Vibrational Circular Dichroism Measurements Research Articles

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.

Matrix effects in MI-VCD spectra of two chiral oxiranes and their potential microscopic origin.

Combining vibrational circular dichroism (VCD) spectroscopy with the matrix isolation (MI) technique opens up interesting possibilities to study chiral molecules. MI involves the isolation of guest species in inert solid matrices at cryogenic temperatures. Hence, MI-VCD measures are solid-state VCD measurements, and as such, can suffer from mostly birefringance-related artefacts in the same way as common solid-state VCD measurements. In this contribution, we demonstrate that the sample preparation condition have tremendous impact on the quality and reliability of the recorded MI-VCD spectra. While MI-IR spectra are basically blind to these artefacts, the variation of deposition temperatures and host flow rates seem to control whether high quality MI-VCD spectra are obtained or if depolarization effects lead to completely obscured spectra. For two selected examples, styrene oxide (SO) and 1-phenyl propylene oxide (PPO), we discuss how the various experimental conditions may lead to the aforementioned effects and give a microscopic description of their origin.

Quantum Cascade Laser-Based Vibrational Circular Dichroism Augmented by a Balanced Detection Scheme.

Vibrational circular dichroism (VCD) constitutes a powerful technique, enabling the determination of the absolute configuration of molecules without the need for specialized reagents. While delivering critical information, VCD signals commonly are several orders of magnitude weaker than classical absorbance signals, which so far necessitated long measurement times to achieve acceptable signal-to-noise ratios (SNRs) in VCD experiments. We present here an improved setup for the measurement of VCD in the range between 5.6 and 6.5 μm. Employing an external cavity quantum cascade laser (EC-QCL) as a high-power light source, we collected spectra with competitive noise levels in less than 5 min. The basis for this improvement was a balanced detection module combined with an optical path catered to VCD measurements. With the stabilization provided by the two-detector setup, noise originating from the laser source could be suppressed effectively. Noise level improvement up to a factor of 4 compared to the classical single detector EC-QCL-VCD could be reported. Compared to commercial Fourier transform infrared (FT-IR) instruments, the presented setup offers measurement time reductions of a factor of at least 6, with comparable noise levels. The applicability of the setup for qualitative and quantitative VCDs was proven. With the comparatively high temporal resolution provided, the monitoring of optically active processes will be possible in future applications.

Optical Resonances of Chiral Metastructures in the Mid‐infrared Spectral Range

AbstractMetamaterials with localized surface plasmon resonances have been the subject of intense research for a wealth of applications ranging from active photonic devices to analytical sensing units that utilizes the local field enhancement to improve sensitivity and decrease acquisition time. Herein, we focus on the modelled properties of a series of spiral metastructures that present multiple chiral plasmon modes in the 1–10 μm mid‐IR spectral range and that could be utilized to enhance vibrational circular dichroism measurements to further identify molecular, supramolecular and structural chirality centres. Finite‐difference time‐domain electromagnetic modelling was conducted to calculate the infrared spectra of the metastructures in response to a circularly polarized excitation. Several geometric parameters were altered to further tune the position of the resonances. In addition, the near‐field distribution of the optical resonances was calculated providing a spatial snapshot of the chiral modes. Preliminary spiral structures were fabricated using electron beam lithography and their vibrational circular response was measured.

Preferred‐handed helical conformation in organic–inorganic hybrid block copolymers with well‐controlled stereoregularity

AbstractA novel block copolymer containing polyhedral oligomeric silsesquioxanes (POSS) features a well‐controlled stereoregularity, that is, isotactic polymethacrylate‐functionalized POSS‐b‐polymethyl methacrylate (it‐PMAPOSS‐b‐PMMA), was prepared via controlled‐anionic polymerization. The stereoregularity of each segment in the it‐PMAPOSS‐b‐PMMA is evaluated using nuclear magnetic resonance (NMR) spectroscopy, which reveals an isotactic configuration in both the PMAPOSS and PMMA segments. The secondary structure of it‐PMAPOSS‐b‐PMMA in a mixture with a small chiral dopant is clarified with electronic circular dichroism and vibrational circular dichroism measurements. By adding a chiral dopant, a controlled preferential helical conformation of the unilateral segment in achiral it‐PMAPOSS‐b‐PMMA is achieved.

Vibrational circular dichroism spectroscopy as a probe for the detection of collagen fibril and fibrillation in solution

The detection of collagen fibrils and fibrillation is crucial for tissue regeneration, biomedical applications of collagen, and for studying the interaction of small molecules with the collagen fibrils in the development of therapeutics for collagen-based diseases. At present, collagen fibrillation is probed by high-value microscopic techniques, which limits real-time monitoring of collagen fibril growth and measurement only in dry form. In this context, a quick sensing method is necessary for detecting these fibrils, especially monitoring fibril growth in solution. Herein, we show that vibrational circular dichroism (VCD), a growing spectroscopic technique, can probe collagen fibrils in aqueous solutions by generating an enhanced amide I VCD signal (intensity) compared to monomer. Unlike regular protein VCD measurements, VCD of collagen fibrils require a very low concentration (5 mg/mL) of collagen and can be probed in a very short time (∼10 min). Moreover, VCD responds to the diameter and height of collagen fibrils as evidenced by an increased VCD intensity (close to 103) with pH and NaCl, making VCD a sensitive tool for monitoring the growth of the fibril diameter and height. VCD was also found to be suitable for detecting the time-dependent increase in fibril diameter under fibrillation conditions. Because no other probe is presently available for rapid detection of collagen fibrils in solution form, the ability of VCD to rapidly probe collagen fibrillation in solution and in real-time opens a new way of studying collagen fibrillation in different fields and applications.

Concurrent Vibrational Circular Dichroism Measurements with Infrared Spectroscopic Imaging.

Vibrational circular dichroism (VCD) spectroscopy has emerged as a powerful platform to quantify chirality, a vital biological property that performs a pivotal role in the metabolism of life organisms. With a photoelastic modulator (PEM) integrated into an infrared spectrometer, the differential response of a sample to the direction of circularly polarized light can be used to infer conformation handedness. However, these optical components inherently exhibit chromatic behavior and are typically optimized at discrete spectral frequencies. Advancements of discrete frequency infrared (DFIR) spectroscopic microscopes in spectral image quality and data throughput are promising for use toward analytical VCD measurements. Utilizing the PEM advantages incorporated into a custom-built QCL microscope, we demonstrate a point scanning VCD instrument capable of acquiring spectra rapidly across all fingerprint region wavelengths in transmission configuration. Moreover, for the first time, we also demonstrate the VCD imaging performance of our instrument for site-specific chirality mapping of biological tissue samples. This study offers some insight into future possibilities of examining small, localized changes in tissue that have major implications for systemic diseases and their progression, while also laying the groundwork for additional modeling and validation in advancing the capability of VCD spectroscopy and imaging.

Strategy for the vibrational circular dichroism study of a glycosylflavonoid evaluated as its peracetate. The case of bioactive 7-O-β-D-glucopyranosylchrysin

As it is difficult to study polyhydroxylated natural products by vibrational circular dichroism (VCD) due to their very limited solubility in infrared transparent solvents, 7-O-β-D-glucopyranosylchrysin (1), isolated from Mimosa rosei, was acetylated to provide 7-O-(2ʹʹ,3ʹʹ,4ʹʹ,6ʹʹ-tetraacetyl-β-D-glucopyranosyl)-5-acetylchrysin (2). Density functional theory (DFT) VCD spectra of 2 were calculated using the B3LYP, B3PW91, and PBEPBE functionals and the DGDZVP basis set to ascertain a good level of theory for calculating this kind of compounds. Statistical comparison of the experimental and calculated VCD spectra revealed that the frequently employed B3LYP and B3PW91 functionals provided good confidence levels with similar statistical results, allowing band assignments for the pyranoside portion of 2. This is the first report on a satisfactory statistical validation of a VCD measurement for such a glycopyranoside. In addition, in vitro cytotoxicity studies of 1 and 2 against the human colon cell line HT-29 and the human melanoma cell line UACC-62 by the MTT method were assayed. Moreover, the immunomodulatory response of 1 and 2 was evaluated by measuring TNF-α, IL-1β, and IL-10 levels by the ELISA technique. The results revealed low cytotoxicity of both compounds against cancer cells. However, flavonoid 1 upregulated the IL-1β production in lipopolysaccharide (LPS)-induced THP-1 macrophage, thus suggesting the potential of this molecule as an immunostimulant for cancer treatment.

Optically tunable terahertz chiral metasurface based on multi-layered graphene

Active manipulation of the polarization states at terahertz frequencies is crucially helpful for polarization-sensitive spectroscopy, having significant applications such as non-contact Hall measurements, vibrational circular dichroism measurements and anisotropy imaging. The weakness of polarization manipulation provided by natural materials can be overcomed by chiral metamaterials. Chiral metamaterials have a huge potential to achieve the necessary polarization effects, hence they provide the basis for applications such as ultracompact polarization components. Terahertz chiral metamaterials that allow dynamic polarization modulation of terahertz waves are of great practical interest and still challenging. Here, we show that terahertz metasurface based on the four conjugated “petal” resonators integrated with multi-layered graphene (MLG) can enable dynamically tunable chiroptical response using optical pumping. In particular, a change of ellipticity angle of 20° is observed around 0.76 THz under optical pumping by a 980 nm continuous wave (CW) laser. Furthermore, using temporal coupled-mode theory, our study also reveals that the chiroptical response of the proposed multi-layered graphene-based metasurface is strongly dependent on the influence of optical pumping on the loss parameters of resonance modes, leading to actively controllable polarization states of the transmitted terahertz waves. The present work paves the way for the realization of fundamental terahertz components capable for active polarization manipulation.

Conformational, configurational, and supramolecular studies of podocephalol acetate from Lasianthaea aurea.

Although podocephalol (1) and its derived acetate 2 were found in Lasianthaea podocephala four decades ago, and 1 was later detected in the essential oils of several vegetal species, its absolute configuration (AC) and conformational preferences remained to be established. The structures of ar-himachalene 1, now isolated from Lasianthaea aurea, and its derived acetate 2, were herein confirmed by extensive 1D and 2D nuclear magnetic resonance (NMR) studies, while the conformational preferences of the cycloheptene was established by density functional theory (DFT) calculations, which in combination with vibrational circular dichroism measurements provided the (R) absolute configuration of the molecules. The structure and AC were further verified through the Flack and Hooft parameters calculations derived from single crystal X-ray diffraction data of 2. In addition, careful evaluation of the crystal data allowed observing supramolecular layers cell package, an uncommon property in natural terpenes that might have potential applications. A transmission electron microscopy analysis of crystal of 2 was also possible, providing its physical characteristics at the micrometric scale.

A Tunable, Fullerene-Based Molecular Amplifier for Vibrational Circular Dichroism.

Vibrational circular dichroism (VCD) studies are reported on a chiral compound in which a fullerene C60 moiety is used as an electron acceptor and local VCD amplifier for an alanine‐based peptide chain. Four redox states are investigated in this study, of which three are reduced species that possess low‐lying electronic states as confirmed by UV/Vis spectroelectrochemistry. VCD measurements in combination with (TD)DFT calculations are used to investigate (i) how the low‐lying electronic states of the reduced species modulate the amplification of VCD signals, (ii) how this amplification depends on the distance between oscillator and amplifier, and (iii) how the spatial extent of the amplifier influences amplification. These results pave the way for further development of tailored molecular VCD amplifiers.

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.

Piscicartone, a Rotenoid FromPiscidia carthagenensis

The bark of the roots of Piscidia carthagenensis afforded the known insecticides rotenone (1) and millettone (2), as well as the new rotenoid piscicartone (3). The structure of 3 followed from nuclear magnetic resonance studies, while its absolute configuration (AC) was determined by vibrational circular dichroism (VCD) measurements in comparison with discrete Fourier transform B3LYP/DGDZVPcalculated spectra using the Compare VOA software. In addition, the AC of 1 and 2 was verified using the same VCD methodology.

In-Situ Monitoring of Sol-Gel Transition by Temperature-dependent VCD Method: Signal Enhancement Induced by Gelation

Temperature-dependent vibrational circular dichroism (VCD) measurements were performed in situ during the sol-gel transformation of a chiral low-molecular weight gelator, N,N′-diperfluoroheptanoyl-...

Unusual Molecular and Supramolecular Structures of Chiral Low Molecular Weight Organogelators with Long Perfluoroalkyl Chains

Gels composed of low molecular weight gelators (LMWGs) are fascinating research targets from the viewpoint of applications because their functionalities are easily modified by designing their molecular structures. Some reliable gelator design approaches have been developed. However, new classes of molecular gelators are sometimes discovered unexpectedly, suggesting that there remain unknown aspects about gelators. To obtain knowledge regarding gelation and crystallization ability, the crystal structure of N,N′-diperfluorooctanoyl-(1R,2R)-1,2-diaminocyclohexane (RR-CF8), which is a derivative of 1,2-diaminocyclohexane, one of the most famous LMWGs, was investigated in addition to the vibrational circular dichroism (VCD) measurements. The crystal structure was solved from powder X-ray diffraction patterns because recrystallization of RR-CF8 afforded no suitable single crystals for single crystal X-ray diffraction measurement. Two unusual structural features were confirmed. One is that the perfluoroalkyl cha...

Stereoselective interactions as manifested by vibrational circular dichroism spectra: the interplay between chiral metal complexes co-adsorbed in a montmorillonite clay

Vibrational circular dichroism (VCD) spectroscopy was applied to study intermolecular interactions on the surface of a clay mineral. A montmorillonite clay loaded with two kinds of chiral molecules was prepared. Firstly a cationic ruthenium(ii) complex, Δ- or Λ-[Ru(phen)3]2+ (phen = 1,10-phenanthroline), was ion-exchanged into sodium montmorillonite (denoted as [Ru(phen)3]2+/mont). Thereafter a neutral metal(iii) complex, Δ- or Λ-[M(acac)3] (acac = acetylacetonato; M = Co and Cr), was adsorbed spontaneously, when the complex was added to an aqueous dispersion of [Ru(phen)3]2+/mont (denoted as [M(acac)3]/[Ru(phen)3]2+/mont). VCD measurements were performed on a KBr disk containing ca. 1 wt% of solid [M(acac)3]/[Ru(phen)3]2+/mont. Both of the chiral components gave clear VCD peaks in the wavelength region of 1200-1800 cm-1. Attention was focussed on the spectral differences between the following two samples: the one containing a pair of co-adsorbed chelates with the same chirality (i.e. Λ-[M(acac)3]/Λ-[Ru(phen)3]2+/mont or Δ-[M(acac)3]/Δ-[Ru(phen)3]2+/mont) and the other containing a pair of co-adsorbed chelates with the opposite chirality (i.e. Δ-[M(acac)3]/Λ-[Ru(phen)3]2+/mont or Λ-[M(acac)3]/Δ-[Ru(phen)3]2+/mont). Notably a new peak shoulder appeared and the relative peak intensity of the couplet peaks varied, depending on the chirality relation between the co-existing complexes. With the help of theoretical calculations, these results were rationalized in terms of the stereoselective intermolecular interactions.

Methodology for the Absolute Configuration Determination of Epoxythymols Using the Constituents of Ageratina glabrata.

A methodology to determine the enantiomeric excess and the absolute configuration (AC) of natural epoxythymols was developed and tested using five constituents of Ageratina glabrata. The methodology is based on enantiomeric purity determination employing 1,1'-bi-2-naphthol (BINOL) as a chiral solvating agent combined with vibrational circular dichroism (VCD) measurements and calculations. The conformational searching included an extensive Monte Carlo protocol that considered the rotational barriers to cover the whole conformational spaces. (+)-(8S)-10-Benzoyloxy-6-hydroxy-8,9-epoxythymol isobutyrate (1), (+)-(8S)-10-acetoxy-6-methoxy-8,9-epoxythymol isobutyrate (4), and (+)-(8S)-10-benzoyloxy-6-methoxy-8,9-epoxythymol isobutyrate (5) were isolated as enantiomerically pure constituents, while 10-isobutyryloxy-8,9-epoxythymol isobutyrate (2) was obtained as a 75:25 (8S)/(8R) scalemic mixture. In the case of 10-benzoyloxy-8,9-epoxythymol isobutyrate (3), the BINOL methodology revealed a 56:44 scalemic mixture and the VCD measurement was beyond the limit of sensitivity since the enantiomeric excess is only 12%. The racemization process of epoxythymol derivatives was studied using compound 1 and allowed the clarification of some stereochemical aspects of epoxythymol derivatives since their ACs have been scarcely analyzed and a particular behavior in their specific rotations was detected. In more than 30 oxygenated thymol derivatives, including some epoxythymols, the reported specific rotation values fluctuate from -1.6 to +1.4 passing through zero, suggesting the presence of scalemic and close to racemic mixtures, since enantiomerically pure natural constituents showed positive or negative specific rotations greater than 10 units.

Absolute configuration of the diterpenoids icetexone and conacytone from Salvia ballotaeflora.

Detailed literature inspections regarding the diterpenoids icetexone (1) and conacytone (3) reveal that the absolute configuration (AC) of these natural occurring compounds is not rigorously proven, despite they were originally isolated in 1976. This task is now completed by single-crystal X-ray diffraction Flack and Hooft parameters determination after processing data collected with Cu Kα graphite monochromated radiation. The AC of both compounds is further determined by vibrational circular dichroism measurements performed on icetexone acetate (2) and conacytone triacetate (4) since the solubility of 1 and 3 is limited. Comparison of the substituent chemical shifts (SCS) induced by acetylation of 1 and 3 to afford 2 and 4, respectively, reveals that in the case of icetexone, all six SCS values of the quinone ring are in excellent agreement with the expected values, while in the case of conacytone, three agree and three do not agree due to the presence of additional acetates near the quinone ring. Density functional theory calculations performed on 3-hydroxythymoquinone (6) and its tautomer 4-hydroxy-1,2-quinone 7, on 6-hydroxythymoquinone (8) and its tautomer ortho-quinone 9, and on icetexone (1) and the claimed natural occurring ortho-quinone tautomer romulogarzone (5) indicate that 2-hydroxy-1,4-quinones are more stable, by some 11-14kcal/mol, than their 4-hydroxy-1,2-quinone tautomers, and therefore, romulogarzone (5) is inexistent.

2-Cyano-2-indolylpropanoic acid as a chiral derivatizing agent for the absolute configuration assignment of secondary alcohols and primary amines by 1H NMR and VCD

A convenient approach for the absolute configuration assignment of secondary alcohols in the (8R,1′R,2′S,5′R)-15,25, (8S,1′R,2′S,5′R)-15,25, (8R,1′R)-21–24, and (8S,1′R)-21–24 ester series, and of primary amines in the (8R,1′R)-32–37 and (8S,1′R)-32–37 amide series, by means of 1H NMR and VCD spectroscopy, using 2-cyano-2-indolylpropanoic acid as a chiral derivatizing agent is presented. DFT calculations were carried out to demonstrate the anisotropic effect of the indole skeleton on the chiral alcohol or the amine fragment. Vibrational circular dichroism (VCD) measurements of the above series indicated a VCD bisignated couplet resulting from the interaction of the ester carbonyl group and the CN group. The absolute configuration assignments were further tested by X-ray diffraction analysis.

A Vibrational Circular Dichroism Microsampling Accessory: Mapping Enhanced Vibrational Circular Dichroism in Amyloid Fibril Films.

We report the first vibrational circular dichroism (VCD) measurement of spatial heterogeneity in a sample using infrared (IR) microsampling. Vibrational circular dichroism spectra are typically measured using a standard IR cell with an IR beam diameter of 10 mm or greater making it impossible to investigate the spatial heterogeneity of a solid film sample. We have constructed a VCD sampling assembly with either 3 mm or 1 mm spatial resolution. An XY-translation stage was used to measure spectra at different spatial locations producing IR and VCD maps of the sample. In addition, a rotating sample stage was employed using a dual photoelastic modulator (PEM) setup to suppress artifacts due to linear birefringence in solid-phase or film samples. Infrared and VCD mapping of an insulin fibril film has been carried out at both 3 and 1 mm spatial resolution, and lysozyme films were mapped at 1 mm resolution. The IR spectra of different spots vary in intensity due primarily to sample thickness. The changes in the VCD intensity across the map largely correlate to corresponding changes in the IR map. Closer inspection of the insulin map revealed changes in the relative intensities of the VCD spectra not present in the parent IR spectra, which indicated differences in the degree of supramolecular chirality of the fibrils in the various spatial regions. For lysozyme films, in addition to different degrees of supramolecular chirality, reversal of the net fibril chirality was observed. The large signal-to-noise ratio observed at 1 mm resolution implies the feasibility of further increasing the spatial resolution by one or two orders of magnitude for protein fibril film samples.