Multidimensional vibrational circular dichroism for insect wings: Comparison of species.

This study reports the microscopic measurements of vibrational circular dichroism (VCD) on four different insect wings using a quantum cascade laser VCD system equipped with microscopic scanning capabilities (named multi-dimensional VCD [MultiD-VCD]). Wing samples, including (i) beetle, Anomala albopilosa (female), (ii) European hornet, Verspa crabro flavofasciata Cameron, 1903 (female), (iii) tiny dragonfly, Nannophya pygmae Rambur, 1842 (male), and (iv) dragonfly, Symetrum gracile Oguma, 1915 (male), were used in this study. Two-dimensional patterns of VCD signals (~10 mm × 10 mm) were obtained at a spatial resolution of 100 µm. Measurements covered the absorption peaks assigned to amides I and II in the range of 1500-1740 cm-1 . The measurements were based on the enhancement of VCD signals for the stereoregular linkage of peptide groups. The patterns were remarkably dependent on the species. In samples (i) and (ii), the wings comprised segregated domains of protein aggregates of different secondary structures. The size of each microdomain was approximately 100 µm. In contrast, no clear VCD spectra were detected in samples (iii) and (iv). One possible reason was that the chain of stereoregular polypeptides was too short to achieve VCD enhancement in samples (iii) and (iv). Notably, the unique features were only observed in the VCD spectra because the IR spectra were nearly the same among the species. The VCD results hinted at the connection of protein microscopic structures with the wing flapping mechanisms of each species.

Thermo-/Mechano-Chromic Chiral Coordination Dimer: Formation of Switchable and Metastable Discrete Structure through Chiral Self-Sorting.

Although strong chiral self-sorting often emerges in extended covalent or supramolecular polymers, the phenomenon is generally weak in discrete assemblies (e.g., dimers and oligomers) of small molecules due to the lack of a cooperative growth mechanism. Consequently, chiral self-sorting has been overlooked in the design of switchable and metastable discrete supramolecular structures. Here, we report a butyl-benzo[h]quinoline-based iridium(III) complex (Bu-Ir) with helical chirality at its metal center, which forms preferentially a homochiral dimer and exhibits thermo-/mechano-chromism based on a monomer-dimer transformation. While a five-coordinate monomer is formed in a racemic or an enantiopure Bu-Ir solution at 25 °C, a six-coordinate homochiral dimer complex is formed almost exclusively at low temperatures, with a higher degree of dimerization in enantiopure Bu-Ir solution. Estimation of apparent dimerization binding constants (K) and thermodynamic parameters (ΔH and ΔS) based on variable temperature ultraviolet-visible (UV-vis) and 1H NMR spectra reveals a strong preference for homochiral dimerization (largest known value for the coordination complex, Khomo/Khetero > 50). Notably, crystals of the homochiral dimer are metastable, undergoing a distinct color change upon grinding (from yellow to red) due to mechanical cleavage of coordination bonds (i.e., a dimer to monomer transformation). A comparison with control compounds having different substituents (proton, methyl, isopropyl, and phenyl groups) reveals that Bu-Ir dimerization involves both strong homochiral self-sorting preference and connected thermo-/mechano-chromic behavior, which is based on matched propeller-shaped chirality and subtle steric repulsion between alkyl substituents that render the homochiral dimer switchable and metastable. These findings provide substantial insights into the emergence of dynamic functionality based on the rational design of discrete chiral assemblies.

Chiroptical switching behavior of heteroleptic ruthenium complexes bearing acetylacetonato and tropolonato ligands.

Four types of tris-chelate ruthenium complexes bearing acetylacetonato (acac) and tropolonato (trop) ligands were synthesized and optically resolved into Δ and Λ isomers: [Ru(acac)3] (Ru-0), [Ru(acac)2(trop)] (Ru-1), [Ru(acac)(trop)2] (Ru-2), and [Ru(trop)3] (Ru-3). Chiral HPLC chromatograms, electronic circular dichroism (ECD), and vibrational circular dichroism (VCD) of the four ruthenium complexes were systematically investigated. As a result, the absolute configurations of the newly prepared enantiomeric complexes Ru-2 and Ru-3 were determined. For the case of Ru-2, its absolute configuration was also confirmed by single crystal X-ray diffraction analysis. The ECD changes upon chemical oxidation were further investigated for the four complexes. An ECD change in enantiomeric Ru-1 was observed upon oxidation, but the oxidized species soon returned to the neutral state within a few minutes. Enantiomers of Ru-3 also showed explicit ECD changes upon oxidation. Further, the lifetime of the oxidized state was the longest among the four investigated complexes, whereas they racemized in solution at room temperature. In contrast, the enantiomers of heteroleptic complexes (Ru-1 and Ru-2) concurrently exhibited ECD changes, relatively long lifetime of the oxidized state, and nil or quite slow racemization behavior. The coexistence of acac and trop ligands was key to making the competing factors compatible in the resultant ruthenium complexes.

Five-coordinate iridium(III) complex with ΔΛ chirality.

The coordinatively unsaturated bis-chelated iridium(III) complex, [Ir(2-Bubzq)2Cl] (2-BubzqH = 2-butyl-benzo[h]quinoline), denoted as complex 1, was obtained by reacting iridium(III) trichloride with 2-BubzqH in a 1 : 2 molar ratio. The results were in contrast to the common view that a chlorine-bridged dimer, [Ir(L)2Cl]2 (L = bis-chelate ligand), is formed under the corresponding conditions. A single-crystal X-ray diffraction structural analysis revealed that complex 1 has a five-coordinate geometry with a distorted square pyramidal configuration. The optical resolution of complex 1 was measured chromatographically on a chiral column, yielding Δ and Λ as enantiomers. The resolved enantiomers were stable enough against racemization in CDCl3 as confirmed by the vibrational circular dichroism measurements. Complex 1 reacted with carbon monoxide (CO) to give [Ir(2-Bubzq)2(CO)Cl] and with 1,10-phenanthroline (phen) to give [Ir(2-Bubzq)2(phen)]Cl within a minute with its absolute configuration (ΔΛ chirality) maintained.

Mapping of Supramolecular Chirality in Insect Wings by Microscopic Vibrational Circular Dichroism Spectroscopy: Heterogeneity in Protein Distribution.

The supramolecular chirality of the hindwing of Anomala albopilosa (male) was investigated using a microscopic vibrational circular dichroism (VCD) system, denoted as MultiD-VCD. The source of intense infrared (IR) light for the system was a quantum cascade laser. Two-dimensional maps of IR and VCD spectra were taken by scanning the surface area (ca. 2 mm × 2 mm) of the insect hindwing tissue. The spectra ranged from 1500 to 1700 cm-1, and the maps have a spatial resolution of 100 µm. The distribution of proteins, including their supramolecular structures, was analyzed from the location-dependent spectral shape of the VCD bands assigned to amides I and II. The results revealed that the hindwing consists of segregated domains of proteins with different secondary structures: an α-helix (in one part of the membrane), a hybrid of α-helix and ß-sheet (in another part of the membrane), and a coil (in a vein).

Vibrational circular dichroism of d-amino acid-containing peptide NdWFamide in the crystal form.

Microcrystals of l-Asn-d-Trp-l-Phe-NH2 (NdWFamide), a tripeptide derived from Aplysia kurodai that exhibits invertebrate cardiac activity, were evaluated by vibrational circular dichroism (VCD). The chirality of the tryptophan residue at the second position in NdWFamide was associated with the conformation and biological characteristics. The VCD spectrum of NdWFamide was a mirror image of its enantiomer; however, it was significantly different from that of its diastereomer, NWFamide, which is its precursor. The obtained VCD signals of NdWFamide were in good agreement with the VCD signals that were calculated based on the optimized aggregates of NdWFamide, which formed a helical-like backbone conformation. The evaluation of the VCD results revealed the conformation of NdWFamide in the crystalline state and succeeded in distinguishing its stereoisomers. Therefore, this study demonstrates VCD as a useful method for the structural analysis of naturally occurring d-amino acid-containing peptides.

Effects of geometrical isomerism on emissive behaviour of heteroleptic cyclometalated Ir(III) complexes.

In this study, we report the emissive properties of a heteroleptic cyclometalated Ir(iii) complex, [Ir(bzq)2(PBO)] (bzqH = benzo[h]quinoline; PBOH = 2-(2-hydroxyphenyl)benzoxazole). The complex, [Ir(C^N)2(N^O)], was synthesised and optically resolved using a chiral column. Two geometrical isomers, trans-(N,N) and cis-(N,N) isomers, were obtained as the major and minor products in an enantiopure form, respectively. Their molecular structures were determined using single crystal X-ray analysis. In the crystalline states, the intermolecular C-Hπ interaction between PBO- and an H atom in bzq- was the main factor influencing molecular packing. When the complexes were dissolved in CH2Cl2 and excited at 430 nm under N2 atmosphere, yellow (λmax = 550 nm) and orange emissions (λmax = 570 nm) were observed for the trans-(N,N) and cis-(N,N) isomers, respectively, with the quantum yield higher for the former than the latter.

Degradation of insulin amyloid by antibiotic minocycline and formation of toxic intermediates.

Insulin balls, localized insulin amyloids formed at subcutaneous insulin-injection sites in patients with diabetes, cause poor glycemic control owing to impairments in insulin absorption. Our previous study has shown that some insulin balls are cytotoxic, but others are not, implying amyloid polymorphism. Interestingly, the patient with toxic insulin balls had been treated with antibiotic minocycline, suggesting a possible relationship between toxicity of insulin balls and minocycline. However, the direct effect of minocycline on the structure and cytotoxicity of the insulin amyloid is still unclear. Herein, we demonstrated that that minocycline at physiological concentrations induced degradation of insulin amyloids formed from human insulin and insulin drug preparations used for diabetes patients. Interestingly, the process involved the initial appearance of the toxic species, which subsequently changed into less-toxic species. It is also shown that the structure of the toxic species was similar to that of sonicated fragments of human insulin amyloids. Our study shed new light on the clarification of the revelation of insulin balls and the development of the insulin analogs for diabetes therapy.

Multidimensional Vibrational Circular Dichroism Apparatus Equipped with Quantum Cascade Laser and Its Use for Investigating Some Peptide Systems Containing d-Amino Acids.

We developed a multidimensional vibrational circular dichroism system with a positional coordinate, making it possible to move in both x- and y-directions using an automatic stage. Quantum cascade laser (QCL) was used as a light source to achieve high intensity and narrow focusing. The QCL emitted light in the wavenumber range of 1500-1740 cm-1, which encompassed absorption bands assigned to the stretching vibrations of amides I and II. The operation of the instrument was analyzed for samples containing amide groups. An aqueous solution of Gly-l-Leu or Gly-d-Leu was measured under the background absorbance as high as 3.5 due to the water medium. The spectra were recorded by scanning at 1.0 cm-1 steps. The time required for performing measurement at each wavenumber was less than 1 s. The mirror-image relation was maintained between the optical antipodes. A peak assigned to amide II appeared clearly at around 1580 cm-1. In the case of KBr pellets containing the same compounds, peaks assigned to amide I and II were observed. For two-dimensional pattering, a KBr pellet comprising two domains of amino acids (or l-Ala and d-Ser) was investigated. The distribution of each component within the pellet was obtained under the two-dimensional alignment at the spatial interval of 2.5 mm.

Vibrational circular dichroism towards asymmetric catalysis: chiral induction in substrates coordinated with copper(II) ions.

Cu(ii) complexes containing RR- or SS-2,2'-isopropylidene-bis(4-phenyl-2-oxazoline) (denoted as [Cu(RR- or SS-oxa)]2+) are known to catalyse many asymmetric organic reactions. Herein, the source of enantioselectivity was investigated by vibrational circular dichroism (VCD) spectroscopy. An achiral ß-diketonato ligand (denoted as LH), such as 1-phenyl-1,3-butanedione and dibenzoylmethane, was added to form [Cu(RR- or SS-oxa)L]+. Clear VCD peaks were obtained from a CDCl3 solution of [Cu(RR- or SS-oxa)]2+ or [Cu(RR- or SS-oxa)L]+ at 1000-1800 cm-1. It is to be noted that when LH was coordinated, a new VCD peak appeared at ∼1380 cm-1, which was assigned to the C-O asymmetric stretching vibration of L-. Theoretical simulation helped rationalise the results in terms of the transformation of coordinated L- into a twisted chiral form. The extent of steric control within the coordination sphere was demonstrated, revealing the first step for enantioselectivity during catalysis.

Enantiomeric Excess Dependent Splitting of NMR Signal through Dynamic Chiral Inversion and Coligand Exchange in a Coordination Complex.

Nuclear magnetic resonance (NMR) spectroscopy cannot be used to discriminate enantiomers, and NMR resonances of enantiomeric mixtures are generally not affected by enantiomeric excess (ee). Here, we report that a coordination complex (L·2Zn·3C), where L is a salen-like prochiral ligand and C is an exchangeable acetate coligand, exhibits symmetrical splitting of one of the 1H NMR resonances of L with the degree of splitting linearly proportional to ee of the chiral guest coligand C, 2-phenoxypropionic acid. Despite the well-defined chirality in the crystal structure of L·2Zn·3C, concurrent fast chiral inversion and coligand exchange in solution renders L·2Zn·3C the primary example of prochiral solvating agent (pro-CSA) based on a coordination complex. Notably, the NMR resonances remain split even in dilute solution due to the lack of chiral guest dissociation in the coligand exchange system. This work provides new insights into chiral transfer events in metal-ligand complexes.

Visualizing the helical stacking of octahedral metallomesogens with a chiral core.

A combination of grazing-incidence X-ray diffraction and molecular dynamics simulation studies led to the visualization of the stacking structure of a helical columnar liquid crystal formed by enantiopure octahedral metallomesogens with ΔΛ chirality. The helical structure was elucidated as a hybrid of two major proposed structures.

Structural Characterization of a Cyclodextrin/l-menthol Inclusion Complex in the Solid-state by Solid-state NMR and Vibrational Circular Dichroism.

Hydrophobic and volatile flavor molecules can be encapsulated inside cyclodextrins (CyDs). Inclusion complexes are frequently used in solid or dispersed states in preserved food and cosmetics. In this study, the solid-state structures of spray-dried inclusion complexes of l-menthol in α-CyD and ß-CyD were analyzed using 13C solid-state NMR and vibrational circular dichroism (VCD). The NMR signals of l-menthol and CyDs were identified in the physical mixture and the l-menthol inclusion complex of α- and ß-CyD. The NMR signal of the isopropyl-methyl group of menthol in the α-CyD inclusion complex exhibited a large low-field shift, which suggested a steric hindrance between menthol and α-CyD. VCD exhibited specific changes in the intensity of bands corresponding to C-C vibrations in α-CyD and O-C stretching vibrations in l-menthol. Our results indicated that l-menthol specifically fitted the narrow space within α-CyD. The combined solid-state NMR and VCD analysis provided structural insights into the flavor inclusion complex in the solid-state.

Solid-state vibrational circular dichroism studies on the conformation of an amino acid molecule in crystalline state.

The present article reviews the results of solid-state vibrational circular dichroism (SD-VCD) measurements on the crystals of amino acids. The investigated series were Ala, Ile, Leu, Phe, Ser, Val, and Thr. Spectra were recorded for the samples containing both D- and L-enantiomers. The molecular conformation in a crystalline state was compared among the series. Molecule optimization of their conformation under intermolecular interactions in crystalline states was revealed. One noteworthy aspect was that some VCD peaks were remarkably enhanced in comparison with the solution states. This fact was rationalized in terms of vibrationally coupled delocalization in a closely stacked pair. In addition, for a molecule with two asymmetric carbons, the signs of the VCD peaks were determined by the interplay between the two chiral centers. Lastly, the roles of the theoretical approach based on density functional theory calculations were described.

Separation of D-amino acid-containing peptide phenylseptin using 3,3'-phenyl-1,1'-binaphthyl-18-crown-6-ether columns.

Several D-amino acid-containing peptides (DAACPs) with antimicrobial, cardio-excitatory, or neuronal activities have been found in several species. Here, we demonstrated the chiral separation of the antimicrobial peptide diastereomers, D-phenylseptin and L-phenylseptin using (S) and (R) 3,3'-phenyl-1,1'-binaphthyl-18-crown-6-ether columns (CR-I (+) and CR-I (-), respectively) and also investigated the underlying mechanism. First, using D-amino acid-containing tripeptide Phe-Phe-Phe-OH, we found that CR-I (+) could be used to recognize diastereomeric tripeptides containing an L-amino acid as the first residue. On the contrary, CR-I (-) enabled separation of a series of diastereomers with D-amino acid as the first residue. Therefore, we achieved separation of the stereoisomers using the chiral columns depending on the position of the D- amino acid in the peptide and demonstrated the orthogonality of separations of the chiral columns. Then, using CR-I (+), we separated amphibian antimicrobial peptide diastereomers, L- and D-phenylseptin, which have the sequences, L-Phe-L-Phe-L-Phe and L-Phe-D-Phe-L-Phe at their N-termini, respectively. In order to understand the host-guest interactions, we performed molecular dynamics simulations for L-Phe-L-Phe-L-Phe tripeptide-CR-I molecule complex systems. Three hydrogen bonds between the N-terminal amine group -NH3+ and the crown ether oxygens were the dominant interactions. The hydrophobic interactions between phenyl-rings in the chiral selector unit of CR-I (+) and the side chains of 2nd and 3rd residues of the peptide also contributed to the affinity. Our results show that the CR-I (+)-column can be applied for the separation of endogenous DAACPs generated by the post-translational modification.

A new horizon for vibrational circular dichroism spectroscopy: a challenge for supramolecular chirality.

Vibrational circular dichroism (VCD) spectroscopy is an extension of circular dichroism spectroscopy into the infrared and near-infrared regions where vibrational transitions occur in the ground electronic state of a molecule. The method offers the advantage of studying the chiroptical properties of a wide range of molecules in non-crystalline states. However, because of the smallness of the signals, one measurement requires several hours to yield reliable results. Accordingly, its targets were limited to a stable molecule in a solution. To overcome this difficulty, our group applied the VCD method to supramolecular systems. The work was launched based on the finding that the VCD signal remarkably enhances when low-molecular mass gelators form gels. By analysing a number of well-resolved VCD peaks, the detailed conformation of a component molecule was deduced. This provided a clue to elucidating the molecular organization in supramolecular architectures. Our final goal was to clarify the route from microscopic molecular chirality to supramolecular chirality. For this purpose, a time-step VCD measurement method was developed for the in situ monitoring of the progress of chirality amplification.

Chiral tectonics toward square planar tetranuclear Pd(ii) complexes: propagation of axial chirality through a long molecular axis.

An approach of molecular tectonics was applied to synthesize a tetranuclear Pd(ii) complex with axial chirality, [{(Lt)Pd(ii)(taet)Pd(ii)}2(tpret)] (LtH = benzoylacetone (bzacH) or 2-naphthoylacetone (npacH), taetH2 = 1,1,2,2-tetraacetylethane and tpretH2 = 1,1,2,2-tetrapropanoylethane). As a first step, a two-handed tecton, [{taetH}Pd(ii)]2(tpret)], was synthesized. Next monomeric Pd(ii) units were connected to both ends of the tecton to form a reactive tetranuclear intermediate, [{(hfac)Pd(ii)(taet)Pd(ii)}2(tpret)] (hfacH = hexafluoroacetylacetone). Finally terminal hfac groups were replaced with the ß-diketonato ligands having one or two bulky groups such as dibenzoylmethane (dbmH), bzacH and npacH. In the case of the symmetrically substituted ligand (dbm), the formed complex was achiral and its structure was determined by single crystal X-ray analysis. In the case of unsymmetrical ligands (bzac and npac), the complexes were axially chiral due to the vertical twisting of the terminal ligands. The complexes were optically resolved chromatographically on a chiral column. Their chiro-optical properties were investigated by means of electronic (ECD) and vibrational circular dichroism (VCD) spectroscopy. Notably the twisting relation between the two terminal ligands of the tetranuclear complexes influenced significantly chiral electronic properties, although they were separated by a distance longer than 3 nm.

Solid-state vibrational circular dichroism studies of L- and D-serine.

d-Serine is considered a key endogenous substance involved in several enzymatic reactions in the human body. In this study, solid-state vibrational circular dichroism (VCD) measurements of enantiomeric serine were performed for spectroscopic distinction between d- and l-serine. The mirror-image VCD signals of the isomers in the KBr pellets were observed in the mid-infrared range of 1800-1250 cm-1. The calculated infrared (IR) and VCD spectra for the optimized serine structures were in good agreement with the corresponding observed spectra. In addition, the chemical shift values estimated from the shielding constants of the optimized structure of serine corresponded with the observed values in 13C and 15N solid-state nuclear magnetic resonance spectra, supporting the VCD assignments. Our results suggest the need for further study of VCD to develop a sensitive and high-resolution spectroscopic technique for the detection of d-amino acids.

Self-assembly of tripeptides into γ-turn nanostructures.

Self-assembling phenylalanine-based peptides have garnered interest owing to their potential for creating new functional materials. Here, we designed four diastereomers, l-Phe-l-Phe-l-Phe (FFF), d-Phe-l-Phe-l-Phe (fFF), l-Phe-d-Phe-l-Phe (FfF) and l-Phe-l-Phe-d-Phe (FFf), to analyze the effect of the d-isomer on the self-assembly. Using SEM, TG, VCD, and solid-state NMR measurements, we found that only FFf forms a γ-turn conformation and self-assembles into a nanoplate with higher thermal stability. The supramolecular structure of FFf consists of intra- and intermolecular hydrogen bonds and π-π stackings. From our results, we have discovered that FFf forms a new type of self-assembling γ-turn conformation, clarifying the structural role of a d-amino acid residue in supramolecular formation.

Fabrication of Perfusable Pseudo Blood Vessels by Controlling Sol-Gel Transition of Gellan Gum Templates.

Construction of three-dimensional (3D) tissues with perfusable vascular networks remains a major challenge in the field of tissue engineering. Although various sacrificial templates have been employed to fabricate the vascular networks, there are some issues with respect to cytocompatibility, structural controllability, and degradability for the achievement of precisely controlled vasculatures without cytotoxicity. Here, we demonstrate a naturally occurring polysaccharide, gellan gum (GG), as a sacrificial template material due to its unique character. GG showed rapid gelation at 30-50 °C during the cooling process depending on the concentration of bivalent calcium ions by intermolecular ionic cross-linking and subsequent double-helix formation of GG molecules. Although chelate agents such as EDTA are generally effective in decomposing ionic cross-linking gels, e.g., alginate gel, they usually show cytotoxicity. In the case of GG gel, the gels could not be decomposed by the chelate agents but were easily decomposed by Tris-HCl buffer (pH = 7.4), which is a basic molecule with high cytocompatibility. We finally fabricated straight vascular tubes in 3D-gelatin gels and then demonstrated perfusion of human whole blood at 3.0 cm/s for 2 h. Since the complex vascular networks were constructed by 3D inkjet printing of the GG solution, GG would be useful as a structurally controllable and easily decomposable sacrificial material with cytocompatibility.