DIRECTION-SPECIFIC BINDING OF AMINO ACIDS ON CALCITE SURFACES

Understanding the interactions of optically active molecules with crystalline surfaces holds a key to comprehending a spectrum of scientific and technological issues related to pharmaceutical synthesis, chemical catalysis, and the design of environmentally friendly materials.  Interactions of amino acids (AAs) with mineral surfaces are of particular interests because of the possible relevance of chiral adsorption to the origin of homochirality in terrestrial biomass.  Previous studies demonstrated the direction-specific nature of the calcite-aspartate (Asp) interactions but left question marks concerning the involvement of functional groups and the geometry of the surface binding.  Unaddressed issues include (1) the roles of the AA side chains and the a-amine group; (2) the effect of electrostatic properties of the side chains; and (3) the coordination of multifunctional groups in surface binding. 

Here we probe the surface binding of AAs on calcite by examining etch pit morphologies revealed in AA-bearing solutions.  Experiments are conducted by using AAs (alanine, glutamic acid, serine, cysteine) and molecules (succinic acid, b-alanine) that differ systematically from Asp in various aspects.  Experimental results show that (1) etch pits retain the inherent rhombohedral morphology in the presence of AAs that have electro-neutral side chains; (2) charged and polar side chains have different morphological effects; (3) the a-amine group exerts strong controls on pit morphology; (4) the separation of carboxyl groups on acidic AAs influences binding directions.

These observations suggest that a side-chain functional group is needed for AAs to show direction-specific binding.  The a-amine group seems critically important for surface binding, but may only be operative in coordination with the other two functional groups.  Combined with the step geometry and the atomic arrangement in the directions preferred by AAs for surface binding, these observations further suggest the binding pattern and binding geometry of AAs on calcite surfaces.