Solid-state Circular Dichroism Research Articles

Synthesis, crystal structures and fluorescence recognition properties of two novel chiral coordination compounds based on alanine derivate

Two novel chiral coordination compounds {[Cd(D-pmala)(2,2′-bipy)(H2O)]⋅1·25H2O}n (1), {[Pb4(D-pmala)4(5,5′-DM-2,2′-bipy)4(H2O)2]⋅7H2O}n (2) (D-pmala = N-(4-carboxyl) benzoyl-d-alanine, 2,2′-bipy = 2,2′-Bipyridine, 5,5′-DM-2,2′-bipy = 5,5′-Dimethyl-2,2′- Bipyridine) have been successfully synthesized under hydrothermal conditions. Their structures were identified using single-crystal X-ray crystallography and characterized by infrared spectroscopy, elemental analysis, thermogravimetric analysis, powder X-ray diffraction and solid-state circular dichroism spectra. Crystallographic analysis indicate that compound 1 features a "Z"-shaped one-dimensional chain structure, which further extends into three-dimensional supramolecular architectures through hydrogen-bonding interactions. Compound 2 displays a two-dimensional layer structure, a two-dimensional double-layer network structures is further formed through the hydrogen-bonding interactions. Moreover, The fluorescence recognition properties of compound 1 indicate that it is highly selective in detecting Cu2+ ions and tyrosine (Tyr) enantiomers in aqueous solutions, which indicates that compound 1 can be expected to be a bifunctional fluorescent recognition sensor to detect metal ions and amino acid enantiomers.

Solid-state vibrational circular dichroism for pharmaceutical applications: Polymorphs and cocrystal of sofosbuvir

X-ray diffraction is a commonly used technique in the pharmaceutical industry for the determination of the atomic and molecular structure of crystals. However, it is costly, sometimes time-consuming, and it requires a considerable degree of expertise. Vibrational circular dichroism (VCD) spectroscopy resolves these limitations, while also exhibiting substantial sensitivity to subtle modifications in the conformation and molecular packaging in the solid state. This study showcases VCD’s ability to differentiate between various crystal structures of the same molecule (polymorphs, cocrystals). We examined the most effective approach for producing high-quality spectra and unveiled the intricate link between structure and spectrum via quantum-chemical computations. We rigorously assessed, using alanine as a model compound, multiple experimental conditions on the resulting VCD spectra, with the aim of proposing an optimal and efficient procedure. The proposed approach, which yields reliable, reproducible, and artifact-free results with maximal signal-to-noise ratio, was then validated using a set comprising of three amino acids (serine, alanine, tyrosine), one hydroxy acid (tartaric acid), and a monosaccharide (ribose) to mimic active pharmaceutical components. Finally, the optimized approach was applied to distinguish three polymorphs of the antiviral drug sofosbuvir and its cocrystal with piperazine. Our results indicate that solid-state VCD is a prompt, cost-effective, and easy-to-use technique to identify crystal structures, demonstrating potential for application in pharmaceuticals. We also adapted the cluster and transfer approach to calculate the spectral properties of molecules in a periodic crystal environment. Our findings demonstrate that this approach reliably produces solid-state VCD spectra of model compounds. Although for large molecules with many atoms per unit cell, such as sofosbuvir, this approach has to be simplified and provides only a qualitative match, spectral calculations, and energy analysis helped us to decipher the observed differences in the experimental spectra of sofosbuvir.

Differentiation of solvatomorphs of active pharmaceutical ingredients (API) by solid-state vibrational circular dichroism (VCD)

Here, we present the new application of solid-state Vibrational Circular Dichroism (VCD) spectroscopy to differentiate several dutasteride (DS) solvatomorphs – the model active pharmaceutical ingredient (API). Several crystalline DS hydrochloride hydrates solvated with methanol, ethanol, acetonitrile, acetone, and acetic acid were prepared. In contrast to almost identical IR spectra, the VCD ones were very sensitive to changes in the sample composition. We marked significant differences in the shape of VCD spectra of studied DS solvatomorphs, DS hydrates, and DS polymorphic forms. Our findings, supported by DFT calculations, show that VCD spectroscopy has the pronounced ability to distinguish their crystal arrangements. We believe that this contribution will extend the use of VCD in the pharmaceutical industry for developing and designing new chiral drug products for the identification, description, and in-depth probing of several pharmaceutical solvatomorphs in the future.

Tellurium(IV) Halide Achieving Effective Nonlinear-Optical Activity: The Role of Chiral Ligands and Lattice Distortion.

Chiral organic-inorganic hybrid metal halides are a promising class of nonlinear-optical materials with unique optical properties and flexible crystal structures. However, the structures and properties of chiral hybrid tellurium halides, especially second harmonic generation (SHG), have not been reported. Here, by introducing chiral organic molecule (R/S)-methylbenzylammonium (R/S-MBA), we synthesized a pair of novel zero-dimensional (0D) chiral tellurium-based hybrid halides with noncentrosymmetric space group C2, (R/S-MBA)2TeCl6 (R/S-Cl). Single-crystal X-ray diffraction analysis and solid-state circular dichroism (CD) spectra confirm that R/S-Cl shows obvious enantiomer enrichment. Moreover, the resulting chiral products present an efficient SHG response. Interestingly, through manipulation of halogen atoms, two pairs of achiral tellurium halides, (R/S-MBA)2TeBr6 (R/S-Br) and (R/S-MBA)2TeI6 (R/S-I), were obtained, both of which crystallize in the centrosymmetric space group R3̅. It is noteworthy that R/S-I has a narrow band gap of 1.55 eV, which is smaller than that of most 0D metal halides and comparable to that of three-dimensional lead halide, showing its potential as a highly efficient light absorber.

Redox-Mediated Enantioselective Interactions in Chiral Metallopolymers Possessing Supramolecular Chirality

Synthetic chiral platforms can be a powerful platform for enantioselective interactions, especially when coupled with redox-mediated electrochemical processes. Redox metallopolymers represent a distinct class of versatile functional materials and have been extensively studied for a wide scope of applications such as membranes, self-assembly, hydrogels, and redox-mediated ion separation. However, many studies have focused on molecularly-selective binding and release of achiral molecules, with limited studies on chirality strategies for enantioselective interactions. Chiral organic anions comprise an important class of biologically-relevant molecules, and their separation and sensing have received intense attention. Here, we introduce new chiral redox-polymers with chiral ferrocene (Fc) building blocks, that can serve as an electro-responsive enantioselective platform for a variety of amino acids and pharmaceutical carboxylates, taking advantage of supramolecular chirality for enhancing electrochemical recognition. Two chiral redox-metallopolymers were designed and synthesized based on Ugi’s amine-inspired chiral monomers containing an amide (poly-1) or amine group (poly-2), and their enantioselective recognitions toward amino acids and pharmaceutical carboxylates were investigated by redox potential shifts using square wave voltammetry. The half potential shift of both poly-(S)-1 and poly-(S)-2 increased more with increasing D-tryptophan concentration than the other enantiomer, L-tryptophan, while the half potential of poly-(R)-1 and poly-(R)-2 shifted more with L-tryptophan than with D-tryptophan, proving the enantioselective interaction of both redox-metallopolymers. Nuclear Overhauser Effect spectroscopy (2D NOESY) and solid-state circular dichroism revealed the emergence of supramolecular chirality of the polymers resulting from the intramolecular interaction between the ferrocene and the alkyl group in the backbone, and the steric hindrance of the ferrocene within the polymer. The half-peak potential shift demonstrated the remarkable enhancement of enantioselective recognition with tryptophan (Trp) and naproxen, provided by the chiral polymers possessing the supramolecular chirality over the single-site, chiral building block itself. For instance, the two (S) polymers showed the potential shift difference 2.4-2.8 times higher than the corresponding monomers upon the binding of Trp enantiomers. The enantioselective potential shift of the chiral metallopolymer electrodes could determine enantiomeric excess (%ee) of Trp over the range of 0–100%ee. Investigation on solvent polarity and pH effect revealed that the enantioselective mechanism of the chiral redox polymers was attributed to the subtle balance between hydrogen bonding and π-π interaction. We also identified the importance of aromatic groups on the analytes for increasing enantioselectivity of the chiral redox polymers. This work highlights the potential of chiral redox-metallopolymers as platforms for electrochemically-modulated enantioselective processes toward a variety of amino acids and pharmaceutical carboxylates. Figure 1

Unlocking Insights into Folding, Structure, and Function of Proteins through Circular Dichroism Spectroscopy—A Short Review

Circular dichroism (CD) spectroscopy has emerged as a powerful tool in the study of protein folding, structure, and function. This review explores the versatile applications of CD spectroscopy in unraveling the intricate relationship between protein conformation and biological activity. A key advantage of CD spectroscopy is its ability to analyze protein samples with minimal quantity requirements, making it an attractive technique for studying proteins that are scarce or difficult to produce. Moreover, CD spectroscopy enables the monitoring of physical and chemical environmental effects on protein structures, providing valuable insights into the dynamic behavior of proteins in different conditions. In recent years, the use of synchrotron radiation as a light source for CD measurements has gained traction, offering enhanced sensitivity and resolution. By combining the advantages of CD spectroscopy, such as minimal sample requirements and the ability to probe environmental effects, with the emerging capabilities of synchrotron radiation (SRCD), researchers have an unprecedented opportunity to explore the diverse aspects of protein behavior. This review highlights the significance of CD spectroscopy in protein research and the growing role of synchrotron radiation in advancing our understanding of protein behavior, aiming to provide novel insights and applications in various fields, including drug discovery, protein engineering, and biotechnology. A brief overview of Solid-State Circular Dichroism (SSCD) is also included.

Solid-state vibrational circular dichroism: Methodology and application for amphetamine derivatives

Amphetamine derivatives are considered most seized substances worldwide. In this study, solid-state vibrational circular dichroism (VCD) measurements of enantiomerically pure substances were performed for spectroscopic discrimination between (S)- and (R)-enantiomers. First, we have developed a universal experimental approach to obtain reliable and reproducible solid-state VCD spectra. First, the samples were prepared as pellets composed of mixtures of camphor as a model compound and a crystalline matrix powder. In order to obtain the best results without artifacts and with a maximum signal-to-noise ratio, the following experimental conditions were optimized: pellet thickness and diameter and sample rotation speed. The optimized parameters were then used for the analysis of amphetamine and its derivatives (methamphetamine and 3,4-methylendioxymethamphetamine). Our high-quality spectra and results suggest that solid-state VCD spectroscopy represents a cost-effective and easy-to-use method for the analysis of conformation changes and molecular packing in solid-state with potential applications in pharmaceutical and forensic practice.

Synthesis of phenylalanine and tartaric acid intercalated layered double hydroxide (LDH): Investigation of two kinds of chiral molecules interaction in interlayer space of LDH using solid-state vibrational circular dichroism

Recently, the applicability of solid-state vibrational circular dichroism (SD-VCD) for the evaluation of organoclay nanohybrids has attracted much attention. Herein, the intercalation of two kinds of chiral molecules, phenylalanine (Phe) and tartaric acid (Tart), into Zn-Al layered double hydroxide (LDH) was carried out by a standard coprecipitation method. The Phe/Tart/LDH was characterized by XRD, FT-IR, TG-DTA, and SD-VCD. The chemical composition and interlayer distance expansion of Phe/Tart/LDH indicated that Phe and Tart were intercalated into the LDH. The VCD and theoretical calculation results showed that Phe and Tart coexisted in the same interlayer space of LDH with the formation of electrostatic interaction and hydrogen bonding between Phe-Tart, LDH-Phe, and LDH-Tart. The results demonstrated the utility of the SD-VCD method for determining the detailed conformation of chiral molecules in the LDH interlayer space.

Thermochromic Behavior of Polydiacetylene Nanomaterials Driven by Charged Peptide Amphiphiles.

The tunability of chromatic phases adapted by chromogenic polymers such as polydiacetylene (PDA) is key to their utility for robust sensing applications. Here, we investigated the influence of charged peptide interactions on the structure-dependent thermochromicity of amphiphilic PDAs. Solid-state NMR and circular dichroism analyses show that our oppositely charged peptide-PDA samples have distinct degrees of structural order, with the coassembled sample being in between the β-sheet-like positive peptide-PDA and the relatively disordered negative peptide-PDA. All solutions exhibit thermochromicity between 20 and 80 °C, whereby the hysteresis of the blue, planar phase is much larger than that of the red, twisted phase. Resonance Raman spectroscopy of films demonstrates that only coassemblies with electrostatic complementarity stabilize coexisting blue and red PDA phases. This work reveals the nature of the structural changes responsible for the thermally responsive chromatic transitions of biomolecule-functionalized polymeric materials and how this process can be directed by sequence-dictated electrostatic interactions.

Crystallization Behavior of Heterotrinuclear ZnII–LnIII–ZnII Complexes Bearing Two Tripodal Nonadentate Ligands: Possibility for Absolute Spontaneous Resolution

Crystal structures and crystallization behaviors of a series of heterotrinuclear ZnII–LnIII–ZnII complexes, [(L)ZnLnZn(L)]NO3 (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, and Y; H3L = 1,1,1-tris[(3-methoxysalicylideneamino)methyl]ethane), were examined. Because the trinuclear complex cation has two asymmetric centers (Δ and Λ) at the ZnII sites resulting from the twist of the tripodal ligand arms, it was confirmed from the single-crystal X-ray diffraction (SC-XRD) analysis that these complexes have homochiral structures (i.e., Λ,Λ- or Δ,Δ-enantiomer) in the solid state. Among the series of compounds, various types of spontaneous resolution were observed, which were classified by the SC-XRD analysis (using the Flack parameter) or solid-state circular dichroism spectroscopy. The latter technique was used to determine the chirality of every single crystal and that of an entire bulk mixture of microcrystals obtained in a particular crystallization process. It was found that the TbIII complex, [(L)ZnTbZn(L)]NO3·2MeOH (Zn–Tb–Zn), exclusively deposited the left-handed Λ-conglomerate in every crystallization experiment. This phenomenon may be termed “absolute spontaneous resolution.” It was also notable that only the lanthanoid(III) ions with an even number of 4f electrons (Eu, Tb, Ho, and Lu) exhibited this unusual phenomenon. It was also attempted to deposit the opposite-handed Zn–Tb–Zn Δ-conglomerate, which could not be isolated from the ambient crystallization. Seeding of a crashed single-crystal Δ-conglomerate of [(L)ZnYZn(L)]NO3·2MeOH (Zn–Y–Zn), which was isomorphic to Zn–Tb–Zn, into a saturated methanolic solution of Zn–Tb–Zn gave, for the first time, crystals of the corresponding Δ-conglomerate.

Base Pair Stacking Modulates Solid-State Charge Transport in DNA Hairpins.

Particular interest has been focused on modulation of solid-state charge transport (CT) in DNA. Nevertheless, it remains challenging to do so in a sensitive and predictive manner due to the lack of a definite relationship between DNA base pair stacking and DNA CT. The challenges can be mainly attributed to the ill-defined systems, which may lead to ambiguous and even contradictory conclusions. Here, we use DNA hairpins to construct the well-defined self-assembled monolayers. We reveal nearly positive-linear correlations between DNA conformation and CT in the DNA hairpins regulated with metal ion chelation and DNA sequence. The correlations have been confirmed by the solid-state current-voltage characteristics and circular dichroism in solution. The enhanced CT via metal ion chelated DNA hairpins is mainly from the improved DNA energy coupling to electrodes, not the almost unchanged energy barrier when Hg ion-induced DNA conformational switches toward the canonical B-form.

Noncentrosymmetric Supramolecular Hydrogen-Bonded Assemblies Based on Achiral Pyrazine-Bridged Zinc(II) Coordination Polymers with Pyrazinedione Derivatives

Reaction of M(OAc)2·xH2O (M, x = Zn, 2 and Co, 4), 1,4-dihydro-5,6-dicyano-2,3-pyrazinedione (H2CN2pyzdione), and pyrazine (pyz) affords two compounds of the same molecular formula {[M(H2O)6][M(CN2pyzdione)2(pyz)]·6H2O}n (M = Zn for 1 and Co for 2) in which discrete units of [M(H2O)6]2+ are linked to one-dimensional chains of [M(CN2pyzdione)2(pyz)]2– via multiple O–H···O hydrogen-bonding interactions and M2+-bound H2O molecules in [M(H2O)6]2+ also serve as linkers of hydrogen-bonded interstitial H2O molecules. Remarkably, 1 crystallizes in the monoclinic crystal system, the similar crystal system and unit cell parameters as 2, but with a space group distinct from 1 and 2, i.e., 1 is the noncentrosymmetric space group C2, whereas 2 is the centrosymmetric space group C2/m. This polar structure for 1 is induced by the presence of alternating arrangements of distinguishable two axial Zn–N bonds within [Zn(CN2pyzdione)2(pyz)]2– chains. Indeed, solid-state circular dichroism spectra of 1 exhibit significant Cotton effects, as evidenced by the polar space group C2. Moreover, these Cotton effects show clear temperature-dependence depending on contents of H2O molecules of 1.

Chiral lanthanide-silver(I) cluster-based metal-organic frameworks exhibiting solvent stability, and tunable photoluminescence.

Due to the lack of effective synthetic strategies, the preparation of chemically stable chiral Ag(I) cluster-based materials for assembly remains challenging. Here, we have developed an approach to synthesize three pairs of chiral Ln-Ag(I) cluster-based metal-organic frameworks (MOFs) named l-LnAg5-3D (Ln = Gd for 1-L, Eu for 2-L, and Tb for 3-L) and d-LnAg5-3D (Ln = Gd for 1-D, Eu for 2-D, and Tb for 3-D) by employing a chiral Ag(I) cluster ({Ag5S6}) as the node and Ln3+ ion as the inorganic linker. Structural analysis revealed that the chiral ligands induced chirality through the entire structure, resulting in a chiral helix arrangement of the C3-symmetric chiral {Ag5S6} nodes and Ln3+ ions. These compounds showed high solvent stability in various polar organic solvents. The solid-state circular dichroism (CD) spectra of compounds l-LnAg5-3D and d-LnAg5-3D exhibited obvious mirror symmetrical peaks. The emission spectra in the solid state revealed that compound 1-L only exhibited the emission peak of {Ag5S6}, while compounds 2-L and 3-L exhibited overlapping peaks of Ln3+ and {Ag5S6} at different excitation wavelengths. This demonstrates the tunable photoluminescence from {Ag5S6} to Ln3+ by introducing different Ln3+ ions and manipulating the excitation wavelengths. The study underscores the enhanced stability of Ag(I) cluster-based MOFs achieved through the incorporation of Ln3+ ions and establishes chiral Ln-Ag(I) cluster-based MOFs as promising candidates for advanced materials with tunable photoluminescence.

Coordination Patterns of the Diphosphate in IDP Coordination Complexes: Crystal Structure and Chirality

The knowledge of accurate geometrical parameters from X-ray diffraction studies in the solid state of metal nucleotide is very important for understanding the relationship between structures and properties, including biochemical processes and even enzyme-metal-substrate interactions. The research is also very necessary to precisely and controllably design the functional materials. Here, seven types of coordination polymers of inosine 5'-diphosphate nucleotide (IDP) with transition metals, {[Zn(HIDP)(azpy)(H2O)2]·4H2O}n (1), {[Cd2(IDP)2(bpda)2]·[Cd(H2O)6]·11H2O}n (2), {[Cd3(IDP)2(4,4'-bipy)2(H2O)3]·6H2O}n (3), {[Cd2(IDP)2(bpe)2(H2O)2]·(H2bpe)·26H2O}n (4), {[Cu3(IDP)2(azpy)2(H2O)5]·5H2O}n (5), {[Cu3(IDP)2(bpe)2(H2O)5]·9H2O}n (6), and {[Co(HIDP)(azpy)(H2O)2]·7H2O}n (7) [4,4'-bipy = 4,4'-bipyridine, azpy = 4,4'-azopyridine, bpe = 1,2-bis(4-pyridyl)ethene, and bpda = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene], were designed, synthesized, and firmly characterized using single-crystal X-ray diffraction. The coordination patterns of the diphosphate group of IDP in these complexes were studied by crystallographic analysis, namely, open, close, and open-close hybrid types. We have investigated the diverse coordination patterns of the diphosphate group and its spatial relationship relative to the pentose ring on the basis of two conformational parameters, the pseudorotation phase angle and the degree of puckering. Crystallographic studies clearly reveal the correlation between the backbone torsion angle (ω' and φ) of the sugar-diphosphate and the conformational preference of the pentose ring, i.e., the signs of the backbone torsion angles ω' and φ are both plus (+) or minus (-), the conformation of the pentose ring is envelope form (E), while when one of the two signs is plus (+) and the other is minus (-), the pentose ring is in the twist form (T). This is the first time elucidation of the coordination pattern of diphosphate relative to the conformation of the pentose ring in nucleotide metal complexes, which are different from the other inorganic or organic diphosphate compounds. The chirality of these coordination polymers was examined by combining solid-state circular dichroism spectroscopy measurements with the explanation of their crystal structures. The results presented in this paper are very important for understanding their nucleotide coordination chemistry, their supramolecular chemistry, and even their biochemistry.

Toxic Misfolded Transthyretin Oligomers with Different Molecular Conformations Formed through Distinct Oligomerization Pathways.

Protein aggregation is initiated by structural changes from native polypeptides to cytotoxic oligomers, which form cross-β structured amyloid. Identification and characterization of oligomeric intermediates are critically important for understanding not only the molecular mechanism of aggregation but also the cytotoxic nature of amyloid oligomers. Preparation of misfolded oligomers for structural characterization is, however, challenging because of their transient, heterogeneous nature. Here, we report two distinct misfolded transthyretin (TTR) oligomers formed through different oligomerization pathways. A pathogenic TTR variant with a strong aggregation propensity (L55P) was used to prepare misfolded oligomers at physiological pH. Our mechanistic studies showed that the full-length TTR initially forms small oligomers, which self-assemble into short protofibrils at later stages. Enzymatic cleavage of the CD loop was also used to induce the formation of N-terminally truncated oligomers, which was detected in ex vivo cardiac TTR aggregates extracted from the tissues of patients. Structural characterization of the oligomers using solid-state nuclear magnetic resonance and circular dichroism revealed that the two TTR misfolded oligomers have distinct molecular conformations. In addition, the proteolytically cleaved TTR oligomers exhibit a higher surface hydrophobicity, suggesting the presence of distinct oligomerization pathways for TTR oligomer formation. Cytotoxicity assays also revealed that the cytotoxicity of cleaved oligomers is stronger than that of the full-length TTR oligomers, indicating that hydrophobicity might be an important property of toxic oligomers. These comparative biophysical analyses suggest that the toxic cleaved TTR oligomers formed through a different misfoling pathway may adopt distinct structural features that produce higher surface hydrophobicity, leading to the stronger cytotoxic activities.

Induction of Chiral Hybrid Metal Halides from Achiral Building Blocks.

Chiral hybrid organic-inorganic metal halides (HOMHs) with intrinsic noncentrosymmetry have shown great promise for broad applications in chiroptoelectronics, spintronics, and ferroelectronics. However, the construction strategies for chiral HOMHs often involve chiral building blocks in their frameworks, which greatly limit their chemical diversity. Here, we take advantage of a chiral induction approach and have successfully constructed a series of chiral HOMHs, DMA4MX7 (DMA = dimethylammonium, M = Sb or Bi, X = Cl or Br), based on achiral precursors. The resulting chiral products demonstrate a clear enantioenrichment, as confirmed by single-crystal X-ray diffraction analysis and solid-state circular dichroism (CD) spectroscopy. The induction of chiral HOMHs enables superior nonlinear optical performances with very high thermal stability and laser resistance. The successful employment of such a chiral induction approach might facilitate the construction of libraries of chiral HOMH crystals from diverse achiral precursors, in particular those into which it is not easy to introduce intrinsic chiral centers, and would thus pave a new way for rational preparation and application of chiral HOMH materials.

Clay Column Chromatography for Optical Resolution: A Series of Derivatized Amino Acids

Abstract Chromatographic resolution of a series of derivatized amino acids was attempted on a column packed with an ion-exchange adduct of Δ-[Ru(phen)3]2+ (phen = 1,10-phenanthroline) and synthetic hectorite. An amino acid was modified to N-3, 5-dinitrobenzoyl amino acid methyl ester (denoted by DNB-aa-me). For aa = Ala, Phe, Leu, Ile, Ser, Val, Thr, Tyr, Asp and Glu, racemic DNB-aa-me was resolved nearly to baseline separation on being eluted with methanol. For aa = Trp and His, racemic DNB-aa-me was resolved partially. No resolution was achieved for Pro and Lys. The mechanism of chiral discrimination was investigated by means of solid-state vibrational circular dichroism spectroscopy.

Homochiral Mn3+ Spin-Crossover Complexes: A Structural and Spectroscopic Study.

Structural, magnetic, and spectroscopic data on a Mn3+ spin-crossover complex with Schiff base ligand 4-OMe-Sal2323, isolated in crystal lattices with five different counteranions, are reported. Complexes of [Mn(4-OMe-Sal2323)]X where X = ClO4– (1), BF4– (2), NO3– (3), Br– (4), and I– (5) crystallize isotypically in the chiral orthorhombic space group P21212 with a range of spin state preferences for the [Mn(4-OMe-Sal2323)]+ complex cation over the temperature range 5–300 K. Complexes 1 and 2 are high-spin, complex 4 undergoes a gradual and complete thermal spin crossover, while complexes 3 and 5 show stepped crossovers with different ratios of spin triplet and quintet forms in the intermediate temperature range. High-field electron paramagnetic resonance was used to measure the zero-field splitting parameters associated with the spin triplet and quintet states at temperatures below 10 K for complexes 4 and 2 with respective values: DS=1 = +23.38(1) cm–1, ES=1 = +2.79(1) cm–1, and DS=2 = +6.9(3) cm–1, with a distribution of E parameters for the S = 2 state. Solid-state circular dichroism (CD) spectra on high-spin complex 1 at room temperature reveal a 2:1 ratio of enantiomers in the chiral conglomerate, and solution CD measurements on the same sample in methanol show that it is stable toward racemization. Solid-state UV–vis absorption spectra on high-spin complex 1 and mixed S = 1/S = 2 sample 5 reveal different intensities at higher energies, in line with the different electronic composition. The statistical prevalence of homochiral crystallization of [Mn(4-OMe-Sal2323)]+ in five lattices with different achiral counterions suggests that the chirality may be directed by the 4-OMe-Sal2323 ligand.

Preparation and characterization of a new chiral metal-organic framework with spiranes

Herein we report the first homochiral IRMOF structure with chiral carbocyclic spirolinkers, basic Zn-(R)-spiro[3.3]heptane-2,6-dicarboxylate, named WIG-5. First, (R)-spiro[3.3]heptane-2,6-dicarboxylic acid was prepared involving a HPLC separation on chiral stationary phase, then further transformed into homochiral WIG-5 by the solvothermal reaction with zinc nitrate hexahydrate in dimethyl formamide (DMF). WIG-5 crystallizes in the P212121 space group and is built of two interpenetrated networks with pcu underlying network topology. The tetranuclear secondary building units (SBUs) of WIG-5 are coordinated to two DMF molecules and six (R)-spiro[3.3]heptane-2,6-dicarboxylic acid linkers, that results in the formation of a homochiral distorted MOF-5-like structure. Extensive characterization by single crystal X-ray diffraction, powder X-ray diffraction, polarized light microscopy, infrared microspectroscopy, solid-state vibrational circular dichroism spectroscopy and thermogravimetry/mass spectroscopy has been performed on the new material. Birefringence of large WIG-5 single crystals enables potential optical applications.

Conformation Locking of the Pentose Ring in Nucleotide Monophosphate Coordination Polymers via π-π Stacking and Metal-Ion Coordination.

The conformation of the pentose ring in nucleotides is extremely important and a basic problem in biochemistry and pharmaceutical chemistry. In this study, we used a strategy to regulate the conformation of pentose rings of nucleotides via the synergistic effect of metal-ion coordination and π-π stacking. Seven types of coordination complexes were developed and characterized using Fourier transform infrared spectroscopy, elemental analysis, thermogravimetric analysis, powder X-ray diffraction, ultraviolet-visible spectroscopy, 1H nuclear magnetic resonance spectroscopy, electrospray ionization mass spectrometry, and single-crystal X-ray diffraction. On the basis of two conformational parameters obtained from single-crystal structure analysis, i.e., the pseudorotation phase angle and degree of puckering, the exact conformation of the furanose ring in these coordination polymers was unequivocally determined. Crystallographic studies demonstrate that a short bridging ligand (4,4'-bipyridine) is conducive to the formation of a twist form, and long auxiliary ligands [1,2-bis(4-pyridyl)ethene and 4,4'-azopyridine] induce the formation of an envelope conformation. However, the longest auxiliary ligands [1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene] cannot limit the flexibility of a nucleotide. Our results demonstrated that the proposed strategy is universal and controllable. Moreover, the chirality of these coordination polymers was examined by combining the explanation of their crystal structures with solid-state circular dichroism spectroscopy measurements.