Local Chirality Research Articles

Theoretical Investigation of Raman Optical Activity Signatures of Tröger’s Base

The Raman and VROA spectra of (S,S)-Tröger's base are simulated. We mainly discuss the peaks in the 1140-1400 cm(-1) wavenumber range where an intense VROA signature is found. In this range, nearly all of the Raman-active bands belong to the irreducible representation A (C(2) point group), whereas no such observation is made for the VROA spectrum. The vibrational normal modes associated with the peaks in this range mainly consist of wagging and twisting motions of the hydrogen atoms. From the atomic contribution patterns (ACPs) and the group coupling matrices (GCMs), one finds that the VROA backward-scattering intensities mainly arise from hydrogen and carbon atoms in the vicinity of the two chiral nitrogen atoms. The VROA signatures in the 1140-1400 cm(-1) range are therefore a fingerprint of the local chirality around the two chiral nitrogen centers.

Determination of Local Chirality in Irregular Single-Walled Carbon Nanotubes Based on Individual Hexagons

We develop a robust theoretical method for the determination of local chirality based on individual hexagons that compose single-walled carbon nanotubes (SWCNTs). The method and code are applied to various SWCNT-like irregular structures that are formed during the growth process. The local chiral index and its distribution are well defined for irregular structures as well as ideal structures of SWCNTs, and can be used to characterize any SWCNT-like irregular structures in terms of local chirality. The present method also permits monitoring the chirality of nanostructures during the growth process.

Spontaneous enrichment of one-handed helices by dissolution of quasiracemic crystals of a tetranuclear single helical complex

The left-handed isomer of the helical complex [LZn(3)La(OAc)(3)] was spontaneously enriched from 50 : 50 to 87 : 13 when the quasiracemate crystals were dissolved. The invertible helicity of [LZn(3)La(OAc)(3)] (global chirality) helps the quasiracemate formation and the fixed point chirality of the R,R-cyclohexanediamine moiety (local chirality) effectively controls the global chirality in solution.

Stereospecific Polymerization of Chiral Oxazolidinone-Functionalized Alkenes

Acryloyl and vinyl monomers functionalized with a chiral oxazolidinone auxiliary have been successfully polymerized in a stereospecific fashion to highly isotactic, optically active polymers, through either the previously established isospecific coordination polymerization (for acryloyl monomers) or a novel isospecific cationic polymerization (for vinyl monomers). Specifically, conjugated chiral acryloyloxazolidinones, N-acryloyl-(R or S)-4-phenyl-2-oxazolidinone [(R or S)-AOZ], are readily polymerized by chiral ansa-zirconocenium coordination catalysts, (R,R-, S,S-, or R,R/S,S)-[C2H4(η5-Ind)2]Zr+(THF)[OC(OiPr)═CMe2][MeB(C6F5)3]− (1), in an isospecific manner through a catalyst-site-controlled mechanism, producing the corresponding optically active chiral polymers, (R or S)-PAOZ. Owing to the nature of stereocontrol dictated by the chiral catalyst site, even the coordination polymerization of the parent AOZ, without the chiral side group, also affords PAOZ with nearly quantitative isotacticity. A series of experiments have shown that the chiral polymers (R or S)-PAOZ exhibit no chiral amplifications, despite having stereoregularly placed stereogenic centers in the main chain, and the optical activity of the polymers arises solely from their chiral auxiliary, a consequence of adopting a random-coil secondary structure and thus having a cryptochiral chain. In sharp contrast, the chiral isotactic polymers derived from nonconjugated chiral vinyl oxazolidinones, N-vinyl-(R)-4-phenyl-2-oxazolidinone [(R)-VOZ] and its p-hexyloxyphenyl derivative (R)-HVOZ (designed to solve the solubility issue of the resulting polymer), exhibit substantial chiral amplifications by virtue of adopting a solution-stable, one-handed helical conformation. The synthesis of such helical vinyl polymers has been accomplished by the development of a novel isospecific cationic polymerization using Lewis and Brønsted acids, such as [Ph3C][B(C6F5)4], BF3·Et2O, and [H(Et2O)2][B(C6F5)4], through a chiral auxiliary-controlled mechanism. Noteworthy is the combination of the near-quantitative isotactic placement of the stereogenic centers of the polymer main chain with the chiral side groups located near those stereocenters that renders one-handed helicity of (R)-PVOZ and (R)-PHVOZ. Significantly, this novel cationic polymerization process, operating at ambient temperature, effectively assembles two elements of polymer local chirality—side-chain chirality and main-chain chirality—into global chirality in the form of excess one-handed helicity. Furthermore, the resulting chiral helical vinyl polymers exhibit considerably higher thermal decomposition temperatures and polymer crystallinity, in comparison to the random-coil chiral acryloyl polymers, having a similarly high degree of main-chain stereoregularity.

Understanding the Mechanism of Chirality Transfer in the Formation of a Chiral MCM-41 Mesoporous Silica

Recently, a series of chiral mesoporous silicas with various pore sizes and nanostructures (including chiral MCM-41, chiral SBA-15, and chiral SBA-16) have been successfully synthesized in our group by using achiral templates and a chiral cobalt complex as cotemplate. Enantioselective discrimination ability of these chiral materials was observed when they were applied on the controlled release of a chiral drug. The enantioselectivity of these silica materials was attributed to the local chirality at a molecular level on the pore wall surfaces. Nevertheless, the formation mechanism of local chirality, i.e., the chirality transferring from chiral cobalt complex to inorganic mesoporous silica matrix, remains indeterminate at this juncture. In this report, by tuning the synthesis parameters based on the previously reported method, chiral MCM-41 with left-handed enantiomeric excess in twisted hexagonal particle morphology was obtained. Enantioselective adsorption of racemic valine corroborated the general chir...

Enantioselectively controlled release of chiral drug (metoprolol) using chiral mesoporoussilica materials

Chiral porous materials have attracted burgeoning attention on account of theirpotential applications in many areas, such as enantioseparation, chiral catalysis,chemical sensors and drug delivery. In this report, chiral mesoporous silica (CMS)materials with various pore sizes and structures were prepared using conventionalachiral templates (other than chiral surfactant) and a chiral cobalt complex asco-template. The synthesized CMS materials were characterized by x-ray diffraction,nitrogen physisorption, scanning electron microscope and transmission electronmicroscope. These CMS materials, as carriers, were demonstrated to be able tocontrol the enantioselective release of a representative chiral drug (metoprolol). Therelease kinetics, as modeled by the power law equation, suggested that the releaseprofiles of metoprolol were remarkably dependent on the pore diameter and porestructure of CMS materials. More importantly, R- and S-enantiomers of metoprololexhibited different release kinetics on CMS compared to the corresponding achiralmesoporous silica (ACMS), attributable to the existence of local chirality on the porewall surface of CMS materials. The chirality of CMS materials on a molecularlevel was further substantiated by vibrational circular dichroism measurements.

Flux state and anomalous quantum Hall effect in the square double-exchange model

The anomalous Hall effect (AHE) around the flux state in square double-exchange model is investigated. By introducing the lattice distortion and local chirality, the system can break the parity symmetry and time-reversal symmetry spontaneously, and thus generate a topological nontriviality in the band structure associated with the AHE. Moreover, a possible realization of this AHE in multiferroic ${\text{TbMnO}}_{3}$ is discussed.

Chiral magnetic effect in SU(2) lattice gluodynamics at zero temperature

The chiral magnetic effect is the appearance of a quark electric current along a magnetic-field direction in topologically nontrivial gauge fields. There is evidence that this effect is observed in collisions between heavy ions at the RHIC collider. The features of the chiral magnetic effect in SU(2) lattice gluodynamics at zero temperature have been investigated. It has been found that the electric current increases in the magnetic-field direction owing to quantum fluctuations of gluon fields. Fluctuations of the local charge density and chirality also increase with the magnetic field strength, which is a signature of the chiral magnetic effect.

Tip‐enhanced Raman spectroscopy of carbon nanotubes

AbstractThis paper presents an overview on the principles of tip‐enhanced Raman spectroscopy (TERS). The technique can be used to record Raman images of carbon nanotubes with high spatial resolution. The high resolution is made possible by local enhancement of the incident and scattered fields. The paper also presents some advances on the science of carbon nanotubes generated by TERS, including the observation of local defects, chirality changes, and dopants. Copyright © 2009 John Wiley & Sons, Ltd.

Amplification of local chirality within a folded dendrimer. An intramolecular ‘sergeants and soldiers’ experiment

A series of pyridine-2,6-dicarboxamide dendrons having varying proportions of chiral and achiral termini have been prepared. Circular dichroism and X-ray crystallographic studies indicate that these folded dendrons propagate terminal chirality via a ‘sergeants and soldiers’ process. The extent of chiral amplification shows a dependence on dendron generation, but also is affected by the relative positioning of the chiral termini at the periphery of the dendron. Analysis of the crystal structures of the G1 dendrons, 4a – 6a , reveals chain–chain and chain–dendron conformational communication, which is likely to be responsible for the chiral amplification observed in solution.

Three-Dimensional Chirality Transfer in Rubrene Multilayer Islands on Au(111)

The growth of rubrene (C(42)H(28), 5,6,11,12-tetraphenylnaphthacene) multilayer islands up to a thickness of six layers on a Au(111) surface has been investigated by scanning tunneling microscopy. The molecules self-organize in parallel twin rows, forming mirror domains of defined local structural chirality. Each layer is composed of twin-row domains of the same structural handedness rotated by 120 degrees with respect to each other. Moreover, this structural chirality is transferred to all successive layers in the island, resulting in the formation of three-dimensional objects having a defined structural chirality. The centered rectangular surface unit cell differs from the one characteristic for the single-crystal orthorhombic phase.

Selection of Optimal Variants of Gō-Like Models of Proteins through Studies of Stretching

The Gō-like models of proteins are constructed based on the knowledge of the native conformation. However, there are many possible choices of a Hamiltonian for which the ground state coincides with the native state. Here, we propose to use experimental data on protein stretching to determine what choices are most adequate physically. This criterion is motivated by the fact that stretching processes usually start with the native structure, in the vicinity of which the Gō-like models should work the best. Our selection procedure is applied to 62 different versions of the Gō model and is based on 28 proteins. We consider different potentials, contact maps, local stiffness energies, and energy scales—uniform and nonuniform. In the latter case, the strength of the nonuniformity was governed either by specificity or by properties related to positioning of the side groups. Among them is the simplest variant: uniform couplings with no i, i + 2 contacts. This choice also leads to good folding properties in most cases. We elucidate relationship between the local stiffness described by a potential which involves local chirality and the one which involves dihedral and bond angles. The latter stiffness improves folding but there is little difference between them when it comes to stretching.

Chiral and magnetic effects in forbidden x-ray scattering from antiferromagnetic hematiteα-Fe2O3and eskolaiteCr2O3

Both magnetic and nonmagnetic x-ray diffraction has been studied near the $\text{Fe}\text{ }K$-edge of hematite $(\ensuremath{\alpha}{\text{-Fe}}_{2}{\text{O}}_{3})$ and the $\text{Cr}\text{ }K$-edge of eskolaite $({\text{Cr}}_{2}{\text{O}}_{3})$ and compared to the symmetry-based calculations. These crystals have identical atomic structures but different magnetic orderings. The observed ``forbidden'' 111 and 333 reflections in both crystals show a resonant peak only in the pre-edge energy region. In eskolaite, the azimuthal angle dependence of the resonant 111 and 333 reflections exhibits threefold symmetry, which is in good agreement with the calculated curves based on electric dipole-quadrupole and quadrupole-quadrupole scattering channels. This threefold symmetry is the first reliable evidence for antisymmetric terms in dipole-quadrupole scattering and hence for local chirality of atoms in centrosymmetric crystals. In hematite, nonresonant and resonant scattering has been observed for the forbidden reflections. The azimuth dependence of the nonresonant intensity shows the twofold symmetry. From the azimuthal symmetry and temperature dependence of the nonresonant diffraction, it is revealed that the nonresonant intensity is due to magnetic scattering caused by the antiferromagnetic structure. The azimuth dependence of the 111 resonant peak in hematite shows almost threefold symmetry similar to eskolaite. On the other hand, the resonant 333 reflection in hematite shows complicated azimuth dependence, nearly mirror symmetry, at room temperature. As a result of least-squares analysis of the azimuth dependence and the low-temperature measurement, we conclude that the nonresonant magnetic scattering has a significant influence on the resonant electric scattering though its intensity is much smaller. Thus the interference between the magnetic and electric scatterings plays a very important role in hematite and opens new ways for studying additional details of the magnetic structure.

Conformationally Driven Asymmetric Induction of a Catalytic Dendrimer

Nature's catalysts promote the reactions necessary for life with extremely high specificity by folding into specific shapes capable of communicating remote structural information to an active site. Achieving this objective in synthetic systems has been hampered by the lack of information concerning how dynamic conformational chirality can influence the stereoselectivity of a catalytic process. Herein, we report the first illustration of a catalytic dendrimer that achieves high enantioselectivity by amplifying/propagating local chirality via a dynamically folded structure. Experimental evidence supports a chiral relay mechanism that propagates local terminal chirality of the dendron to the axial chirality of the biphenyl core through the helical secondary structure of the dendron.

Molecular Quantum Similarity and Chirality: Enantiomers with Two Asymmetric Centra

Molecular quantum similarity is evaluated for enantiomers possessing two asymmetric carbon atoms, namely halogen substituted ethanes. This study is an extension of previous work performed on molecules with a single asymmetric carbon atom and molecules possessing a chiral axis. Global similarity and its local counterpart based on the Hirshfeld partitioning are evaluated. By these means we quantify the dissimilarity of enantiomers and illustrate Mezey's holographic electron density theorem in chiral systems. Furthermore, the relation between the optical activity and the dissimilarity is studied. Special attention is drawn to the meso compounds, since these isomers enable us to examine local chirality in an achiral system.

Can Raman Optical Activity Separate Axial from Local Chirality? A Theoretical Study of Helical Deca‐Alanine

Motivated by experimental work on the distinction of protein secondary structure motifs by Raman optical activity (ROA) spectroscopy, we demonstrate using density functional theory that axial chirality in structures with different local chirality can be filtered out by ROA spectroscopy. To this purpose, two diastereomers of right-handed helical deca-alanine, the (all-S) and the (R,S,R,S,R,S,R,S,R,S) form, are compared. Furthermore, we suggest to interpret calculated ROA spectra of large molecules in terms of vibrational bands rather than individual peaks. This is due to the non-homogeneous effect of the harmonic approximation as well as of the chosen electronic structure method onto the vibrational frequencies, which in a dense region of many vibrations will strongly determine the shape of the spectrum. In addition, the calculated ROA spectrum of (all-S)-deca-alanine is compared to the experimental spectrum of poly-(L)-alanine in solution.

Chirality and protein folding

There are several simple criteria of folding to a native state in model proteins. One of theminvolves crossing of a threshold value of the root mean square deviation distance away fromthe native state. Another checks whether all native contacts are established, i.e. whether theinteracting amino acids come closer than some characteristic distance. We useGo-like models of proteins and show that such simple criteria may prompt oneto declare folding even though fragments of the resulting conformations have awrong sense of chirality. We propose that a better condition of folding shouldaugment the simple criteria with the requirement that most of the local values of thechirality should be nearly native. The kinetic discrepancy between the simple andcompound criteria can be substantially reduced in the Go-like models by providing theHamiltonian with a term which favours native values of the local chirality. We studythe effects of this term as a function of its amplitude and compare it to othermodels such as ones with side groups and ones with angle-dependent potentials.

Double-exchange model on the triangle chain

We study ground state properties of the double-exchange model on a triangle chain in the classical limit on ${t}_{2g}$ spins. The ground state is determined by a competition among the kinetic energy of the ${e}_{g}$ electron, the antiferromagnetic exchange energy between the ${t}_{2g}$ spins, and frustration due to a geometric structure of the lattice. The phase diagrams are obtained numerically for two kinds of the models which differ only in the transfer integral being real or complex. The properties of the states are understood from the viewpoint of the spin-induced Peierls instability. The results suggest the existence of a chiral glass phase which is characterized by a local spin chirality and a continuous degeneracy.

Improved Measure of Local Chirality

It is popular to probe the structure of the QCD vacuum indirectly by studying individual fermion eigenmodes, because this provides a natural way to filter out UV fluctuations. The double-peaking in the distribution of the local chiral orientation parameter (X) has been offered as evidence, by some, in support of a particular model of the vacuum. Here we caution that the X-distribution peaking varies significantly with various versions of the definition of X. Furthermore, each distribution varies little from that resulting from a random reshuffling of the left-handed (and independently the right-handed) fields, which destroys any QCD-induced left-right correlation; that is, the double-peaking is mostly a phase-space effect. We propose a new universal definition of the X parameter whose distribution is uniform for randomly reshuffled fields. Any deviations from uniformity for actual data can then be directly attributable to QCD-induced dynamics. We find that the familiar double peak disappears.

Erratum: Realization of chiral symmetry in the domain model of QCD [Phys. Rev. D69, 074029 (2004)

The domain model for the QCD vacuum has previously been developed and shown to exhibit confinement of quarks and strong correlation of the local chirality of quark modes and duality of the background domain-like gluon field. Quark fluctuations satisfy a chirality violating boundary conditions parametrized by a random chiral angle $\alpha_j$ on the $j-th$ domain. The free energy of an ensemble of $N\to\infty$ domains depends on $\{\alpha_j, j=1... N\}$ through the logarithm of the quark determinant. Its parity odd part is given by the axial anomaly. The anomaly contribution to the free energy suppresses continuous axial U(1) degeneracy in the ground state, leaving only a residual axial Z(2) symmetry. This discrete symmetry and flavour $SU(N_f)_L\times SU(N_f)_R$ chiral symmetry in turn are spontaneously broken with a quark condensate arising due to the asymmetry of the spectrum of Dirac operator. In order to illustrate the splitting between the $\eta'$ from octet pseudoscalar mesons realised in the domain model, we estimate the masses of light pseudoscalar and vector mesons.