we demonstrate the conflicting, yet simultaneous influences on biological crystal formation, namely the ordered template and crystal nucleation and crystal inhibition.