Enantioselective Separation on Naturally Chiral Metal Surfaces:d,l-Aspartic Acid on Cu(3,1,17)R&SSurfaces

Kink-Controlled Gold Nanoparticles for Electrochemical Glucose Oxidation

Enzymes in nature efficiently catalyze chiral organic molecules by elaborately tuning the geometrical arrangement of atoms in the active site. However, enantioselective oxidation of organic molecules by heterogeneous electrocatalysts is challenging because of the difficulty in controlling the asymmetric structures of the active sites on the electrodes. Here, we show that the distribution of chiral kink atoms on high-index facets can be precisely manipulated even on single gold nanoparticles; and this enabled stereoselective oxidation of hydroxyl groups on various sugar molecules. We characterized the crystallographic orientation and the density of kink atoms and investigated their specific interactions with the glucose molecule due to the geometrical structure and surface electrostatic potential.

Molecular Origins of Chiral Amplification on an Achiral Surface: 2D Monolayers of Aspartic Acid on Cu(111)

Recent experiments have demonstrated an intriguing phenomenon in which adsorption of a nonracemic mixture of aspartic acid (Asp) enantiomers onto an achiral Cu(111) metal surface leads to autoamplification of surface enantiomeric excess, ee_s, to values well above those of the impinging gas mixtures, ee_g. This is particularly interesting because it demonstrates that a slightly nonracemic mixture of enantiomers can be further purified simply by adsorption onto an achiral surface. In this work, we seek a deeper understanding of this phenomena and apply scanning tunneling microscopy to image the overlayer structures formed by mixed monolayers of d- and l-Asp on Cu(111) over the full range of surface enantiomeric excess; ee_s = -1 (pure l-Asp) through ee_s = 0 (racemic dl-Asp) to ee_s = 1 (pure d-Asp). Both enantiomers of three chiral monolayer structures are observed. One is a conglomerate (enantiomerically pure), another is a racemate (equimolar mixture of d- and l-Asp); however, the third structure accommodates both enantiomers in a 2:1 ratio. Such solid phases of enantiomer mixtures with nonracemic composition are rare in 3D crystals of enantiomers. We argue that, in 2D, the formation of chiral defects in a lattice of one enantiomer is easier than in 3D, simply because the stress associated with the chiral defect in a 2D monolayer of the opposite enantiomer can be dissipated by strain into the space above the surface.

2D Ising Model for Enantiomer Adsorption on Achiral Surfaces: L- and D-Aspartic Acid on Cu(111)

The 2D Ising model is well-formulated to address problems in adsorption thermodynamics. It is particularly well-suited to describing the adsorption isotherms predicting the surface enantiomeric excess, ees, observed during competitive co-adsorption of enantiomers onto achiral surfaces. Herein, we make the direct one-to-one correspondence between the 2D Ising model Hamiltonian and the Hamiltonian used to describe competitive enantiomer adsorption on achiral surfaces. We then demonstrate that adsorption from racemic mixtures of enantiomers and adsorption of prochiral molecules are directly analogous to the Ising model with no applied magnetic field, i.e., the enantiomeric excess on chiral surfaces can be predicted using Onsager’s solution to the 2D Ising model. The implication is that enantiomeric purity on the surface can be achieved during equilibrium exposure of prochiral compounds or racemic mixtures of enantiomers to achiral surfaces.

Chiral Recognition on Bare Gold Surfaces by Quartz Crystal Microbalance

Quartz crystal microbalance (QCM) is one of the powerful tools for the studies of molecular recognition and chiral discrimination. Its efficiency mainly relies on the design of the functional sensitive layer on the electrode surface. However, the organic sensitive layer may easily cause dissipation of oscillation or detachment and weaken the signal transfer during the molecular recognition processes. In this work, we reveal for the first time that the bare metal surface without the organic selector layer has the capability for chiral recognition in the QCM system. During the adsorption of various chiral amino acids, relatively higher selectivity of D-enantiomers on gold (Au) surface was shown by the QCM detection. Based on analyses of the surface crystalline structure and density functional theory calculations, we demonstrate that the chiral nature of Au surface plays an important role in the selective binding of specific D-amino acids. These results may open new insights on chiral detection by QCM system. It will also promote the construction of novel chiral sensing systems with both efficient detection and separation capability.

Benzoate ester as a new species for supramolecular chiral assembly

In this work, a benzoate ester molecule, dodecamethylnonacosane-2,28-diyl dibenzoate (DMNDB), has been discovered as a new species that aggregates into chiral nano-assemblies. In the tetrahydrofuran (THF)/water system, the benzoate ester, DMNDB, could self-assemble into left-handed twisted nanowires, and the most suitable THF/water volume ratio to obtain uniform twisted nanowires was 3 : 7. The driving forces of assembly and the molecular packing type in assemblies for the twisted nanowires were explored, and a possible assembly mechanism was proposed to understand the generation of chiral assemblies. Interestingly, the left-handed nanowires could cross-link and immobilize the solvent in the isopropanol (iPrOH)/water (2 : 8) system to form chiral gels. When the iPrOH/water ratio was increased to 6 : 4, the left-handed nanowires as structural units were found to evolve to right-handed nanofibers. Accordingly, the intermolecular interactions and the molecular packing type also changed with the solvent ratio. What is more, the xerogel could be obtained by drying the gel and left-handed twisted nanowires could form in the THF/water system again, showing the recyclability of chiral nanoassemblies. Also, these DMNDB chiral nanostructures exhibited potential for application in enantioselective separation by co-assembling with tetra-aniline.

2D Ising Model for Adsorption-induced Enantiopurification of Racemates

Mechanisms for the spontaneous transformation of achiral chemical systems into states of enantiomeric purity have important ramifications in modern pharmacology and potential relevance to the origins of homochirality in life on Earth. Such mechanisms for enantiopurification are needed for production of chiral pharmaceuticals and other bioactive compounds. Previously proposed chemical mechanisms leading from achiral systems to near homochirality are initiated by a symmetry-breaking step resulting in a minor excess of one enantiomer via statistical fluctuations in enantiomer concentrations. Subsequent irreversible processes then amplify the majority enantiomer concentration while simultaneously suppressing minority enantiomer production. Herein, equilibrium adsorption of amino acid enantiomer mixtures onto chiral and achiral surfaces reveals amplification of surface enantiomeric excess relative to the gas phase; i. e. enantiopurification of chiral adsorbates by adsorption. This adsorption-induced amplification of enantiomeric excess is shown to be well-describe by the 2D Ising model. More importantly, the 2D-Ising model predicts formation of homochiral monolayers from adsorption of racemic mixtures or prochiral molecules on achiral surfaces; i. e. enantiopurification with no apparent chiral driving force.

Chiral Surface and Geometry of Metal Nanocrystals

Chirality is a basic property of nature and has great importance in photonics, biochemistry, medicine, and catalysis. This importance has led to the emergence of the chiral inorganic nanostructure field in the last two decades, providing opportunities to control the chirality of light and biochemical reactions. While the facile production of 3D nanostructures has remained a major challenge, recent advances in nanocrystal synthesis have provided a new pathway for efficient control of chirality at the nanoscale by transferring molecular chirality to the geometry of nanocrystals. Interestingly, this discovery stems from a purely crystallographic outcome: chirality can be generated on high-Miller-index surfaces, even for highly symmetric metal crystals. This is the starting point herein, with an overview of the scientific history and a summary of the crystallographic definition. With the advance of nanomaterial synthesis technology, high-Miller-index planes can be selectively exposed on metallic nanoparticles. The enantioselective interaction of chiral molecules and high-Miller-index facets can break the mirror symmetry of the metal nanocrystals. Herein, the fundamental principle of chirality evolution is emphasized and it is shown how chiral surfaces can be directly correlated with chiral morphologies, thus serving as a guide for researchers in chiral catalysts, chiral plasmonics, chiral metamaterials, and photonic devices.

Chiral metal surfaces for enantioselective processes

Chiral surfaces are critical components of enantioselective heterogeneous processes such as those used to prepare enantiomerically pure pharmaceuticals. While the majority of chiral surfaces in practical use are based on achiral materials whose surfaces have been modified with enantiomerically pure chiral adsorbates, there are many inorganic materials with valuable surface properties that could be rendered enantiospecific, if their surfaces were intrinsically chiral. This Perspective discusses recent developments in the fabrication of intrinsically chiral surfaces exhibiting enantiospecific adsorption, surface chemistry and electron emission. We propose possible paths to the scalable fabrication of high-surface-area, enantiomerically pure surfaces and discuss opportunities for future progress.

Theoretical investigation of amino-acid adsorption on hydroxylated quartz surfaces: dispersion can determine enantioselectivity

Chiral mineral surfaces, such as quartz, are attractive substrates for use in enantioselective separation and may have contributed to the origin of biological homochirality. In this work, we apply density-functional theory and the exchange-hole dipole moment (XDM) dispersion model to study the adsorption of 5 amino acids (glycine, serine, alanine, valine, and phenylalanine) on a hydroxylated α-quartz (0001) surface. It is demonstrated that London dispersion is responsible for 30-50% of the total adsorption energies and its inclusion or omission can reverse predictions of enantioselectivity. Differing dispersion stabilization, caused by the opposing side-chain placements relative to the quartz surface, lead to differences of 1.0 and 1.8 kcal mol^-1 in the adsorption energies of the alanine and phenylalanine enantiomers, respectively. These results are consistent with a 3-point model, with the hydrogen-bonding sites conserved and variations in the dispersion interactions determining enantioselectivity.

A Most Enantioselective Chiral Surface: Tartaric Acid on All Surfaces Vicinal to Cu(110)

Enantioselective chemistry on intrinsically chiral surfaces is the quintessential form of structure-sensitive surface chemistry, arising purely from the dissymmetry of the surface structure. Identification or design of chiral surface structures that maximize enantioselectivity for a given processes is extremely challenging because of the limited magnitude of the enantiospecific interaction energetics of chiral molecules with chiral surfaces. Using spherical Cu single crystals exposing surfaces with a continuous two-dimensional distribution of crystallographic orientations, we mapped the enantiospecific surface reaction kinetics of tartaric acid decomposition across the surface orientation space. These measurements reveal both the mechanistic origin of enantioselectivity and identify the structural features of the most enantiospecific surface orientation.

Kinetics and Mechanism of Aspartic Acid Adsorption and Its Explosive Decomposition on Cu(100)

The mechanism and kinetics of aspartic acid (Asp, HO₂CCH(NH₂)CH₂CO₂H) decomposition on Cu(100) have been studied using X-ray photoemission spectroscopy and temperature-programmed reaction spectroscopy. We investigate the Asp decomposition mechanism in detail using unlabeled d-Asp and isotopically labeled l-Asp-4-¹³C (HO₂CCH(NH₂)CH₂¹³CO₂H), l-Asp- d₇ (DO₂CCD(ND₂)CD₂CO₂D), l-Asp-2,3,3- d₃ (HO₂CCD(NH₂)CD₂CO₂H), and l-Asp-¹⁵N-2,3,3- d₃ (HO₂CCD(¹⁵NH₂)CD₂CO₂H). The monolayer of Asp adsorbed on the Cu(100) surface is in a doubly deprotonated bi-aspartate form (-O₂CCH(NH₂)CH₂CO₂-). During heating, Asp decomposes on Cu(100) with kinetics consistent with a vacancy-mediated explosion mechanism. The mechanistic steps yield CO₂ by sequential cleavage of the C3-C4 and C1-C2 bonds, and N≡CCH₃ and H₂ via decomposition of the remaining CH(NH₂)CH₂ intermediate. Deuterium labeling has been used to demonstrate that scrambling of H(D) occurs during the decomposition to acetonitrile of the CD(NH₂)CD₂ intermediate formed by decarboxylation of l-Asp-2,3,3- d₃ and l-Asp-¹⁵N-2,3,3- d₃.

Enantiomer surface chemistry: conglomerate versus racemate formation on surfaces

Research on surface chirality is motivated by the need to develop functional chiral surfaces for enantiospecific applications. While molecular chirality in 3D has been the subject of study for almost two centuries, many aspects of 2D chiral surface chemistry have yet to be addressed. In 3D, racemic mixtures of chiral molecules tend to aggregate into racemate (molecularly heterochiral) crystals much more frequently than conglomerate (molecularly homochiral) crystals. Whether chiral adsorbates on surfaces preferentially aggregate into heterochiral rather than homochiral domains (2D crystals or clusters) is not known. In this review, we have made the first attempt to answer the following question based on available data: in 2D racemic mixtures adsorbed on surfaces, is there a clear preference for homochiral or heterochiral aggregation? The current hypothesis is that homochiral packing is preferred on surfaces; in contrast to 3D where heterochiral packing is more common. In this review, we present a simple hierarchical scheme to categorize the chirality of adsorbate-surface systems. We then review the body of work using scanning tunneling microscopy predominantly to study aggregation of racemic adsorbates. Our analysis of the existing literature suggests that there is no clear evidence of any preference for either homochiral or heterochiral aggregation at the molecular level by chiral and prochiral adsorbates on surfaces.

Chirality in adsorption on solid surfaces

In the present review we survey the main advances made in recent years on the understanding of chemical chirality at solid surfaces. Chirality is an important topic, made particularly relevant by the homochiral nature of the biochemistry of life on Earth, and many chiral chemical reactions involve solid surfaces. Here we start our discussion with a description of surface chirality and of the different ways that chirality can be bestowed on solid surfaces. We then expand on the studies carried out to date to understand the adsorption of chiral compounds at a molecular level. We summarize the work published on the adsorption of pure enantiomers, of enantiomeric mixtures, and of prochiral molecules on chiral and achiral model surfaces, especially on well-defined metal single crystals but also on other flat substrates such as highly ordered pyrolytic graphite. Several phenomena are identified, including surface reconstruction and chiral imprinting upon adsorption of chiral agents, and the enhancement or suppression of enantioselectivity seen in some cases upon adsorption of enantiomixtures of chiral compounds. The possibility of enhancing the enantiopurity of adsorbed layers upon the addition of chiral seeds and the so-called "sergeants and soldiers" phenomenon are presented. Examples are provided where the chiral behavior has been associated with either thermodynamic or kinetic driving forces. Two main approaches to the creation of enantioselective surface sites are discussed, namely, via the formation of supramolecular chiral ensembles made out of small chiral adsorbates, and by adsorption of more complex chiral molecules capable of providing suitable chiral environments for reactants by themselves, via the formation of individual adsorbate:modifier adducts on the surface. Finally, a discussion is offered on the additional effects generated by the presence of the liquid phase often required in practical applications such as enantioselective crystallization, chiral chromatography, and enantioselective catalysis.

Adsorption differences between low coverage enantiomers of alanine on the chiral Cu{421}R surface

Chiral separation using heterogeneous methods has long been sought after. Chiral metal surfaces have the potential to make it possible to model these systems using small amino acids, the building blocks for proteins. A comparison of submonolayer concentrations of alanine enantiomers adsorbed onto Cu{421}^R has revealed a large geometrical differences between the two molecules as compared to the saturated coverage. Large differences were observed in HR-XPS and NEXAFS and complemented by theoretical DFT calculations. At approximately one third of a monolayer a comparison of the C1s XPS signal showed a shift in the methyl group of more than 300 meV indicating that the two enantiomers are in different chemical environments. NEXAFS spectroscopy confirmed the XPS variations and showed large differences in the orientation of the adsorbed molecules. Our DFT results show that the l-enantiomer is energetically the most stable in the {311} microfacet configuration. In contrast to the full monolayer coverage, these lower coverages showed enhanced selectivity.

Structure-sensitive enantiospecific adsorption on naturally chiral Cu(hkl) R&S surfaces

The desorption kinetics of a chiral compound, R-3-methylcyclohexanone (R-3MCHO), have been measured on both enantiomers of seven chiral Cu(hkl) ^R&S surfaces and on nine achiral Cu single crystal surfaces with surface structures that collectively span the various regions of the stereographic triangle. The naturally chiral surfaces have terrace-step-kink structures formed by all six possible combinations of the three low Miller index microfacets. The chirality of the kink sites is defined by the rotational orientation of the (1 1 1), (1 0 0) and (1 1 0) microfacets forming the kink. R-3MCHO adsorbs reversibly on these Cu surfaces and temperature programmed desorption has been used to measure its desorption energetics from the chiral kink sites. The desorption energies from the R- and S-kink sites are enantiospecific, [Formula: see text], on the chiral surfaces. The magnitude of the enantiospecificity is [Formula: see text]  ≈  1 kJ mol^-1 on all seven chiral surfaces. However, the values of [Formula: see text] are sensitive to elements of the surface structure other than just their sense of chirality as defined by the rotational orientation of the low Miller index microfacets forming the kinks; [Formula: see text] changes sign within the set of surfaces of a given chirality.

Chiral nanoscale pores created during the surface explosion of tartaric acid on Cu(111)

The autocatalytic decomposition of tartaric acid on Cu(111) exhibits unique kinetics, which are linked to a hexagonal surface structure adopted at high coverage.

Simple rules and the emergence of complexity in surface chirality

Surface chirality arising from self-organized molecular monolayers may manifest both a handedness and footedness, leading to a dual level of chiral expression.

Influence of step faceting on the enantiospecific decomposition of aspartic acid on chiral Cu surfaces vicinal to Cu{111}

The crystallographic orientation of chiral step facets created by l-aspartic acid adsorption dictates enantioselectivity on chiral surfaces vicinal to Cu{111}.

Explosive enantiospecific decomposition of aspartic acid on Cu surfaces

R- and S-enantiomorphs of the Cu(643) surface catalyze the enantiospecific explosive decomposition of d- and l-aspartic acid.

Emerging enantiomeric resolution materials with homochiral nano-fabrications

The major scientific challenge of enantiomeric separation is to develop simple, rapid, and sensitive routine analytical methods. Generally, enantio-resolution is still based on "three-point interaction" theory, which indicates that homochiral sites are needed for enantio-selective interaction. However, in recent years, advanced materials with precise homochiral fabrication at the nanoscale have been synthesized, and have shown great potential in development of high-throughput enantio-resolution methods. This tutorial review summarizes fabrication and applications of homochiral materials for enantio-selective recognition and separation. These materials, which include intrinsic and restructured chiral metal surfaces, plasmonic nanostructures, coordination polymers, organic polymer sensors, and molecularly imprinted polymers, have been applied as sensors or chiral stationary phases (CSPs) for efficient enantio-resolution.

Enantiospecific Adsorption of Amino Acids on Naturally Chiral Cu{3,1,17}R&S Surfaces

Gas-phase equilibrium adsorption of D- and L-serine (Ser) mixtures and D- and L-phenylalanine (Phe) mixtures has been studied on the naturally chiral Cu{3,1,17}(R&S) surfaces. (13)C labeling of the l enantiomers (*L-Ser and *L-Phe) has enabled mass spectrometric enantiodiscrimination of the species desorbing from the surface following equilibrium adsorption. On the Cu{3,1,17}(R&S) surfaces, both equilibrium adsorption and the thermal decomposition kinetics of the D and *L enantiomers exhibit diastereomerism. Following exposure of the surfaces to D/*L mixtures, the relative equilibrium coverages of the two enantiomers are equal to their relative partial pressures in the gas phase, θ(D)/θ(*L) = P(D)/P(*L). This implies that adsorption is not measurably enantiospecific. The decomposition kinetics of Ser are enantiospecific whereas those of Phe are not. Comparison of these results with those for aspartic acid, alanine, and lysine suggests that enantiospecific adsorption on the naturally chiral Cu surfaces occurs for those amino acids that have side chains with functional groups that allow strong interactions with the surface. There is no apparent correlation between amino acids that exhibit enantiospecific adsorption and those that exhibit enantiospecific decomposition kinetics.

Chiral selectivity of amino acid adsorption on chiral surfaces--the case of alanine on Pt

We study the binding pattern of the amino acid alanine on the naturally chiral Pt surfaces Pt(531), Pt(321), and Pt(643). These surfaces are all vicinal to the {111} direction but have different local environments of their kink sites and are thus a model for realistic roughened Pt surfaces. Alanine has only a single methyl group attached to its chiral center, which makes the number of possible binding conformations computationally tractable. Additionally, only the amine and carboxyl group are expected to interact strongly with the Pt substrate. On Pt(531), we study the molecule in its pristine as well as its deprotonated form and find that the deprotonated one is more stable by 0.47 eV. Therefore, we study the molecule in its deprotonated form on Pt(321) and Pt(643). As expected, the oxygen and nitrogen atoms of the deprotonated molecule provide a local binding "tripod" and the most stable adsorption configurations optimize the interaction of this "tripod" with undercoordinated surface atoms. However, the interaction of the methyl group plays an important role: it induces significant chiral selectivity of about 60 meV on all surfaces. Hereby, the L-enantiomer adsorbs preferentially to the Pt(321)(S) and Pt(643)(S) surfaces, while the D-enantiomer is more stable on Pt(531)(S). The binding energies increase with increasing surface density of kink sites, i.e., they are largest for Pt(531)(S) and smallest for Pt(643)(S).

Chirally-modified metal surfaces: energetics of interaction with chiral molecules

Imparting chirality to non-chiral metal surfaces by adsorption of chiral modifiers is a highly promising route to create effective heterogeneously catalyzed processes for the production of enantiopure pharmaceuticals.

Synthesis of homochiral tris-indanyl molecular rods

By Ti-mediated alkyne trimerization and subsequent Sonogashira and Ohira–Bestman reactions, homochiral molecular rod molecules were prepared for surface self-assembly studies.

Enantioselective extraction mediated by a chiral cavitand–salen covalently assembled on a porous silicon surface

A chiral organic–inorganic hybrid device shows enantioselection properties in water solution towards bromine-marked alkyl-ammonium salts.