Vibrational Circular Dichroism Measurements Research Articles

Direct Observation of Siloxane Chirality on Twisted and Helical Nanometric Amorphous Silica.

Synthesis of chiral inorganic or hybrid nanomaterials through sol-gel transcription of chiral organic templates has attracted a great deal of interest for more than a decade. However, the chiral nature of these inorganic matrices has never been directly observed. For the first time, we report a direct evaluation of chirality on noncrystalline silica chiral nanoribbons by vibrational circular dichroism (VCD) measurements. Strong Cotton effect around 1150-1000 cm-1 from Si-O-Si asymmetric stretching vibration was observed. Surprisingly, calcination of these hybrid nanoribbons doubled the intensity of Cotton effects. On the basis of transmission electron microscopy observations, IR, VCD, NMR, and Raman spectroscopies, we demonstrate that the silica chirality originates from twisted siloxane network composed of chiral arrangement of the Si-O-Si bonds. Our findings clearly prove the presence of chiral organization of amorphous silica network, making them very promising chiral platforms for chiral recognition, optical applications, or asymmetric catalysis.

Absolute configuration assignment of oxindole derivatives by vibrational circular dichroism exciton coupling

A convenient and reliable approach for the absolute configuration assignment of oxindole derivatives 2a–m by vibrational circular dichroism (VCD) measurements and evaluation of the VCD bisignated couplet resulting from the interaction of the C2 and C9 carbonyl groups is presented. The absolute configuration assignments were further tested by 1H NMR measurements and by X-ray diffraction analysis.

IR and Vibrational Circular Dichroism Spectroscopy of Matrine- and Artemisinin-Type Herbal Products: Stereochemical Characterization and Solvent Effects.

Five Chinese herbal medicines--matrine, oxymatrine, sophoridine, artemisinin, and dihydroartemisinin--were investigated using vibrational circular dichroism (VCD) experiments and density functional theory calculations to extract their stereochemical information. The three matrine-type alkaloids are available from the dry roots of Sophora flavescens and have long been used in various traditional Chinese herbal medicines to combat diseases such as cancer and cardiac arrhythmia. Artemisinin and the related dihydroartemisinin, discovered in 1979 by Professor Youyou Tu, a 2015 Nobel laureate in medicine, are effective drugs for the treatment of malaria. The VCD measurements were carried out in CDCl3 and DMSO-d6, two solvents with different dielectric constants and hydrogen-bonding characteristics. A "clusters-in-a-liquid" approach was used to model both explicit and implicit solvent effects. The studies show that effectively accounting for solvent effects is critical to using IR and VCD spectroscopy to provide unique spectroscopic features to differentiate the potential stereoisomers of these Chinese herbal medicines.

The Clusters-in-a-Liquid Approach for Solvation: New Insights from the Conformer Specific Gas Phase Spectroscopy and Vibrational Optical Activity Spectroscopy.

Vibrational optical activity spectroscopies, namely vibrational circular dichroism (VCD) and Raman optical activity (ROA), have been emerged in the past decade as powerful spectroscopic tools for stereochemical information of a wide range of chiral compounds in solution directly. More recently, their applications in unveiling solvent effects, especially those associated with water solvent, have been explored. In this review article, we first select a few examples to demonstrate the unique sensitivity of VCD spectral signatures to both bulk solvent effects and explicit hydrogen-bonding interactions in solution. Second, we discuss the induced solvent chirality, or chiral transfer, VCD spectral features observed in the water bending band region in detail. From these chirality transfer spectral data, the related conformer specific gas phase spectroscopic studies of small chiral hydration clusters, and the associated matrix isolation VCD experiments of hydrogen-bonded complexes in cold rare gas matrices, a general picture of solvation in aqueous solution emerges. In such an aqueous solution, some small chiral hydration clusters, rather than the chiral solutes themselves, are the dominant species and are the ones that contribute mainly to the experimentally observed VCD features. We then review a series of VCD studies of amino acids and their derivatives in aqueous solution under different pHs to emphasize the importance of the inclusion of the bulk solvent effects. These experimental data and the associated theoretical analyses are the foundation for the proposed “clusters-in-a-liquid” approach to account for solvent effects effectively. We present several approaches to identify and build such representative chiral hydration clusters. Recent studies which applied molecular dynamics simulations and the subsequent snapshot averaging approach to generate the ROA, VCD, electronic CD, and optical rotatory dispersion spectra are also reviewed. Challenges associated with the molecular dynamics snapshot approach are discussed and the successes of the seemingly random “ad hoc explicit solvation” reported before are also explained. To further test and improve the “clusters-in-a-liquid” model in practice, future work in terms of conformer specific gas phase spectroscopy of sequential solvation of a chiral solute, matrix isolation VCD measurements of small chiral hydration clusters, and more sophisticated models for the bulk solvent effects would be highly valuable.

Absolute configuration of stegane lignans by vibrational circular dichroism

Novel steganes (−)-(P,8R,8′R)-5′-desmethoxystegane 4, (−)-(P,7R,8R,8′R)-5′-desmethoxysteganol 5, (−)-(P,7R,8R,8′R)-5′-desmethoxysteganacin 6, and (−)-(P,8R,8′R)-5′-desmethoxysteganone 7 were synthesized from (+)-(M,8R,8′R)-5′-desmethoxyisostegane 2, which in turn was obtained from natural (−)-(8R,8′R)-5′-desmethoxyyatein 1 together with the novel (+)-(M,8R,8′R)-3,4-dihydroxy-3′,4′-dimethoxyisosteganolide 3. Their stereostructures were determined by 1D and 2D NMR spectroscopy, while their absolute configuration was assigned by vibrational circular dichroism (VCD) measurements in combination with density functional theory calculations. Thus, the VCD spectrum of 2 was contrasted with that of its atropisomeric derivative 4 obtained by thermal inversion of the chiral axis. These compounds exhibited numerous opposite VCD transitions due to the predominance of the biaryl core, thus allowing the identification of bands arising from local (8R,8′R) and non-local (M/P) chirality. The conformational analysis of these compounds revealed that the eight-membered ring of 2 adopts a single twist-boat–chair conformation, while in 4 it exists in an equilibrium between two different twist-boat conformations. The introduction of an acetyl group at C-7 in 4 to give 6 shifted the conformational equilibrium toward a single twist-boat conformation which was verified by VCD spectroscopy.

The interplay of aggregation, fibrillization and gelation of an unexpected low molecular weight gelator: glycylalanylglycine in ethanol/water.

Hydrogels formed by polypeptides could be much-favored tools for drug delivery because their main ingredients are generally biodegradable. However, the gelation of peptides in aqueous solution generally requires a minimal length of the peptide as well as distinct sequences of hydrophilic and hydrophobic residues. The aggregation of short peptides like tripeptides, which are relatively cheap and offer a high degree of biodegradability, are generally thought to require a high hydrophobicity of their residues. We found that contrary to this expectation cationic glycylalanylglycine in 55 mol% ethanol/45 mol% water forms a gel below a melting temperature of ca. 36 °C. A pure hydrogel state can be obtained after allowing the ethanol component to evaporate. The gel phase consists of crystalline fibrils of several 100 μm, which form a sample-spanning network. Rheological data reveal a soft elastic solid gel. We investigated the kinetics of the various processes that lead to the final gel state of the ternary mixture by a unique combination of UV circular dichroism, infrared, vibrational circular dichroism (VCD) and rheological measurements. A mathematical analysis of our data show that gelation is preceded by the formation of peptide β-sheet like tapes or ribbons, which give rise to a significant enhancement of the amide I' VCD signal, and the subsequent formation of rather thick and long fibrils. The VCD signals indicate that the tapes exhibit a right-handed helicity at temperatures above 16 °C and a left-handed helicity below. The tapes'/ribbons' helicity change occurs at a temperature where the UVCD data reflect a relatively long nucleation process. The kinetics of gel formation probed by the storage and loss moduli are composed of a fast process that follows tape/ribbon/fibril formation and is clearly identifiable in a movie that shows the gelation process and a slow process that causes an additional gel stabilization. The rheological data indicate that left-handed fibrils observed at low temperatures form a more solid-like structure than their right-handed counterparts formed at higher temperatures. Taken together our data reveal GAG as an unexpected gelator, the formation of which is underlied by a set of distinguishable kinetic processes.

Enantiomeric switching of chiral metamaterial for terahertz polarization modulation employing vertically deformable MEMS spirals.

Active modulation of the polarization states of terahertz light is indispensable for polarization-sensitive spectroscopy, having important applications such as non-contact Hall measurements, vibrational circular dichroism measurements and anisotropy imaging. In the terahertz region, the lack of a polarization modulator similar to a photoelastic modulator in the visible range hampers expansion of such spectroscopy. A terahertz chiral metamaterial has a huge optical activity unavailable in nature; nevertheless, its modulation is still challenging. Here we demonstrate a handedness-switchable chiral metamaterial for polarization modulation employing vertically deformable Micro Electro Mechanical Systems. Vertical deformation of a planar spiral by a pneumatic force creates a three-dimensional spiral. Enantiomeric switching is realized by selecting the deformation direction, where the polarity of the optical activity is altered while maintaining the spectral shape. A polarization rotation as high as 28° is experimentally observed, thus providing a practical and compact polarization modulator for the terahertz range.

A Solid Phase Vibrational Circular Dichroism Study of Polypeptide-Surfactant Interaction.

We studied the interaction of poly-l-lysine (PLL) and poly-l-arginine (PLAG) with sodium dodecyl sulfate (SDS) surfactant and the interaction of poly-l-glutamic acid (PLGA) and poly-l-aspartic acid (PLAA) with tetradecyltrimethylammonium bromide (TTAB) surfactant using vibrational circular dichroism (VCD) spectroscopy in the region of C-H stretching vibration and in the Amide I region both in solution and in mulls. A chirality transfer from polypeptides to achiral surfactants was observed in the C-H stretching region, where measurements in solution were impossible. This observation was enabled by a special sample treatment technique using lyophilization and the preparation of mulls. This technique demonstrated itself as an interesting and beneficial tool for VCD measurements. In addition, we observed that SDS changed the secondary structure of PLL to the β-sheet and of PLAG to the α-helix. TTAB disrupted the PLGA and PLAA structure. These results were obtained in the mull but were confirmed by the VCD spectra measured in solution and by electronic circular dichroism. The chirality transfer from the polypeptides to SDS was caused by polypeptides ordered into a specific conformation during the interaction, while in the TTBA system it was induced primarily by the chirality of the amino acid residues.

Structure and antimicrobial activity of phloroglucinol derivatives from Achyrocline satureioides.

The new prenylated phloroglucinol α-pyrones 1-3 and the new dibenzofuran 4, together with the known 23-methyl-6-O-demethylauricepyrone (5), achyrofuran (6), and 5,7-dihydroxy-3,8-dimethoxyflavone (gnaphaliin A), were isolated from the aerial parts of Achyrocline satureioides. Their structures were determined by 1D and 2D NMR spectroscopic studies, while the absolute configuration of the sole stereogenic center of 1 was established by vibrational circular dichroism measurements in comparison to density functional theory calculated data. The same (S) absolute configuration of the α-methylbutyryl chain attached to the phloroglucinol nucleus was assumed for compounds 2-6 based on biogenetic considerations. Derivatives 7-16 were prepared from 1 and 5, and the antimicrobial activities of the isolated metabolites and some of the semisynthetic derivatives against a selected panel of Gram-positive and Gram-negative bacteria, as well as a set of yeast molds, were determined.

Optical Activity Measurements by Interferometry-Diffraction Technique: Test with BSO Crystals

To measure the optical rotation of a material, we propose a technique which we couple Mach-Zehnder interferometer and an optical Fourier processor 2F. The method uses the dependence of the contrast of the interference fringes with the angle between the planes of polarization of the two beams of the interferometer. The technique allows following the dynamic variations of optical activity, directly observing the changes in the intensity of the diffraction orders in the exit plane of the Fourier processor 2F. We present results. Full Text: PDF References L. A. Nafie, Vibrational Optical Activity: Principles and Applications (Noida, John Wiley & Sons Ltd 2011). CrossRef W. Yang, P R. Griffiths, G. J. Kemeny, "Vibrational Circular Dichroism Measurements by Optical Subtraction FT-IR Spectrometry", Applied Spectroscopy 38, 37 (1984). DirectLink L. P. Polavarapu, Vibrational spectra: principles and applications with emphasis on optical activity (Amsterdam, Elsevier 1998). L. Barron, Molecular light scattering and optical activity (New York, Cambridge University Press 2004). D. Tsankov, A. Polyanichko, H. Wieser, 1 Vibrational Circular Dichroism: Ensuring Quality of Pharmaceutical Products. Pharmaceutical Sciences Encyclopedia. 1, (2011). S. Zhang, Z. Tian, Q. Wang, X. Yin, S. Li, "Optimal length of an electro-optical Q-switch with optical activity crystal La3Ga5SiO14", Optics & Laser Technology 39, 507 (2007). CrossRef E. Shamonina V.P. Kamenov, K.H. Ringhofer, G. Cedilnik, A. Kießling, R. Kowarschik, D.J. Webb, "Optical activity in photorefractive Bi12TiO20 ", Optics Communications 146, 62 (1998). CrossRef V. Marinova, "Optical properties of Bi12TiO20 doped with Al, P, Ag, Cu, Co and co-doped with Al+P single crystals", Optical Materials 15, 149 (2000). CrossRef R.M. Ribeiro, A.B.A. Fiasca, P.A.M. dos Santos, M.R.B. Andreeta, A.C. Hernandes, "Optical activity measurements in the photorefractive Bi12TiO20 single crystal fibers", Optical Materials 10, 201 (1998). CrossRef D.G. Papazoglou A.G. Apostolidis, E.D. Vanidhis, "Index of refraction, optical activity and electro-optic coefficient of bismuth titanium oxide (Bi12TiO20)", Applied Physics B 65, 499 (1997). CrossRef X. He, H. Zhou, W. Zhou, Z. Hu, C. Zhang, H. Huo, J. Wang, Y. Zuo, F. Lu, "Solubility and optical activity of Bi12SiO20 crystals", Journal of Crystal Growth 351, 182 (2012). CrossRef V. Tassev G. Diankov, M. Gospodinov, "Optical activity of doped sillenite crystals", Materials Research Bulletin 30, (10), 1263 (1995). CrossRef V. Tassev, M. Gospodinov, M. Veleva, "Optical activity of BSO crystals doped with Cr, Mn and Cu", Optical Materials 13, 249 (1999). CrossRef J. E. Rueda-P, A. L. Becerra, "Characterization of Reflective TN-LCD, Tuned in Phase-Only Modulation and to Six Wavelengths", Photonics Letters of Poland, 2(4), 174 (2010). CrossRef L.A. Guerra, J.E. Rueda, Master's thesis in Physics, Dynamic Holography using photorefractive materials Bi12SiO20 and LiNbO3 (Universidad de Pamplona, Colombia, 2011) E. Hecht, Optics, Chapter 12. Basics of coherence theory (Addison Weley, 2002).

Amyloid Fibrils: the Eighth Wonder of the World in Protein Folding and Aggregation

Protein folding, misfolding, and aggregation are among the central topics of molecular biology in the postgenome era. Increasing evidence indicates that protein aggregation plays a vital role in a variety of biological processes. Specifically, fibrillar aggregates, i.e., filamentous assemblies of peptides and proteins that possess a common cross-β sheet structure, have been found in a wide range of organisms, from bacteria to mammals, and have been shown to possess different physiological functions.

Reaction Monitoring Using Mid‐Infrared Laser‐Based Vibrational Circular Dichroism

Changes in vibrational circular dichroism (VCD) were recorded on-line during a chemical reaction. The chiral complex nickel-(-)-sparteine chloride was hydrolyzed to free (-)-sparteine base in a biphasic system of sodium hydroxide solution and chloroform (CHCl(3)). Infrared (IR) and VCD spectra were iteratively recorded after pumping a sample from the CHCl(3) phase through a lab-built VCD spectrometer equipped with a tunable mid-IR quantum cascade laser light source, which allows for VCD measurements even in the presence of strongly absorbing backgrounds. Time-dependent VCD spectra were analyzed by singular value decomposition and global exponential fitting. Spectral features corresponding to the complex and free (-)-sparteine could be clearly identified in the fitted amplitude spectrum, which was associated with an exponential decay with an apparent time constant of 127 min (t(½) = 88 min).

Determination of absolute configuration of salvic acid, an ent-labdane from Eupatorium salvia, by vibrational circular dichroism

The relative stereochemistry at C13 and the absolute configuration of salvic acid, a constituent of the leaves of Eupatorium salvia, were established as the 13-(R)-ent-labdane 1. The results follow from vibrational circular dichroism measurements of the derived O-methyl ether methyl ester 3 which were compared to DFT B3LYP/DGDZVP calculated spectra. The relative stereochemistry of salvic acid at C13 was independently verified by single crystal X-ray diffraction measurements of 1, and of its derived diol 4.

Absolute configuration and stereochemical analysis of 3α,6β-dibenzoyloxytropane

Both enantiomers of 3α,6β-dibenzoyloxytropane (1) have been prepared from optical active 6β-hydroxyhyoscyamines establishing their absolute configurations as (−)-(3R,6R) and (+)-(3S,6S)-dibenzoyloxytropane. Independent stereochemical confirmation was obtained by vibrational circular dichroism measurements, since bands characteristic of (3R,6R) and (3S,6S) configurations of tropanediols derivatives were observed. In addition, a chiral HPLC method was developed for determining absolute configurations of tropane-related natural substances at the microgram (μg) level. The complete 1H NMR characterization of the scaffold of 1 is also reported.

Determination of the absolute configurations using electronic and vibrational circular dichroism measurements and quantum chemical calculations

The stereochemistry of products obtained via a chemical reaction may not always be obvious from the reaction scheme utilized. The identification of convenient methods to determine the stereochemistry in such cases is highly desirable. To identify these methods, we considered a substituted 4-vinyl-1-azabicyclo[3.2.0]hept-3-en-7-one that undergoes spontaneous oxidation in the atmosphere at room temperature, yielding an epoxide with unknown absolute configuration. To determine the absolute configuration of the resulting epoxide, three different approaches have been utilized: (a) experimental NOE measurements; (b) experimental electronic circular dichroism (ECD) spectroscopic measurements and their analysis using corresponding quantum chemical predictions at the B3LYP/aug-cc-pVDZ level; (c) experimental vibrational circular dichroism (VCD) spectroscopic measurements and their analysis using corresponding quantum chemical predictions at the B3LYP/aug-cc-pVDZ level. It was found that the NOE data could not provide enough proof for assigning the absolute configuration, while ECD data could not provide enough discrimination to distinguish between the two possible stereoisomers. On the other hand, VCD spectroscopic analysis provided enough discrimination to distinguish between the two possible stereoisomers, and the absolute configuration could be assigned with confidence.

Comparison of mid‐infrared Fourier transform vibrational circular dichroism measurements with single and dual polarization modulations

Fourier transform infrared vibrational circular dichroism (FTIR-VCD) measurements have gone through major advances in the last decade. A major thrust in these advances was to find ways that can minimize the VCD spectral artifacts and obtain the VCD signals at enhanced signal quality. However, all these advances are not incorporated in FTIR-VCD instruments manufactured by some commercial manufacturers. In this article, we compare the measurements obtained with single and dual polarization modulations to seek the attention of the users of single polarization modulation method in minimizing the artifacts in such measurements. We also report that the VCD spectrum of a home-made collagen film can serve as a simple and convenient sample for routine verification of the correct functioning of the VCD spectrometers.

Quantum-Cascade Laser-Based Vibrational Circular Dichroism

Vibrational circular dichroism (VCD) spectra were recorded with a tunable external-cavity quantum-cascade laser (QCL). In comparison with standard thermal light sources in the IR, QCLs provide orders of magnitude more power and are therefore promising for VCD studies in strongly absorbing solvents. The brightness of this novel light source is demonstrated with VCD and IR absorption measurements of a number of compounds, including proline in water.

Enantioselectively controlled release of chiral drug (metoprolol) using chiral mesoporoussilica materials

Chiral porous materials have attracted burgeoning attention on account of theirpotential applications in many areas, such as enantioseparation, chiral catalysis,chemical sensors and drug delivery. In this report, chiral mesoporous silica (CMS)materials with various pore sizes and structures were prepared using conventionalachiral templates (other than chiral surfactant) and a chiral cobalt complex asco-template. The synthesized CMS materials were characterized by x-ray diffraction,nitrogen physisorption, scanning electron microscope and transmission electronmicroscope. These CMS materials, as carriers, were demonstrated to be able tocontrol the enantioselective release of a representative chiral drug (metoprolol). Therelease kinetics, as modeled by the power law equation, suggested that the releaseprofiles of metoprolol were remarkably dependent on the pore diameter and porestructure of CMS materials. More importantly, R- and S-enantiomers of metoprololexhibited different release kinetics on CMS compared to the corresponding achiralmesoporous silica (ACMS), attributable to the existence of local chirality on the porewall surface of CMS materials. The chirality of CMS materials on a molecularlevel was further substantiated by vibrational circular dichroism measurements.

Absolute configuration of tropane alkaloids from Schizanthus species by vibrational circular dichroism

The absolute configuration (AC) of 6 β-hydroxy-3 α-senecioyloxytropane ( 1), 3 α-hydroxy-6 β-tigloyloxytropane ( 2), 3 α-hydroxy-6 β-senecioyloxytropane ( 3), and 3 α-hydroxy-6 β-angeloyloxytropane ( 4) was assigned as (1 R,3 R,5 S,6 R) using density functional theory (DFT) calculations at the B3LYP/DGDZVP level of theory in combination with experimental vibrational circular dichroism (VCD) measurements and comparison with the spectra of similar tropanes. The AC of 1 followed from a sample isolated from Schizanthus grahamii, while those of the mixture of 2 and 3, isolated from the same source, were determined by comparing the VCD measurement to a weighted calculation of the individual VCD spectra according to a 69:31 ratio of 2: 3 determined by 1H NMR signal integration. In turn, Schizanthus pinnatus provided a 7:3 mixture of 1: 4 whose AC was determined using the experimental VCD absorptions in the 1150–950 cm −1 spectral region which were compared with those observed for 1– 3 and with those described for other 3 α,6 β-tropanediol derivatives.

Conformations of Phenylalanine in the Tripeptides AFA and GFG Probed by Combining MD Simulations with NMR, FTIR, Polarized Raman, and VCD Spectroscopy

Conformational properties of small, flexible peptides are a matter of ongoing interest since they can be considered as models for unfolded proteins. However, the investigation of the conformations of small peptides is challenging as they are ensembles of rapidly interconverting conformers; moreover, the different methods used are prone to different approximations and errors. In order to obtain more reliable results, it is prudent to combine different techniques; here, molecular dynamics (MD) simulations together with nuclear magnetic resonance (NMR), Fourier transform IR (FTIR), polarized Raman, and vibrational circular dichroism (VCD) measurements were used to study the conformational propensity of phenylalanine in the tripeptides AFA and GFG, motivated by the relevance of phenylalanine for the self-aggregation of peptides. The results of this analysis indicate that the F residue predominantly populates the beta-strand (beta) and polyproline II (PPII) conformations in both AFA and GFG. However, while phenylalanine exhibits a propensity for beta-strand conformations in GFG (0.40 < or = beta population < or = 0.69 and 0.29 < or = PPII population < or = 0.42), the substitution of terminal glycines with alanine residues induces a higher population of PPII (0.31 < or = beta population < or = 0.50 and 0.37 < or = PPII population < or = 0.57).