2, New York University, Manhattan, New York, United States
Biomineral-occluded proteins are known to play an important role in precursor phase stabilization, hierarchical structuring, and strengthening of the forming mineral. Obviously, these influences occur at the protein interface with the forming nuclei. Quantifying these effects may help us understanding the mechanisms behind the natural production of biominerals and open the door for more efficient fabrication methods and new engineering tools. One example is the process of S. purpuratus sea urchin spiculogenesis, which exhibits smooth and controlled single crystal growth in its embryonic endoskeleton stage. It is thought that the small amount of protein inclusions (1% w/w) causes these extremely different traits then in-vitro inorganic grown calcite. Unfortunately, our understanding on the specific function of these proteins is very limited. Herein we focus on the spicule matrix (SM)30 protein, which is the most abundant acidic glycoprotein involved in S. purpuratus spiculogenesis, and study the effect of protein concentration on the nucleation kinetics, polymorph selection, and morphology using a state-of-the-art microfluidic chip. Interestingly, our experiments suggest decreasing the protein concentration increases the nucleation rate, and favors vaterite formation. On the other hand, high protein concentration yields exclusively calcite, providing evidence that the protein plays a significant role in polymorph selection. Further studies will reveal the protein location in the forming crystals, giving us additional insight into the role of SM30 in calcium carbonate crystal formation.