The Effect of Amino Acids on the Formation of Amorphous Calcium Carbonate Nanoparticles

Numerous calcifying marine organisms build biominerals through crystallization by particle attachment of amorphous calcium carbonate (ACC), a process that requires a closely controlled nanoparticle size distribution. Using synchrotron‐based in situ small‐angle X‐ray scattering (SAXS) combined with a stopped‐flow device, the formation of synthetic magnesium‐stabilized ACC (Mg‐ACC) nanoparticles in the presence of amino acids is tracked with a 10 ms time resolution. It is shown that amino acids narrow the Mg‐ACC size distribution by modulating the growth stage following initial nanoparticle formation. Temperature‐dependent kinetic analysis indicates that this effect arises from kinetic regulation rather than a thermodynamic shift. Specifically, amino acids adsorb to the particle–water interface, likely impeding monomer addition and selectively slowing the growth of older particles, resulting in a more uniform size distribution. While previous studies linked size control only to larger oligomeric additives, it is demonstrated that even small, monomeric amino acids can significantly influence Mg‐ACC nanoparticle growth. This effect is particularly pronounced under low‐supersaturation conditions, as commonly encountered in biomineralization. Moreover, it is shown that pH‐induced broadening of the size distribution can be mitigated by amino acid additives. These findings suggest that organic molecules may help biomineralizing organisms maintain particle‐attachment‐based mineralization under environmentally variable pH conditions.