Enantioselectivity on Surfaces with Nanoscale Chiral Structures
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Chirality is a critical property of amino acids that form proteins, the building blocks of life, and of DNA, which encodes the genetic traits passed from one generation to the next. Furthermore, chirality can be observed throughout nature in objects ranging from spiral seashells to the human hands. If an object and its mirror image are nonsuperimposable, then they have the property of chirality. This property is found in nearly every biological molecule and in many synthetic bioactive molecules such as pharmaceuticals and agrochemicals. From a practical perspective, the importance of molecular chirality arises from the fact that the two mirror images of a chiral molecule, known as enantiomers, can have vastly different physiological impacts when ingested by living organisms. As a result, it is necessary to produce many chiral compounds in enantiomerically pure form. Understanding chirality and enantioselectivity is critical to the development of the separations and reactions used to achieve enantiopurity.
Many processes used in chemical production involve solid surfaces and can be made enantioselective by using chiral surfaces. There are three common types of chiral surfaces: surfaces modified by chiral organic adsorbates, surfaces of any naturally chiral bulk crystalline solid, and chiral surfaces prepared from achiral bulk crystals. A number of studies have shown that such chiral surfaces exhibit enantiospecific properties when exposed to chiral species in either the gas phase or in solution. Although chirality is simply a symmetry property of an object or an extended lattice, the enantiospecificity of surfaces is derived from the local structure of nanoscale features such as molecular adsorbates, or specific arrangements of atoms at a crystal surface. Experimental and theoretical work to elucidate the properties of these nanostructured surfaces is an active area of research in surface chemistry. Although a number of aspects of the enantioselectivity of chiral surfaces are now understood, many unanswered questions remain as topics of ongoing research in the field.