Insights into Mollusk Shell Formation: Interlamellar and Lamellar-Specific Nacre Protein Hydrogels Differ in Ion Interaction Signatures.

In the mollusk shell nacre layer, there exist hydrogelator proteomes that play important roles in the formation of the mineral phase. Two of these proteomes, the intracrystalline and the framework, reside in the interior and exterior, respectively, of the nacre tablets. To date there is no clear evidence of what distinguishes an intracrystalline protein from a framework protein regarding the nucleation process. Using Eu(III), phosphate anions, and recombinant versions of the intracrystalline protein, AP7 and the framework protein, n16.3 we probed each protein hydrogel for its interactions with these model ions. Fluorescence spectroscopy of Eu(III) interactions with both protein hydrogels revealed that r-AP7 exhibited enhanced effects on Eu(III) fluorescence compared to r-n16.3, and, 31 P NMR experiments demonstrated that r-AP7 had a more significant impact on phosphate anions compared to r-n16.3. Thus, r-AP7 was found to be more of an ion "disruptor" than r-n16.3. Interestingly, these findings correlate with the particle size distributions and internal structure of the hydrogel particles themselves, suggesting that the physical and chemical properties of the hydrogels dictate hydrogel-ion interactions. In conclusion, we confirm that hydrogelator proteomes possess distinguishable ion interaction properties that may impact the nucleation processes in these regions and control the overall formation of mesoscale nacre tablets.