Formation of chiral morphologies through selective binding of amino acids to calcite surface steps

Many living organisms contain biominerals and composites with finely tuned properties, reflecting a remarkable level of control over the nucleation, g rowth and shape of the constituent crystals(1-6). Peptides and proteins pla y an important role in achieving this control(1,7,8). But the general view that organic molecules affect mineralization through stereochemical recogni tion, where geometrical and chemical constraints dictate their binding to a mineral, seems difficult to reconcile(4) with a mechanistic understanding, where crystallization is controlled by thermodynamic and kinetic factors(9 ). Indeed, traditional crystal growth models emphasize the inhibiting effec t of so-called 'modifiers' on surface-step growth, rather than stereochemic al matching to newly expressed crystal facets. Here we report in situ atomi c force microscope observations and molecular modelling studies of calcite growth in the presence of chiral amino acids that reconcile these two seemi ngly divergent views. We rnd that enantiomer-specific binding of the amino acids to those surface-step edges that offer the best geometric and chemica l rt changes the step-edge free energies, which in turn results in macrosco pic crystal shape modifications. Our results emphasize that the mechanism u nderlying crystal modification through organic molecules is best understood by considering both stereochemical recognition and the effects of binding on the interfacial energies of the growing crystal.