Chiral Materials: Progress,Applications, and Prospects

Chirality is a universal phenomenon in molecular and biological systems, denoting an asymmetric configurational property where an object cannot be superimposed onto its mirror image by any kind of translation or rotation, which is ubiquitous on the scale from neutrinos to spiral galaxies. Chirality plays a very important role in the life system. Many biological molecules in the life body show chirality, such as the “codebook” of the earth's biological diversity-DNA, nucleic acid, etc. Intriguingly, living organisms hierarchically consist of homochiral building blocks, for example, l -amino acids and d -sugars with unknown reason. When molecules with chirality interact with these chiral factors, only one conformation favors the positive development of life, that is, the chiral host environment can only selectively interact with chiral molecules of one of the conformations. The differences in chiral interactions are often manifested by chiral recognition, mutual matching, and interactions with chiral molecules, which means that the stereoselectivity of chiral molecules can produce changes in pharmacodynamics and pathology. Here, the latest investigations are summarized including the construction and applications of chiral materials based on natural small molecules as chiral source, natural biomacromolecules as chiral sources, and the material synthesized by design as a chiral source.     

Chiral switching by spontaneous conformational change in adsorbed organic molecules

Self-assembly of adsorbed organic molecules is a promising route towards functional surface nano-architectures, and our understanding of associated dynamic processes has been significantly advanced by several scanning tunnelling microscopy (STM) investigations. Intramolecular degrees of freedom are widely accepted to influence ordering of complex adsorbates, but although molecular conformation has been identified and even manipulated by STM, the detailed dynamics of spontaneous conformational change in adsorbed molecules has hitherto not been addressed. Molecular surface structures often show important stereochemical effects as, aside from truly chiral molecules, a large class of so-called prochiral molecules become chiral once confined on a surface with an associated loss of symmetry. Here, we investigate a model system in which adsorbed molecules surprisingly switch between enantiomeric forms as they undergo thermally induced conformational changes. The associated kinetic parameters are quantified from time-resolved STM data whereas mechanistic insight is obtained from theoretical modelling. The chiral switching is demonstrated to enable an efficient channel towards formation of extended homochiral surface domains. Our results imply that appropriate prochiral molecules may be induced (for example, by seeding) to assume only one enantiomeric form in surface assemblies, which is of relevance for chiral amplification and asymmetric heterogenous catalysis.     

Supramolecular Nanostructures of Chiral Perylene Diimides with Amplified Chirality for High‐Performance Chiroptical Sensing

Chiral supramolecular nanostructures with optoelectronic functions are expected to play a central role in many scientific and technological fields but their practical use remains in its infancy. Here, this paper reports photoconductive chiral organic semiconductors (OSCs) based on perylene diimides with the highest electron mobility among the chiral OSCs and investigates the structure and optoelectronic properties of their homochiral and heterochiral supramolecular assemblies from bottom‐up self‐assembly. Owing to the well‐ordered supramolecular packing, the homochiral nanomaterials exhibit superior charge transport with significantly higher photoresponsivity and dissymmetry factor compared with those of their thin film and monomeric equivalents, which enables highly selective detection of circularly polarized light, for the first time, in visible spectral range. Interestingly, the heterochiral nanostructures assembled from co‐self‐assembly of racemic mixtures show extraordinary chiral self‐discrimination phenomenon, where opposite enantiomeric molecules are packed alternately into heterochiral architectures, leading to completely different optoelectrical performances. In addition, the crystal structures of homochiral and heterochiral nanostructures have first been studied by ab initio X‐ray powder diffraction analysis. These findings give insights into the structure–chiroptical property relationships of chiral supramolecular self‐assemblies and demonstrate the feasibility of supramolecular chirality for high‐performance chiroptical sensing.     

Enantiomeric interactions between nucleic acid bases and amino acids on solid surfaces

Molecular interaction between nucleic acid bases and amino acids is a fundamental process in biology. The adsorption of the molecules on surfaces provides the opportunity to study such interactions in great detail by exploiting the high-resolution imaging capabilities of scanning tunnelling microscopy (STM). The chemisorption of prochiral molecules, such as adenine, on a metal surface causes the adsorbed species to become chiral. Subsequent interactions with inherently chiral molecules may then lead to the formation of diastereoisomers, if the enantiomeric interaction process is sufficiently strong. In the case of adenine adsorption on Cu[110], the chiral adsorbates form homochiral chains. Here, we show that the adenine chain direction is fully correlated with the chirality, and that the alpha-amino acid, phenylglycine, shows a strong chiral preference in its interaction with these chains. STM images clearly demonstrate that S-phenylglycine (R-phenylglycine) binds only to chains rotated 19.5 degrees (anti-) clockwise from the [001] direction. Closer examination reveals that the enantiomeric interaction involves double rows of phenylglycine molecules and the adenine chains. This is the first observation at the molecular level of diastereoisomeric interaction, and demonstrates that STM is a powerful method for studying the details of these interactions.     

Biased Symmetry Breaking in the Formation of Intercalated Layered Double Hydroxides: toward Control of Homochiral Supramolecular Assembly

Homochiral supramolecular assembly (HSA) based on achiral molecules has provided important clues to understand the origin of biological homochirality from the aspect of symmetry breaking. However, planar achiral molecules still face the challenge of forming HSA due to the lack of driving force for twisted stacking, which is a prerequisite for homochirality. Here, with the benefit of the formation of 2D intercalated layered double hydroxide (LDH, host–guest nanomaterials) in vortex motion, planar achiral guest molecules can form the chiral units with spatially asymmetrical structure in the confinement space of LDH. Once the LDH is removed, these chiral units are in a thermodynamic non-equilibrium state, which can be amplified to HSA by self-replicating. Especially, the homochiral bias can be predicted in advance by controlling the vortex direction. Therefore, this study breaks the bottleneck of complicated molecular design and provides a new technology to achieve HSA made of planar achiral molecules with definite handedness.     

Controlling Chirality across Length Scales using DNA

Nano‐objects with chiral properties attract growing interest due to their relevance for a wide variety of technological applications. For example, chiral nano‐objects may be used in characterization platforms that involve chiral molecular recognition of proteins or in the fabrication of nanomechanical devices such as screw‐gears or nanoswimmers. Spatial ordering of emitters of circularly polarized light might greatly benefit from the utilization of chiral shapes. Tools developed in DNA nanotechnology now allow precise tailoring of the chiral properties of molecules and materials at various length scales. Among others, they have already been applied to control the handedness of helical shapes (configurational chirality) or the chiral positioning of different‐sized nanoparticles at the vertices of tetrahedra (compositional chirality). This work covers some of the key advances and recent developments in the field of chiral DNA nanoarchitectures and discusses their future perspectives and potential applications.     

Characterization of chiral host-guest complexation in fast atom bombardment mass spectrometry

A new method has been developed for the characterization of complexion between host and guest molecules. Adduct formation between chiral crown ethers 1 and 2 and enantiomeric ammonium ions 4 and 5 was examined. The reference compound 3 (achiral host) was chosen to be similar in structure to the chiral crown ethers for quantitative measurements. Our approach is based on a formalism assuming an equilibrium: [chiral host + H](+) + [achiral host + chiral guest](+) ? [chiral host + chiral guest](+) + [achiral host + H](+). The equlibrium constant for this process was calculated using the relative peak intensities of the corresponding species in the FAB mass spectra. It was found that these provide significantly better reproducibility and more reliable results than the relative peak intensity method described before (Sawada, M.; et al. J. Am. Chem. Soc. 1992, 114, 4405; 1993, 115, 7381; Org. Mass Spectrom. 1993, 28, 1525).(1)(-)(3) In the examples studied, the equilibrium constants corresponding to the formation of heterochiral adducts (S,S-R or R,R-S) were higher than those for the formation of homochiral aggregates (S,S-S or R,R-R).     

新型纳米多孔材料——金属有机配位聚合物的包结作用及其应用研究进展

金属有机配位聚合物(MOCPs)具有纳米多孔的特殊结构,且结构具有可设计性,通过MOCPs活性位点与客体分子的包结作用能够选择性地包结多种客体分子,表现出特有的分子识别能力.MOCPs作为一种新型多孔材料在择形及手性催化、吸附分离、气体储存、分子识别与传感、生物模拟、微反应器等研究应用方面具有诱人的潜力.综述了MOCPs的设计、合成及主-客体相互作用方式,并展望了这种聚合物的应用前景.     

A Dual Modulated Homochiral Helical Nanofilament Phase with Local Columnar Ordering Formed by Bent Core Liquid Crystals: Effects of Molecular Chirality

Helical nanofilament (HNF) phases form as a result of an intralayer mismatch between top and bottom molecular halves in bent‐core liquid crystals (BC‐LCs) that is relieved by local saddle‐splay geometry. HNFs are immensely attractive for photovoltaic and chiral separation applications and as templates for the chiral spatial assembly of guest molecules. Here, the synthesis and characterization of two unichiral BC‐LCs and one racemic mixture with tris‐biphenyl‐diester cores featuring chiral (R,R) and (S,S) or racemic 2‐octyloxy aliphatic side chains are presented. In comparison to the achiral compound with linear side chains forming an intralayer modulated HNF phase (HNF_mod), synchrotron small angle X‐ray diffraction indicates that the unichiral derivatives form a dual modulated HNF phase with intra‐ as well as interlayer modulations (HNF_mod2) suggesting a columnar local structure of the nanofilaments. Transmission electron microscopy and circular dichroism spectropolarimetry confirm that the unichiral materials exclusively form homochiral HNFs with a twist sense‐matching secondary twist. A contact preparation provides the first example of two identical chiral liquid crystal phases only differing in their handedness that do not mix and form an achiral liquid crystal phase with an entirely different structure in the contact zone.     

Achieving chiral resolution in self-assembled supramolecular structures through kinetic pathways

Chiral phase transitions were studied in a self-assembled 2,6-dibromoanthraquinones supramolecular system prepared on Au(111) using scanning tunneling microscopy. As the molecules were deposited at about 150?K, they formed heterochiral chevron structures (a racemate) consisting of two alternating prochiral molecular rows. When the as-deposited sample was warmed to 300?K followed by cooling to 80?K, phase-separated homochiral structures (a conglomerate), as well as the chevron structures, were observed. We propose molecular models for the structures that are in good agreement with ab?initio studies and can be explained by hydrogen bonds and halogen bonds. We found that heterochiral chevron structures were more stable than homochiral structures due to two additional [Formula: see text] halogen bonds per molecule. We considered kinetic pathways for the phase transitions that were made possible via a disordered liquid phase entropically stabilized at 300?K. We show how chiral resolution can be achieved by exploiting kinetic paths allowed in supramolecular systems.     

Chirality of magneto-electrodeposited metal film electrodes

The chiral electrode behaviors of magneto-electrodeposited (MED) Ag and Cu films were examined for the electrochemical reactions of D-glucose, L-glucose and L-cysteine. The Ag and Cu films were electrodeposited under a magnetic field of 2 T parallel (+2 T) or antiparallel (-2 T) to the faradaic current. For MED films of both Ag and Cu, the oxidation current of L-glucose was larger than that of D-glucose on the +2 T-film electrodes, and the results were opposite on the - 2 T-film electrodes. These facts demonstrate that the MED metal films possess the ability of chiral recognition for D- and L-glucoses. The MED Ag film electrodes also exhibited chiral behavior for the oxidation of L-cysteine.     

Chirality of magneto-electrodeposited metal film electrodes

Abstract

The chiral electrode behaviors of magneto-electrodeposited (MED) Ag and Cu films were examined for the electrochemical reactions of D-glucose, L-glucose and L-cysteine. The Ag and Cu films were electrodeposited under a magnetic field of 2 T parallel (+2 T) or antiparallel (-2 T) to the faradaic current. For MED films of both Ag and Cu, the oxidation current of L-glucose was larger than that of D-glucose on the +2 T-film electrodes, and the results were opposite on the - 2 T-film electrodes. These facts demonstrate that the MED metal films possess the ability of chiral recognition for D- and L-glucoses. The MED Ag film electrodes also exhibited chiral behavior for the oxidation of L-cysteine.     

基于甲壳素的手性分离材料

甲壳素是一种丰富的可再生资源,将甲壳素在碱性条件下脱去乙酰基得到壳聚糖,甲壳素和壳聚糖分子骨架有大量的手性碳原子存在,且含有较多的羟基、乙酰氨基或氨基活性官能团,容易进行化学改性得到有较好手性识别能力的衍生物,且它们能以膜、纤维、凝胶及微球等不同形式出现,可作为各种手性分离介质。概述了近年来甲壳素衍生物作为手性分离介质的重要研究进展,主要包括甲壳素及其衍生物的色谱手性固定相、手性分离膜、分子印迹聚合物。重点介绍了这些手性分离介质的结构、制备及性能,并展望了甲壳素衍生物手性分离介质的研究前景。     

酰胺类手性固定相的研究进展

近年来手性拆分成为新的研究热点,手性固定相的研究发展异常迅速.立足于国内外手性固定相的研究成果,介绍了手性固定相的分类及其应用,并重点综述了基于酰胺的手性固定相的发展及其制备方法.同时简要介绍了作为手性固定相对不同异构体的识别机理,并对该领域未来的发展方向进行了展望.     

Chiral recognition of mandelic acid by L-MA derivative-modified sensor in liquid environment

This study demonstrates a new approach for the highly selective molecular recognition of chiral mandelic acid using an L-mandelic acid derivative with an optically active hydroxyl group as the selector. The proposed method is based on quartz crystal microbalance (QCM) detection combined with functional multilayer film techniques in a liquid environment. The present chiral recognition results suggest that the L-mandelic acid derivative is an excellent resolving agent for detecting chiral mandelic acid.     

Chiroselective self-directed octamerization of serine: implications for homochirogenesis

Serine undergoes chiroselective self-directed oligomerization to form a singly protonated octamer under positive ion electrospray conditions, as identified by ion trap tandem mass spectrometry. The experiments also show a series of higher-order clusters (metaclusters) corresponding to [(Ser8H)n]n+, n = 1, 2, 3. There is a magic number effect favoring formation of the protonated octamer over its homologues and also a strong preference for octamer formation from homochiral serine molecules. Collision-induced dissociation suggests that the protonated octamer is composed of four hydrogen-bonded dimers, stabilized by further extensive hydrogen bonding. Density functional calculations support this model and show that the protonated homochiral octamer is energetically stabilized relative to its possible fragments (dimer plus protonated hexamer, etc). The calculations also show that heterochiral octamers are less stable than homochiral octamers (e.g., the protonated 7:1 cluster is 2.1 kcal/mol less stable than the 8:0 analogue). The implications of these results for the origin of homochirality are discussed.     

A Method for Improving CNN-Based Image Recognition Using DCGAN

Image recognition has always been a hot research topic in the scientific community and industry. The emergence of convolutional neural networks(CNN) has made this technology turned into research focus on the field of computer vision, especially in image recognition. But it makes the recognition result largely dependent on the number and quality of training samples. Recently, DCGAN has become a frontier method for generating images, sounds, and videos. In this paper, DCGAN is used to generate sample that is difficult to collect and proposed an efficient design method of generating model. We combine DCGAN with CNN for the second time. Use DCGAN to generate samples and training in image recognition model, which based by CNN. This method can enhance the classification model and effectively improve the accuracy of image recognition. In the experiment, we used the radar profile as dataset for 4 categories and achieved satisfactory classification performance. This paper applies image recognition technology to the meteorological field.     

Empirical procedure that uses molecular structure to predict enantioselectivity of chiral stationary phases.

A total of 121 racemic compounds were separated in the normal-phase mode on a (S)-(1-naphthylethyl)carbamoylated beta-cyclodextrin (S-NEC-beta-CD) bonded phase and 74 on the R equivalent (R-NEC) chiral stationary phase (CSP). All compounds are of the type that have four substituents on a stereogenic center, rather than an "axis of chirality". It is shown that the binary solvent pair used as the mobile phase has a significant influence on chiral recognition. However, the proportions of the components of a specific pair have little effect. From the results, the individual contributions to chiral recognition by these CSPs were estimated for 81 different substituents of the stereogenic center. Varying the arrangement of these 81 substituents could produce over 1.6 million compounds. Hydrogen was chosen as the reference substituent and was assigned a 0 cal/mol free energy. The chiral recognition increased when sp2-hybridized carbons were connected to the stereogenic center. Conversely, sp3-hybridized carbons decreased the enantioselectivity. Amido groups increased the chiral recognition, especially when associated with pi-acid (3,5-dinitrobenzoyl) or pi-basic (naphthyl) groups. This approach does not allow one to know which enantiomer elutes first. However, the "substituent energy" list for chiral compounds can be used to obtain an estimated value for the enantioselectivity of a compound by adding the energy contributions of the four substituents connected to the stereogenic center. In this way one can predict a priori whether or not a compound will separate on a CSP and estimate its separation factor (alpha). Theoretically, this approach can be used for most CSPs, provided a sufficient data base is generated on them.     

Devnagari numeral recognition by combining decision of multiple connectionist classifiers

This paper is concerned with recognition of handwritten Devnagari numerals. The basic objective of the present work is to provide an efficient and reliable technique for recognition of handwritten numerals. Three different types of features have been used for classification of numerals. A multi-classifier connectionist architecture has been proposed for increasing reliability of the recognition results. Experimental results show that the technique is effective and reliable.     

Amino acid clusters formed by sonic spray ionization

A novel ion source based on the principle of sonic spray ionization has been built and used to optimize mass spectrometric conditions for generating amino acid clusters. The ion source employs a simple pneumatic spray operated at extremely high nebulizing gas flow rates. Several factors that affect the performance of the cluster source are identified, and information from these observations provides insights into the mechanisms of gas phase ion formation. Serine is used as a model system in optimizing instrumental and sample parameters to maximize cluster ion formation. The sonic spray results for this oligomer compare favorably with electrospray data, showing an order of magnitude better signal intensity and excellent signal-to-noise ratios. The performance of the system for the protonated serine octamer includes a limit of detection of 10 nM and a linear dynamic range of 4 orders of magnitude. Ion formation was observed to go into saturation above 1 mM. This result and data on pH, electrolyte concentration, and solvent composition are interpreted as supporting a charge residue model of sonic spray ionization. Other amino acids can be substituted for serine in the octamer, with a strong chiral preference in favor of homochiral cluster formation in the cases of threonine and cysteine. These amino acids show a preference for substitution of more than two serine molecules. Phenylalanine, asparagine, tryptophan, and tyrosine also substitute into the serine octamer; however, the process yields only two incorporations and only small chiral effects.