Single Chirality Research Articles

Tailoring Homochirality at Surfaces: Going Beyond Molecular Handedness

Chirality can be bestowed upon a surface by the adsorption of molecules and is usually discussed in terms of the molecular handedness. However, the adsorption process often leads to a new manifestation of chirality in the form of the adsorption footprint, which can also be chiral and generate mirror-images in 2-D. Therefore, in describing the chirality of the interface, one must consider both the handedness and the adsorption 'footedness' of the system. Specifically, the creation of a truly homochiral surface must ensure that a single chirality is expressed for each aspect, and requires not only the control of molecule handedness but also direct control over footedness. Here, we demonstrate the ability to engineer homochiral footedness by a structural modification of enantiopure (S)-proline, which normally creates a (4 × 2) organization on a Cu(110) surface with heterochiral footedness. This modification of proline via the addition of a double bond within the pyrrolidine ring, yielding 3-pyrroline-2-carboxylic acid (PCA), is sufficient to drive the footprints of the entire (4 × 2) assembly from heterochiral to homochiral, leading to the creation of a truly homochiral interface The effects of modifications upon the footprint arrangements were characterized at the single-molecule level by scanning tunnelling microscopy, reflection absorption infrared spectroscopy and periodic density functional theory calculations. The control of adsorption footprints is not only pivotal to tailoring chirality at surfaces but also plays a key role in dictating the organization, the outward facing functionalities and the response of the organic-inorganic interface.

A route to enantiopure RNA precursors from nearly racemic starting materials

The single-handedness of biological molecules is critical for molecular recognition and replication processes and would seem to be a prerequisite for the origin of life. A drawback of recently reported synthetic routes to RNA is the requirement for enantioenriched reactants, which fails to address the puzzle of how the single chirality of biological molecules arose. Here, we report the synthesis of highly enantioenriched RNA precursor molecules from racemic starting materials, with the molecular asymmetry derived solely from a small initial imbalance of the amino-acid enantiomers present in the reaction mixture. Acting as spectators to the main reaction chemistry, the amino acids orchestrate a sequence of physical and chemical amplification processes. The emergence of molecules of single chirality from complex, multi-component mixtures supports the robustness of this synthesis process under potential prebiotic conditions and provides a plausible explanation for the single-handedness of biological molecules before the emergence of self-replicating informational polymers.

Directly observable G band splitting in Raman spectra from individual tubular graphite cones

Individual tubular graphite cones (TGCs) are characterized using Raman spectroscopy at room temperature. A split G band at 1569–1587 cm −1 is directly observed from the root to the tip of TGCs without requiring surface electromagnetic and chemical enhancement effects. The G band can be deconvoluted into two Lorentzian peaks at 1571 ± 2 and 1584 ± 3 cm −1, which may be attributed to the resonance enhancement of a single chirality excitation in the innermost tubes and all constituent tubes of the TGCs, respectively. Results suggest that the splitting of G band can be envisaged as a fingerprint for monochirality of tubular carbon nanomaterials.

Electrodynamic and Excitonic Intertube Interactions in Semiconducting Carbon Nanotube Aggregates

The optical properties of selectively aggregated, nearly single chirality single-wall carbon nanotubes were investigated by both continuous-wave and time-resolved spectroscopies. With reduced sample heterogeneities, we have resolved aggregation-dependent reductions of the excitation energy of the S(1) exciton and enhanced electron-hole pair absorption. Photoluminescence spectra revealed a spectral splitting of S(1) and simultaneous reductions of the emission efficiencies and nonradiative decay rates. The observed strong deviations from isolated tube behavior are accounted for by enhanced screening of the intratube Coulomb interactions, intertube exciton tunneling, and diffusion-driven exciton quenching. We also provide evidence that density gradient ultracentrifugation can be used to structurally sort single-wall carbon nanotubes by aggregate size as evident by a monotonic dependence of the aforementioned optical properties on buoyant density.

First enantiopure phosphapalladacycle with a palladium bonded stereogenic carbon as the sole chirality source

First enantiomerically pure phosphapalladacycle bearing an asymmetric carbon directly bonded to a palladium atom as a single chirality source was prepared by the resolution of racemic dimer {Pd(η 2-L)(μ-Cl)} 2 ( 1), where L = 2 - Bu 2 t PC 6 H 4 CHMe , using ( R,R)-stilbendiamine as an auxiliary ligand. Double recrystallization of a 1:1 mixture of diastereomers ( S,RR)- 4a and ( R,RR)- 4b from MeCN affords one of them, ( S,RR)- 4a, as a single isomer (>98% de, 31P{ 1H} NMR data). Another diastereomerically pure complex, ( R,SS)- 4c, was isolated by the same method using ( S,S)-stilbendiamine as a resolving agent. This diastereomer ( R,SS)- 4c was converted into the enantiopure dimer ( R,R)- 1 (>98% ee, 31P{ 1H} NMR data) by the diamine protonation with dilute acetic acid at ∼5 °C. Dimer ( R,R)- 1 is configurationally stable at high temperatures up to at least 110 °C. The absolute configuration of the C∗-stereocenter in dimer ( S,S)- 1 was established by an X-ray diffraction study of its precursor ( S,RR)- 4a.

Density Gradient Ultracentrifugation of Nanotubes: Interplay of Bundling and Surfactants Encapsulation

Density gradient ultracentrifugation (DGU) has emerged as a promising tool to prepare chirality enriched nanotube samples. Here, we assess the performance of different surfactants for DGU. Bile salts (e.g., sodium cholate (SC), sodium deoxycholate (SDC), and sodium taurodeoxycholate (TDC)) are more effective in individualizing Single Wall Carbon Nanotubes (SWNTs) compared to linear chain surfactants (e.g., sodium dodecylbenzene sulfonate (SDBS) and sodium dodecylsulfate (SDS)) and better suited for DGU. Using SC, a narrower diameter distribution (0.69−0.81 nm) is achieved through a single DGU step on CoMoCAT tubes, when compared to SDC and TDC (0.69−0.89 nm). No selectivity is obtained using SDBS, due to its ineffectiveness in debundling. We assign the reduced selectivity of dihydroxy bile salts (SDC and TDC) in comparison with trihydroxy SC to the formation of secondary micelles. This is determined by the number and position of hydroxyl (−OH) groups on the α-side of the steroid backbone. We also enrich CoMoCAT SWNTs in the 0.84−0.92 nm range using the Pluronic F98 triblock copolymer. Mixtures of bile salts (SC) and linear chain surfactants (SDS) are used to enrich metallic and semiconducting laser-ablation grown SWNTs. We demonstrate enrichment of a single chirality, (6,5), combining diameter and metallic versus semiconducting separation on CoMoCAT samples.

Orbital Specific Chirality and Homochiral Self-Assembly of Achiral Molecules Induced by Charge Transfer and Spontaneous Symmetry Breaking

We study the electronic mechanisms underlying the induction and propagation of chirality in achiral molecules deposited on surfaces. Combined scanning tunneling microscopy and ab initio electronic structure calculations of Cu-phthalocyanines adsorbed on Ag(100) reveal the formation of chiral molecular orbitals in structurally undistorted molecules. This effect shows that chirality can be manifest exclusively at the electronic level due to asymmetric charge transfer between molecules and substrate. Single molecule chirality correlates with attractive van der Waals interactions, leading to the propagation of chirality at the supramolecular level. Ostwald ripening provides an efficient pathway for complete symmetry breaking and self-assembly of homochiral supramolecular layers.

Carbon Nanotubes: Measuring Dispersion and Length

Advanced technological uses of single-walled carbon nanotubes (SWCNTs) rely on the production of single length and chirality populations that are currently only available through liquid-phase post processing. The foundation of all of these processing steps is the attainment of individualized nanotube dispersions in solution. An understanding of the colloidal properties of the dispersed SWCNTs can then be used to design appropriate conditions for separations. In many instances nanotube size, particularly length, is especially active in determining the properties achievable in a given population, and, thus, there is a critical need for measurement technologies for both length distribution and effective separation techniques. In this Progress Report, the current state of the art for measuring dispersion and length populations, including separations, is documented, and examples are used to demonstrate the desirability of addressing these parameters.

Chirality dependence of theK-momentum dark excitons in carbon nanotubes

Using a collection of twelve semiconducting carbon nanotube samples, each highly enriched in a single chirality, we study the chirality dependence of the $K$-momentum dark singlet exciton using phonon sideband optical spectroscopy. Measurements of bright absorptive and emissive sidebands of this finite momentum exciton identify its energy as 20 - 38 meV above the bright singlet exciton, a separation that exhibits systematic dependencies on tube diameter, $2n+m$ family, and semiconducting type. We present calculations that explain how chiral angle dependence in this energy separation relates to the Coulomb exchange interaction, and elaborate the dominance of the $K_{A_1'}$ phonon sidebands over the zone-center phonon sidebands over a wide range of chiralities. The Kataura plot arising from these data is qualitatively well described by theory, but the energy separation between the sidebands shows a larger chiral dependence than predicted. This latter observation may indicate a larger dispersion for the associated phonon near the $K$ point than expected from finite distance force modeling.

Solution‐Phase Racemization in the Presence of an Enantiopure Solid Phase

Solution-phase racemization drives the evolution of single chirality in the solid phase by the "chiral amnesia" process first described by Viedma. The current investigations lay the basis for a better understanding of the mechanism of the solid-phase deracemization by uncoupling the chemical rate processes associated with the interconversion of enantiomers in the solution phase from the physical processes associated with solution-solid phase transfer via dissolution and reaccretion of molecules onto crystals. In addition, the enantiomer concentration profiles presented in this work, together with an analytical treatment of the racemization process in the presence of excess enantiopure solid, unequivocally reconfirm the validity of the Meyerhoffer double solubility rule for systems under solution racemization conditions.

Single magnetic chirality in the magnetoelectricNdFe3(B11O3)4

We have performed an extensive study of single crystals of the magnetoelectric ${\text{NdFe}}_{3}{({^{11}\text{B}\text{O}}_{3})}_{4}$ by means of a combination of single-crystal neutron diffraction and spherical neutron polarimetry. Our investigation did not detect significant deviations at low temperatures from space group $R32$ concerning the chemical structure. With respect to magnetic ordering our combined results demonstrate that in the commensurate magnetic phase below ${T}_{N}\ensuremath{\approx}30\text{ }\text{K}$ all three magnetic Fe moments and the magnetic Nd moment are aligned ferromagnetically in the basal hexagonal plane but align antiferromagnetically between adjacent planes. The phase transition to the low-temperature incommensurate (IC) magnetic structure observed at ${T}_{\text{IC}}\ensuremath{\approx}13.5\text{ }\text{K}$ appears to be continuous. By means of polarized neutron studies it could be shown that in the incommensurate magnetic phase the magnetic structure of ${\text{NdFe}}_{3}{({^{11}\text{B}\text{O}}_{3})}_{4}$ is transformed into a long-period antiferromagnetic helix with single chirality. Close to the commensurate-incommensurate phase transition third-order harmonics were observed, which in addition indicate the formation of magnetic solitons.

ChemInform Abstract: From Ostwald Ripening to Single Chirality.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Growth potential

Could carbon nanotubes of a single chirality be grown from the bottom up using a common organic reaction?

Supernovae and the Chirality of the Amino Acids

A mechanism for creating amino acid enantiomerism that always selects the same large-scale chirality is identified, and subsequent chemical replication and galactic mixing that would populate the Galaxy with the predominant species is described. This involves (1) the spin of the 14N in the amino acids, or in precursor molecules from which amino acids might be formed, that couples to the chirality of the molecules; (2) the neutrinos emitted from the supernova, together with the magnetic field from the nascent neutron star or black hole formed from the supernova, which selectively destroy one orientation of the 14N and thus select the chirality associated with the other 14N orientation; (3) chemical evolution, by which the molecules replicate and evolve to more complex forms of a single chirality on a relatively short timescale; and (4) galactic mixing on a longer timescale that mixes the selected molecules throughout the Galaxy.

From Ostwald Ripening to Single Chirality

A century ago Wilhelm Ostwald received the Nobel Prize for Chemistry. Although Ostwald was never significantly involved with the phenomenon of chirality, one of his discoveries, Ostwald ripening, is thought to be involved in a recently discovered method in which grinding-induced attrition is used to transform racemic conglomerates virtually quantitatively into a single enantiomer. In this Minireview the basic concepts developed by Ostwald will be introduced, followed by a summary of the current status of grinding-induced asymmetric transformations. We will see how close Ostwald himself came to discovering this technique.

Attrition-enhanced preferential crystallization combined with racemization leading to redissolution of the antipode nuclei

Preferential crystallization is an effective and cheap resolution method which is limited by the nucleation of the antipode, even in the case of in situ racemization. Grinding (during an in situ racemization experiment) was done to promote the crystals stemming from the seeds by enhancing the dissolution step of smaller ones (resulting from nucleation). Thus, the solid remains of a single chirality even if the supersaturation exceeds the Ostwald limit of the antipode.

Spontaneous mirror symmetry breaking in the aldol reaction and its potential relevance in prebiotic chemistry.

The origin of the single chirality of most biomolecules is still a great puzzle. Carbohydrates could form in the formose reaction, which is proposed to be autocatalytic and contains aldol reaction steps. Based on our earlier observation of organoautocatalysis and spontaneous enantioenrichment in absence of deliberate chiral influences in the aldol reaction of acetone and p-nitrobenzaldehyde we suggest that a similar effect might be present also in the aldol reactions involved in gluconeogenesis. Herein we show that reactant precipitation observed in our earlier reported experiments does not affect the asymmetric autocatalysis in the aldol reaction we studied. We explain the phenomenon of spontaneous mirror symmetry breaking in such organocatalytic homogenous systems qualitatively by non-linear reaction network kinetics and classical transition state theory.

Complete chiral symmetry breaking of an amino acid derivative directed by circularly polarized light

Circularly polarized light (CPL) emitted from star-forming regions is an attractive candidate as a cause of single chirality in nature. It has remained difficult, however, to translate the relatively small chemical effects observed on irradiation of molecular systems with CPL into high enantiomeric excesses. Here we demonstrate that irradiation of a racemic amino acid derivative with CPL leads to a small amount of chiral induction that can be amplified readily to give an enantiopure solid phase. A racemate composed of equal amounts of left- and right-handed crystals in contact with the irradiated solution is converted completely into crystals of single-handedness through abrasive grinding when racemization is effected in the solution. The rotation sense of the CPL fully determines the handedness of the final solid state. These findings illustrate the potential effectiveness of CPL in the control of molecular asymmetry, which is relevant for the origin of the single chirality inherent to many biological molecules.

HAND OVER HAND

RETURN TO ISSUEPREVNewsNEXTHAND OVER HANDIdeas aplenty still vie to explain how LIFE’S SINGLE CHIRALITY emergedCARMEN DRAHLView Author Information C&EN WASHINGTONCite this: Chem. Eng. News 2009, 87, 12, 38–41Publication Date (Print):March 23, 2009Publication History Published online15 November 2010Published inissue 23 March 2009https://doi.org/10.1021/cen-v087n012.p038Copyright © 2009 AMERICAN CHEMICAL SOCIETYArticle Views16Altmetric-Citations2LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (2 MB) SUBJECTS:Crystals Get e-Alerts

Magnetization reversal in circularly exchange-biased ferromagnetic disks

We investigate the reversal behavior of circularly exchange-biased micron-sized bilayer disks of Permalloy (Py)/IrMn and CoFe/IrMn. A circular exchange bias is induced by imprinting the vortex configuration of the ferromagnetic layer into the IrMn when the disks are cooled in zero external field through the blocking temperature of IrMn. The resulting circular exchange bias has a profound effect on the reversal behavior of the ferromagnetic magnetization. In Py/IrMn disks the reversal takes place via vortex motion only, and the behavior is controlled by the exchange bias; it is reversible over a range of small fields and the vortex maintains a single chirality throughout reversal, determined by the chirality of the exchange bias. In CoFe/IrMn disks the non-negligible magnetocrystalline anisotropy causes a reversal via both vortices and domain walls resulting in a finite coercivity, and the behavior is controlled by microstructure. We verify that circular exchange bias does not give rise to a hysteresis loop shift. It lowers coercivity with respect to the field-cooled case, and in Py/IrMn disks it even causes completely reversible magnetic behavior. In both Py/IrMn and CoFe/IrMn disks, circular exchange bias removes the randomness (i.e., stochastic processes due to thermal activation) inherent in single-layer ferromagnetic disks and causes the magnetic behavior to be reproducible over time.