Naturally Chiral Surfaces

Chiral surfaces are of growing importance as a result of their potential for use in enantioselective chemical processes. By far the most widely used and commonly studied chiral surfaces are those prepared by templating of achiral surface with chiral organic ligands. It is possible, however, to prepare naturally chiral surfaces by a number of means. This paper describes the various types of chiral surfaces. In addition data are presented to suggest that naturally chiral surface of metals can be prepared by a process of imprinting in which chiral adsorbates induce reconstructions that create chiral kinks on


Introduction
Any two objects which are nonsuperimposable mirror images of one another are chiral. Chirality manifests itself in many different forms and at all length scales throughout nature. Ones' left and right hands are the most familiar examples, however, there are many more. In molecules chirality appears most frequently in the form of carbon atoms that are tetrahedrally coordinated to four different substituents. Two such molecules with different arrangements of their four substituents are nonsuperimposable images of one another. Although these two molecules have many identical properties this seemingly innocuous structural difference can have profound effects when they interact with other chiral molecules or chiral environments. This is critically important in living organisms because almost all bioactive molecules such as proteins and enzymes are chiral and exist in only one enantiomeric form. Thus the left and right handed enantiomers of a chiral compound will have different physiological effects in the human body simply because they interact in different ways with the enzymes and proteins, all of which are of one handedness.
Many pharmaceuticals have structures containing tetrahedrally coordinated carbon atoms that are chiral. As a consequence they exist in left and right handed forms and it is only one of the two enantiomers that is therapeutically active when ingested. More often than not, the other enantiomer is toxic. Thus there are enormous real and regulatory pressures for pharmaceutical manufacturers to produce single enantiomer forms of most pharmaceuticals.
Under most conditions a chemical reaction that produces a chiral product will produce both enantiomers in equal quantities, forming what is known as a racemic mixture. To produce pharmaceuticals that are enantiomerically pure requires chemical processing steps that are enantioselective. These could be synthesis steps that produce only one of the two enantiomers or separation steps that are enantioselective. In either case these enantioselective chemical processes must occur in chiral environments of a single handedness: enantiomerically pure reagents, chiral solvents, enzymes, chiral adsorbents, or chiral catalysts. Furthermore, the chirality of these environments must occur on length scales that are of molecular dimensions in order for the interaction of the reagents to be enantiospecific. Many chemical processing steps involve surfaces as catalytic sites or for adsorption. If these surfaces can be prepared such that they have chiral structures, then catalysis or adsorption on them can be enantiospecific and can lead to enantioselectivity. This paper discusses the possible origins of chirality at the surfaces of solid materials and means for preparing homochiral surfaces. Some such surfaces occur by cleaving of materials that have bulk chiral structures, however, it is also possible for materials such as common catalytic metals which have achiral bulk structures to expose surfaces that are naturally chiral.

Bulk Chiral Crystals
Naturally occurring mineral crystals that do not have a center of symmetry in their bulk structures can expose chiral surfaces. Quartz is the most commonly found chiral material in nature, and its chirality arises from the helical arrangement of SiO 4 tetrahedra in the bulk structure 1 . Since the bulk structure of quartz is chiral its surfaces must be chiral and must exhibit enantiospecific interaction with the two enantiomers of a chiral compound. Bonner et al. examined the adsorption of radioactive D-and L-alanine hydrochloride on D-and L-quartz crystals 2 . The radioactivity of powdered samples of the chiral quartz crystals after exposure to the alanine was used to quantify adsorption of the amino acids onto the quartz surfaces. D-alanine adsorbed preferentially onto the Dquartz while L-alanine adsorbed preferentially onto the L-quartz, with an enantiomeric excess adsorption of between 1.0 and 1.8 percent in each case. Enantioselective adsorption has also been observed on other mineral surfaces with even higher selectivity.

Chirally templated surfaces
The most common method for preparation of chiral surfaces is by adsorption of a form chiral domains with long range order on the Cu(110) surface whose chirality can be observed using low energy electron diffraction 12 . The chiral amino acid S-proline forms an ordered overlayer structure on the Cu(110) surface, but does not exhibit the variety of phases observed for alanine 13 .
Most surface science studies of chirally templated surfaces have been motivated by the desire to understand the behavior of chirally templated heterogeneous catalysts and have focused on the surface chemistry and structure of the templating agent itself.
Although the surfaces described above are chiral, none have yet been used to demonstrate enantioselective surface chemistry. One example of an enantiospecific process that has been studied on a chirally templated single crystal metal surface is the enantioselective chemisorption of propylene oxide on a Pd(111)surface modified by adsorption of chiral 2-butanoxy groups 14  HtBDC persist after the removal of the HtBDC. Thus imprinting appears to be a viable route to the production of naturally chiral metal surfaces.  Nonetheless, these experiments have provided additional evidence of imprinting of chiral structures onto otherwise achiral surfaces and suggest that this is a potential route to the formation of chiral surfaces.

Summary
Chiral surfaces are used in a number of enantioselective chemical processes and will have growing importance in the production of enantiomerically pure materials such as pharmaceuticals. Naturally chiral surfaces can be created from materials with naturally chiral bulk crystal structures or from achiral materials such as metals that are cleaved to expose planes that are naturally chiral. Alternatively, chiral surface can be made from achiral materials by either templating with chiral organic ligands or by imprinting.
Imprinting can create structures on surfaces similar to those exposed by naturally chiral planes.     Desorption Rate (a.u.)

Kink s
Step s