CHIRAL ADSORPTION OF AMINO ACIDS ON CALCITE AND QUARTZ: IMPLICATIONS FOR THE ORIGINS OF LIFE’S HOMOCHIRALITY

Strong chiral preference for amino acids, sugars and other essential biomolecules is a defining characteristic of biological systems. Abiotic chiral selection processes are thus central to geochemical models of life’s origin and evolution. Chiral recognition and separation of molecules, furthermore, is vital to the pharmacological activity of many drugs, the biodegradation of packaging materials, the development of improved polymers, and many other applications in science and industry. The ability of some inorganic crystalline surfaces to adsorb selectively chiral molecules provides insights to the origins of biochemical homochirality, as well as a promising avenue for the development of industrial processes for chiral purification. Common rock-forming minerals provide a rich variety of chiral solid surfaces for study. Our experiments demonstrate molecular selectivity on crystal growth faces of calcite and quartz. Chiral surfaces of calcite, for example, selectively adsorb D- and L-aspartic acid, as well as other amino acids. Adsorption is strongly influenced by the difference between the mineral surface’s point of zero charge (pH_pzc) and the molecule’s isoelectric point (pI). We find, for example, that quartz crystal surfaces (pH_pzc ~ 2.9) strongly adsorb lysine (pI ~ 9.7) compared to other amino acids with lower pI. Mineral surfaces thus had the potential to select and concentrate molecular species in prebiotic environments.