Surfaces enantiomorphism

Chiral surfaces Enantiomorphous crystals Homochiral peptides Mirror symmetry breaking Non-linear kinetics Self-replication of peptides... 

Another hypothesis on homochirality involves interaction of biomolecules with minerals, either at rock surfaces or at the sea bottom thus, adsorption processes of biomolecules at chiral mineral surfaces have been studied. Klabunovskii and Thiemann (2000) used a large selection of analytical data, provided by other authors, to study whether natural, optically active quartz could have played a role in the emergence of optical activity on the primeval Earth. Some researchers consider it possible that enantioselective adsorption by one of the quartz species (L or D) could have led to the homochirality of biomolecules. Asymmetric adsorption at enantiomor-phic quartz crystals has been detected L-quartz preferentially adsorbs L-alanine. Asymmetrical hydrogenation using d- or L-quartz as active catalysts is also possible. However, if the information in a large number of publications is averaged out, as Klabunovskii and Thiemann could show, there is no clear preference in nature for one of the two enantiomorphic quartz structures. It is possible that rhomobohedral... 

Crystals composed of the R and S enantiomers of the same racemic mixture must be related by mirror symmetry in terms of both their internal structure and external shape. Enantiomorphous crystals may be sorted visually only if the crystals develop recognizable hemihedral faces. [Opposite (hid) and (hkl) crystal faces are hemihedral if their surface structures are not related to each other by symmetry other than translation, in which case the crystal structure is polar along a vector joining the two faces. Under such circumstances the hemihedral (hkl) and (hkl) faces may not be morphologically equivalent.] It is well known that Pasteur s discovery of enantiomorphism through die asymmetric shape of die crystals of racemic sodium ammonium tartrate was due in part to a confluence of favorable circumstances. In the cold climate of Paris, Pasteur obtained crystals in the form of conglomerates. These crystals were large and exhibited easily seen hemihedral faces. In contrast, at temperatures above 27°C sodium ammonium tartrate forms a racemic compound. 

The etching of an enantiomorphous pair of asn crystals in the presence of (/ )-aspartic acid and () )-A-methylasparagine is illustrated in Figure 12. Under these conditions, only the R crystals are etched (Figure 12a), whereas the S crystals dissolve smoothly (Figure 12b). The dramatic differences in the surfaces of the two enantiomorphous crystals after dissolution again make it possible to perform a manual Pasteur-type sorting of the R and S crystals with a quantitative enantiomeric yield. 

The observed adsorbate lattice structures show enantiomorphism, that is, adsorption of the right-handed P-heptahehcene (P stands for positive) leads to structures which are mirror images of those observed for M-heptahelicene. This effect can be clearly observed in the high-resolution STM images of Fig. 4.19. Furthermore, the enantiomeric lattices form opposite angles with respect to the [lIO] substrate surface direction. The combined molecule-substrate systems thus exhibit extended... 

If instead of enantiomeric heptahelicene a racemic mixture is sublimated onto the surface, the molecules self-organize in enantiomorphous mirror domains. Thus, as in the case of cysteine on Au(l 10), the surface is stereoselective. 

Careful stepwise crystallization of cobalt acetylacetonate from solutions of the partially resolved chelate produced surprising results (14). A typical experiment is summarized in Table VI. The molecular rotation of the filtrates steadily increased as each crystal crop was removed until no solute remained in solution— at this time all optical activity had, of course, been lost. All crystal crops were racemic It seems that the racemate is being preferentially crystallized from solution and at the same time a surface racemization is taking place to make up the deficient enantiomorph as the d, l crystals are formed. 

The enantiomorph of abequose is colitose, a sugar found in surface antigens of E. coli. 

Keywords 2D self-assembly Chiral surfaces Crystallization Enantiomorphism Molecular monolayers... 

Chiral recognition at surfaces occurs at different levels and in different ways. Chiral expression becomes especially obvious in the formation of chiral motifs. That is, chirality is transferred from the single molecule into a supramolecular enantiomorphous structure. Moreover, enantioselective interactions between identical or different species are decisive for spontaneous resolution or play an important role in cooperative phenomena. 

Fig. 27 NP on Au(lll) forms enantiomorphous homochiral domains. Grey areas represent up-lifted atoms of the reconstructed clean surface. Reprinted in part with permission from [90], Copyright (2003) American Chemical Society...

In addition to the chiral interactions of enantiomers, interactions of dissimilar chiral species are fundamentally important. As described above, adenine becomes chiral upon adsorption and forms homochiral dimer chains on Cu(110). These chains are tilted 19.5° with respect to the [001] surface direction. This tilt makes the sites right next to an enantiomorphous supramolecular chain chiral Consequently, the interactions of inherently chiral molecules with both chain types are energetically nonequivalent. This... 

The coadsorption of chiral molecules into racemic layers is an efficient way to induce further asymmetrization towards single handedness. While in heterogeneous chiral catalysis the stationary ratio of modifier and reactant at the surface is assumed to be one, a small amount of chiral dopant can be sufficient for induction of homochirality on the entire surface SU on Cu( 110), for example, forms two enantiomorphous domains in its bisuccinate phase [27]. 

Similar 2-D enantiomorphous domains were obtained from the non-chiral nucleic acid base adenine deposited on copper [85] and on M0S2 surface [86,87] and the deposition of cysteine on gold [88]. Evidence for strong chiral preference in interactions of nucleic acid bases and amino acids has been shown for the self-assembly of phenylglycine molecules on gold surfaces on which adenine molecules had been previously deposited [89]. 

Fig. 10 a STM images of two-dimensional chiral decamers (denoted L and R) formed by 1-nitronaphthalene molecules on the Au (111) surface at 50 K b Surface with 0.1 mono-layer coverage exhibits about 85% of enantiomorphous decameric clusters (Reproduced from [83], Copyright 1999, Wiley)... 

GIXD studies have demonstrated that racemic N -alkanoyl-lysincs and their corresponding AP-carboxyanhydrides, for example N, -slearoyl-lysinc-NCA, undergo spontaneous segregation of the enantiomers into enantiomorphous 2-D crystalline domains at the surface of water [194], Polymerization reactions within such enantiomorphous crystallites, using nickel acetate as... 

A particular class of modified electrodes consists of those containing a layer of asymmetric compounds, and such electrodes are termed chiral. If one uses these electrodes in organic synthesis, the compound produced may also be asymmetric and optically active. One of the better-known examples of such phenomena is called the Sharpless process (Finn and Sharpless, 1986 Katsuki, 1996). In such processes, the electrode is modified by asymmetric compounds that lead to epoxidation and dihy-droxylation of olefenic compounds, but in an asymmetric form. An example is shown in Fig. 11.5, in which the hydroxylation occurs either on the top or the bottom of the enantiomorphic surface. 

Monolayers of Racemic Mixtures Poly (alanine), Poly (y-benzyl Glutamate), Poly ( -benzyl Aspartate), Poly (Benzyloxy carbonyl Lysine). Experimental Results. The pressure-area and surface potential results for the two enantiomorphic forms of a given polymer were virtually identical except for poly (benzyl aspartate) where the plateau of poly( -benzyl-D-aspartate) was about 2 dynes/cm higher than that of the L enantiomorph (Figure 6). This may result from the incomplete ben-zylation of the o-aspartate. 

The effect of the Bonnet transformation on IPMS is to transform the lattice of catenoidal channels - characteristic of IPMS - into helicoidal strips, through a screw operation on the whole surface. For example, the channels in both the P- and D-surfaces are transformed into spiral tunnels in the g)u oid. Due to its intermediate Bonnet angle with respect to the P- and D-surfaces, it lacks straight lines (2-fold axes) and mirror planes. The labyrinths on both sides are enantiomorphic one labyrinth is left-handed and the other right-handed (Fig. 1.21). 

The substitution of chiral molecules onto the crystal surfaces has been investigated by Mein Lahav, Lia Addadi, Leslie Leiserowitz and co-workers.The chiral molecule is recognized by the molecules already oriented in the crystal lattice. This can be true for either an enantiomorphous crystal composed of molecules with only one chirality sense, or for a centrosymmetric racemic crystal in which a face that is composed only of one enantiomer interacts with a chiral agent. 

The influence of a surface on an adsorbed species is well-accepted. The TA/Ni(l 10) system demonstrates how much the molecule can influence the behaviour of the surface. How far can an adsorbate like tartaric acid induce such effects Work by Switzer and co-workers on the electrodeposition of CuO films in the presence of tartaric acid showed that chirality could be induced in a normally achiral inorganic material [25]. In a standard electrochemical cell, a Au(OOl) crystal is placed in a solution containing Cu(II) ions, tartrate ions and NaOH. At a certain potential, CuO will deposit, as a thin-film on the Au Surface. Characterization by diffraction revealed that the deposited CuO film has no mirror or inversion elements, i.e. it is chiral. The chirality of the film is controlled by the chirality of the tartrate ions in the solution (/ ,/ )-tartrate yielding a chiral CuO(-lll) fihn while presence of (S,S )-tartrate produces the mirror Cu(l-l-l) enantiomorph. Switzer et al, by catalyzing the oxidation of tartaric acid, demonstrate that not only the bulk, but also the surface of the CuO film is chiral the CuO electrode surface grown in the presence of (/ ,/ )-tartrate is more effective at oxidizing (/ ,/ )-TA, while the surface deposited in the presence of (S,S )-tartrate is more effective at oxidizing (S,S )-TA. 

De Feyter S, De Schryver FC (2003) Two-dimensional supramolecular self-assembly probed by scanning tunneling microscopy. Chem Soc Rev 32 393 Ernst K-H (2006) Supramolecular surface chirahty. Top Curr Chem 265 209 Perez-Garcia L, Amabrlino DB (2007) Spontaneous resolution, whence and wither from enantiomorphic solids to chiral liquid crystals, monolayers and macro- and supramolecular polymers and assemblies. Chem Soc Rev 36 941... 

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