Enantioseparation Membranes: Research Status,Challenges, and Trends

The purity of enantiomers plays a critical role in human health and safety. Enantioseparation is an effective way and necessary process to obtain pure chiral compounds. Enantiomer membrane separation is a new chiral resolution technique, which has the potential for industrialization. This paper mainly summarizes the research status of enantioseparation membranes including membrane materials, preparation methods, factors affecting membrane properties, and separation mechanisms. In addition, the key problems and challenges to be solved in the research of enantioseparation membranes are analyzed. Last but not least, the future development trend of the chiral membrane is expected.     

Synthesis of Bitten Gold Nanoparticles with Single-Particle Chiroptical Responses (Small 26/2023)

The introduction of structural complexity to nanoparticles brings them interesting properties. Regularity breaking has been challenging in the chemical synthesis of nanoparticles. Most reported chemical methods for synthesizing irregular nanoparticles are complicated and laborious, largely hindering the exploration of structural irregularity in nanoscience. In this study, the authors have combined seed-mediated growth and Pt(IV)-induced etching to synthesize two types of unprecedented Au nanoparticles, bitten nanospheres and nanodecahedrons, with size control. Each nanoparticle has an irregular cavity on it. They exhibit distinct single-particle chiroptical responses. Perfect Au nanospheres and nanorods without any cavity do not show optical chirality, which demonstrates that the geometrical structure of the bitten opening plays a decisive role in the generation of chiroptical responses.     

Substrate Enolate Intermediate and Mimic Captured in the Active Site of Streptomyces coelicolor Methylmalonyl-CoA Epimerase**

Methylmalonyl-CoA epimerase (MMCE) is proposed to use general acid-base catalysis, but the proposed catalytic glutamic acids are highly asymmetrical in the active site unlike many other racemases. To gain insight into the puzzling relationships between catalytic mechanism, structure, and substrate preference, we solved Streptomyces coelicolor MMCE structures with substrate or 2-nitropropionyl-CoA, an intermediate/transition state analogue. Both ligand bound structures have a planar methylmalonate/2-nitropropionyl moiety indicating a deprotonated C2 with ≥4 Å distances to either catalytic acid. Both glutamates interact with the carboxylate/nitro group, either directly or through other residues. This suggests the proposed catalytic acids sequentially catalyze proton shifts between C2 and carboxylate of the substrate with an enolate intermediate. In addition, our structures provide a platform to design mutations for expanding substrate scope to support combinatorial biosynthesis.     

Chiral Transport in Nanochannel Based Artificial Drug Transporters

The precise regulation of chiral drug transmembrane transport can be achieved through drug transporters in living organisms. However, implementing this process in vitro is still a formidable challenge due to the complexity of the biological systems that control drug enantiomeric transport. Herein, a facile and feasible strategy is employed to construct chiral L-tyrosine-modified nanochannels (L-Tyr nanochannels) based on polyethylene terephthalate film, which could enhance the chiral recognition of propranolol isomers (R-/S-PPL) for transmembrane transport. Moreover, conventional fluorescence spectroscopy, patch-clamp technology, laser scanning confocal microscopy, and picoammeter technology are employed to evaluate the performance of nanochannels. The results show that the L-Tyr nanochannel have better chiral selectivity for R-/S-PPL compared with the L-tryptophan (L-Trp) channel, and the chiral selectivity coefficient is improved by about 4.21-fold. Finally, a detailed theoretical analysis of the chirality selectivity mechanism is carried out. The findings would not only enrich the basic theory research related to chiral drug transmembrane transport, but also provide a new idea for constructing artificial channels to separate chiral drugs.     

Tuning the Chiral Structures from Self-Assembled Carbohydrate Derivatives (Small 25/2023)

Carbohydrates have been regarded as one of the most ideally suited candidates for chirality study via self-assembly owning to their unique chemical structures, abundance, and sustainability. Much efforts have been devoted to design and synthesize diverse carbohydrate derivatives and self-assemble them into various supermolecular morphologies. Nevertheless, still inadequate attention is paid to deeply and comprehensively understand how the carbohydrate structures and self-assembly approaches affect the final morphologies and properties for future demands. Herein, to fulfill the need, a range of recently published studies relating to the chirality of carbohydrates is reviewed and discussed. Furthermore, to tune the chirality of carbohydrate-based structures on both molecular and superstructural levels via chirality transfer and chirality expression, the designing of the molecules and choosing of the proper approaches for self-assembly are elucidated.     

Chiral Plasmonic Response of 2D Ti3C2Tx Flakes: Realization and Applications

The circularly polarized light sensitive materials response can be reached at plasmon wavelengths through the coupling of intrinsically non-chiral plasmonic nanostructure with chiral organic molecules. As a plasmonic background, the different types of metal nanoparticles of various shapes and sizes are successfully tested and an apparent circular dichroism (CD) signal is measured in both, nanoparticles suspensions and after nanoparticle immobilization in substrate. In this work, the creation of plasmon-active 2D flakes of MXenes (Ti₃C₂T_x) is proposed, with the apparent CD response at plasmon wavelength, through the coupling of intrinsically non-chiral flakes with helically shaped helicene enantiomers. This work provides the first demonstration of chiral and plasmon-active 2D material, which shows the absorption sensitive to light intrinsic circular polarization even in plasmon wavelengths range. The appearance of the induced CD signal is additionally confirmed by several theoretical calculations. After the experimental and theoretical confirmation of the optical chirality at plasmon wavelengths, the flakes are utilized for the polarization sensitive conversion of light to heat, as well as for polarization dependent triggering of plasmon-assisted chemical transformation.