Peptide Controls on Calcite Mineralization:  Polyaspartate Chain Length Affects Growth Kinetics and Acts as a Stereochemical Switch on Morphology

Polyaspartate domains are a prominent feature of proteins associated with biogenic carbonates and have been implicated in modifying crystal morphology through specific interactions with step edges. Here, we show that the morphology and growth kinetics of calcite are modified in a systematic way when a series of poly- l -aspartates, Asp 1 - 6, are introduced into solution. In-situ measurements of step propagation rates by atomic force microscopy reveal these effects are strongly chain-length dependent and specific to the crystallographically distinct, obtuse and acute step types. Direct observations of differential roughening and rounding of the step edges demonstrate that, while Asp 1 and Asp 2 have stronger effects on acute step edges, a crossover occurs for the longer Asp 4,5,6 peptides that preferentially affect obtuse steps. Independent analysis of Asp n −step edge interactions by semiempirical quantum mechanical modeling gives estimates of aspartate−step edge binding energies and predicts that the crossover should occur at n = 2. The switch occurs because, upon Asp n binding, the energy required to dehydrate acute steps is greater than that at the obtuse steps when n = 3−6. Step velocity measurements show that the concentration of Asp n needed to stop growth scales exponentially and inversely with the calculated binding energies. A simple model of Asp n adsorption to the steps is derived from these results. These findings suggest a process by which small fluctuations in primary structure in proteins can control calcite shape.