FROM PEPTIDES TO PROTEINS: CONTROL OF NET MOLECULAR CHARGE AND HYDROPHILICITY ON THE KINETICS OF CALCITE GROWTH

Many studies have shown that proteins isolated from sites of biomineralization are unusually enriched in the acidic amino acids, notably aspartic and glutamic. This has led to hypotheses that the physiochemical properties of biomolecules influence biomineral growth. Using in situ AFM to measure kinetics of growth in characterized solution compositions, we show that low solution concentrations of aspartate and peptides promote the growth of calcite (CaCO3) by a systematic relation that scales with net molecular charge and hydrophilicity of the biomolecule.

Data analysis shows the degree of enhancement is independent of amino acid sequence but not composition. The relation is general to a range of functional chemistries and explains recent reports that natural proteins isolated from abalone nacre enhance calcite growth by 5-fold over rates measured in the pure system and under constant supersaturation. The rate enhancement arises from increases in the kinetic coefficient. We interpret the mechanism to be a catalytic process whereby biomolecules reduce the diffusive barrier, Ek, by perturbations that displace water molecules. The result is a decrease in the repulsive barrier for attachment of solutes to the solid phase.

The new relationship suggests that peptide model systems may be scaled up to better understand the growth-modifying properties of more complex biomolecules, including proteins. Because impurity signatures are influenced by growth rate, the findings raise the question of whether local biochemistry modulates the contents of minor and trace elements in calcite. Could organism-specific differences in compositional signatures and other vital effects, arise, in-part, from local biochemical differences at sites of mineralization.