Naturally chiral surfaces

Abstract Enantioselective heterogeneous catalysis requires surfaces with structures that are chiral at the atomic level. It is possible to obtain naturally chiral surfaces from crystalline inorganic materials with chiral bulk structures. It is also possible to create naturally chiral surfaces from achiral materials by exposing surfaces that have atomic stractures with no mirror symmetry planes oriented perpendicular to the surface. Over the past decade there have been a number of experimental and theoretical demonstrations of the enantiospecific physical phenomena and surface chemistry that arise from the adsorption of chiral organic compounds on the naturally chiral, high Miller index places of metals. 

Naturally chiral surfaces can be created from achiral crystalline materials. The bulk structures of many crystalline materials such as metals are highly symmetric, contain one or more mirror symmetry elements and thus, are not chiral. Although it may seem counterintuitive, such achiral bulk structures can, nonetheless, expose surfaces with chiral atomic structures. These are planes whose normals do not lie in one of the bulk mirror planes. The classification of the symmetry of surfaces of a variety of bulk crystal structures has recently been reviewed by Jenkins et al. and they have identified all planes in those crystal structures that are chiral [9,10]. As a simple example consider the two surfaces illustrated in Fig. 4.1. These are the two enantiomers of the (643) surfaces of a face centered cubic lattice. 

Acknowledgments The author s work in the field of naturally chiral surfaces has been supported by the National Science Foundation, the US Department of Energy and by Merck Corp. The author is also indebted to his collaborators D.S. ShoU, E.C. Sykes, S.S. Perry, and P.A. Salvador. The work on this problem in his research group has been performed by C.F. McFadden, P. Cremer, J.D. Horvath, A.J. Koritnik, Y. Huang, V. Pushkarev, L. Baker, and W.C. Cheong. 

Horvath J, Kamakoti P, Koiitnik A, ShoU DS, GeUman A1 (2004) Enantioselective separation on a naturally chiral surface. 1 Am Chem Soc 126 14998... 

The screw models of Figure 7.4 are, of course, an abstraction, and real macromolecules are much more complex. In particular, apart from DNA and G-wires, which have sugar-phosphate backbones as external ridges, but are also charged, most natural and synthetic polymers have external side chains which could generate a secondary chiral surface, which might interfere with the chirality of the polymer backbone.28 Even if the qualitative estimate of Sq based on the models of Figure 7.4 seems rather primitive, the estimate of Hq... 

Up until 1977, the non-covalent polymeric assemblies found in biological membranes rarely attracted any interest in supramolecular organic chemistry. Pure phospholipids and glycolipids were only synthesized for biophysical chemists who required pure preparations of uniform vesicles, in order to investigate phase transitions, membrane stability and leakiness, and some other physical properties. Only very few attempts were made to deviate from natural membrane lipids and to develop defined artificial membrane systems. In 1977, T. Kunitake published a paper on A Totally Synthetic Bilayer Membrane in which didodecyl dimethylammonium bromide was shown to form stable vesicles. This opened the way to simple and modifiable membrane structures. Since then, organic chemists have prepared numerous monolayer and bilayer membrane structures with hitherto unknown properties and coupled them with redox-active dyes, porous domains and chiral surfaces. Recently, fluid bilayers found in spherical vesicles have also been complemented by crystalline mono-... 

Broadly speaking, there are two types of chiral surfaces those that are templated [2] and those that are naturally chiral. The most commonly used and studied chiral surfaces are those that are templated with chiral organic ligands. These are achiral substrates of any solid that have been modified by the adsorption of a chiral organic compound. The presence of the chiral ligand renders the local structure chiral, and... 

In this crystal lattice system, all surfaces with Miller indices, (hkl), satisfying the conditions h x k x 1 and h k l h are chiral [11]. Although such high Miller index surfaces have been studied for decades, it was not until recently that McFadden et al. specifically pointed out and demonstrated that their low synunetry structures render them chiral and, therefore, that they might have enantiospecific interactions with chiral adsorbates [12]. There has been a growing interest in the enantiospecific properties of naturally chiral metal surfaces and in the possibility of using such surfaces for enantioselective chemical processes. 

The thermal roughening of naturally chiral metal surfaces must have some impact on their enantiospecific interactions with chiral adsorbates. Sholl et al. have also studied the effects of step roughening on the adsorption of small chiral molecules [11, 17]. Molecular simulation of small chiral alkanes adsorbed on ideal and roughened Pt(643) surfaces revealed that enantioselectivity is preserved during... 

The enantioselectivities of reactions on chiral surfaces are of interest from a practical standpoint and are the result of enantiospecific differences in reaction energetics and reaction barriers. Another manifestation of the enantiospecific interaction between a chiral adsorbate and a chiral surface is adsorbate orientation. Enantiospecific orientations of chiral adsorbates on naturally chiral metal surfaces have been predicted by molecular simulation studies. The first studies using Monte Carlo methods to study chiral cycloalkanes adsorbed on chiral surfaces pre-... 

Shell DS, Asthagiri A, Power TD (2001) Naturally chiral metal surfaces as enantiospecific adsorbents. 1 Phys Chem B 105 4771... 

Power TD, Asthagiri A, ShoU DS (2002) Atomically detailed models of the effect of thermal roughening on the enantiospedficity of naturally chiral platinum surfaces. Langmuir 18 3737... 

Power TD, ShoU DS (2002) Effects of surface relaxation on enantiospecific adsorption on naturally chiral Pt surfaces. Top Catal 18 201... 

The preparation of the conventional chiral stationary phases can be realized both by surface immobilization of natural chiral selectors (polysaccharides, proteins) on chromatographic supports, and by direct synthesis of stationary phases composed of polyacrylates with pendant chiral groups, amides, or helical polymers. Even if they are quite expensive and poorly resistant to chemical and biological attack, these materials are largely used to separate racemic mixtures for preparative and... 

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