A vibrational circular dichroism (VCD) methodology for the measurement of enantiomeric excess in chiral compounds in the solid phase and for the complementary use of NMR and VCD techniques in solution: the camphor case

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.

Vibrational circular dichroism of adenosine crystals

Adenosine is one of the building blocks of nucleic acids and other biologically important molecules. Spectroscopic methods have been among the most utilized techniques to study adenosine and its derivatives. However, most of them deal with adenosine in solution. Here, we present the first vibrational circular dichroism (VCD) spectroscopic study of adenosine crystals in solid state. Highly regular arrangement of adenosine molecules in a crystal resulted in a strongly enhanced supramolecular VCD signal originating from long-range coupling of vibrations. The data suggested that adenosine crystals, in contrast to guanosine ones, do not imbibe atmospheric water. Relatively large dimensions of the adenosine crystals resulted in scattering and substantial orientational artifacts affecting the spectra. Several strategies for tackling the artifacts have been proposed and tested. Atypical features in IR absorption spectra of crystalline adenosine (e.g., extremely low absorption in mid-IR spectral range) were observed and attributed to refractive properties of adenosine crystals.

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.

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.

Terahertz spectroscopy for quantitatively elucidating the crystal transformation of chiral histidine enantiomers to racemic compounds

d-Histidine (d-His), l-Histidine (l-His), and their racemic compound dl-Histidine (dl-His) have different stereo chirality, making them intrinsic diverse functionalities to the living system. Identifying the configuration and crystal structures of enantiomers and the racemic compound is always the foremost requirement in processing protein foods. Although these features can be analyzed by spectroscopic technologies individually, it remains challenging to incorporate these complex methods into a facile analytical strategy. Herein, we propose a terahertz spectroscopy with solid-state density functional theory to both distinguish the configurational difference and quantify the crystal transformation from l-His and d-His to dl-His. By comparison with X-ray diffraction analysis, the validity of the crystal transformation evaluation based on terahertz spectroscopy is verified. A normalized fitting line regarding the terahertz absorption frequency and intensity is calculated to quantitatively elucidate the crystal transformation from enantiomers to dl-His. Our findings provide a new analytical approach to the research on food chemistry.

Demystifying racemic natural products in the homochiral world

Natural products possess structural complexity, diversity and chirality with attractive functions and biological activities that have significantly impacted drug discovery initiatives. Chiral natural products are abundant in nature but rarely occur as racemates. The occurrence of natural products as racemates is very intriguing from a biosynthetic point of view; as enzymes are chiral molecules, enzymatic reactions generating natural products should be stereospecific and lead to single-enantiomer products. Despite several reports in the literature describing racemic mixtures of stereoisomers isolated from natural sources, there has not been a comprehensive review of these intriguing racemic natural products. The discovery of many more natural racemates and their potential enzymatic sources in recent years allows us to describe the distribution and chemical diversity of this ‘class of natural products’ to enrich discussions on biosynthesis. In this Review, we describe the chemical classes, occurrence and distribution of pairs of enantiomers in nature and provide insights about recent advances in analytical methods used for their characterization. Special emphasis is on the biosynthesis, including plausible enzymatic and non-enzymatic formation of natural racemates, and their pharmacological significance.

Racemic natural products display a wealth of bioactivities and chemical diversity. Their derivation from intriguing racemization processes, through enzymatic or non-enzymatic pathways, are discussed here, as well as their pharmacological properties and the analytical techniques developed for their identification, resolution and characterization.

Accurate Determination of Enantiomeric Excess Using Raman Optical Activity

The optical purity of a chiral sample is of particular importance to the analytical chemistry and pharmaceutical industries. In recent years, the vibrational optical activity (VOA) has become established as a sensitive and nondestructive technique for the analysis of chiral molecules in solution. However, the relatively limited accuracy in the range of about 1–2% reported in published papers and the relatively small spread of experimental facilities to date have meant that vibrational spectroscopy has not been considered a common method for determining enantiomeric excess. In this paper, we attempt to describe, in detail, a methodology for the determination of enantiomeric excess using Raman optical activity (ROA). This method achieved an accuracy of 0.05% for neat α-pinene and 0.22% for alanine aqueous solution, after less than 6 h of signal accumulation for each enantiomeric mixture, which we believe is the best result achieved to date using vibrational optical activity techniques. An algorithm for the elimination of systematic errors (polarization artifacts) is proposed, and the importance of normalizing ROA spectra to correct for fluctuations in excitation power is established. Results comparable to those obtained with routinely used chemometric analysis by the partial least squares (PLS) method were obtained. These findings show the great potential of ROA spectroscopy for the quantitative analysis of enantiomeric mixtures.

The solid state VCD of a novel N-acylhydrazone trifluoroacetate

A novel N-acylhydrazone with pharmaceutical importance was subject of structural and IR/VCD investigations in the solid state. In the crystal structure, dimers of anion-cation pairs are stabilized by H-bonding and ionic interactions. Some less common interaction types, like C=N···C-NH₃⁺ (σ-hole) interactions, hydrazone-aromatic interactions and dispersive contacts of the CF₃ groups are also present in the crystal. Satisfactory reproduction of the solid state IR and VCD spectra required that quantum-chemical calculations be done on a tetramer (four cation-anion pairs) cut out from the crystal structure, exhibiting key intermolecular interactions. Ten DFT functionals were assessed as to the agreement between the calculated and experimental spectra. Various approaches to scaling of the calculated frequencies were applied. The best results were yielded with individual (optimized) frequency scaling factors (FSFs) and band half-widths at half maximum-(HWHM) for four separate spectral subregions. The best matching between the experimental and theoretical spectra (according to SimIR, SimVCD and SimVDF indices) was found for the B3PW91 functional, however, a few other functionals follow closely in the ranking. Based on the quantum chemical calculations, spectral assignments have been made.

Identification of Enantiomers Using Low-Frequency Raman Spectroscopy

Distinguishing between d and l enantiomers is of important scientific interest, especially for the pharmaceutical industry. Enantiomeric differentiation in the solid form is repeatedly presented as a challenge in the research community. Raman spectroscopy is a nondestructive tool, widely used for the characterization of different materials by probing their vibrational modes. The low-frequency region of the Raman spectrum reveals lattice-level interactions and global fluctuations in the molecule. Lower frequencies correspond to vibrations arising from weaker bonds and long-range interactions and hence are very susceptible to polarization changes. This work presents low-frequency Raman (LFR) spectroscopy as a facile technique to identify enantiomers. The optical setup of conventional Raman spectroscopy is engineered such that the excitation and collection geometries use an asymmetrical focal cone. In addition, a half-wave retarder is added to the excitation path and a Glan-Taylor polarizer is added to the collection path, and these modifications allow us to select the polarization plane for both excitation and collection geometries. The asymmetry in the foci when using a polarized beam for excitation provides different intensities of the collected signal for each polarization plane. In a calibrated system, one can define the chirality of an analyte by comparing the intensity of the LFR signal along orthogonal sets of polarization planes. For nonchiral molecules, the spectral intensity is always higher in the co-polarized plane when compared to the orthogonally depolarized plane, as expected. This contrast in the intensity of Raman spectra serves as a distinct tool for identifying enantiomers.

Qualitative and quantitative recognition of chiral drugs based on terahertz spectroscopy

Chiral drugs are drugs with chiral or asymmetric centres in their molecular structure. Different enantiomers of the same chiral drug have noticeably different pharmacological activities and pharmacokinetic properties. However, its distinction has been perplexing scholars for many years in the qualitative and quantitative detection of antagonistic drugs. Conventional detection methods, such as polarimetry, circular dichroism, and high-performance liquid chromatography, are time consuming, cause sample loss and have cumbersome operations, and they can be applied only to the sampling method. In this paper, we propose a fast, accurate, qualitative and quantitative method for the study of chiral drugs based on linearly polarized terahertz (THz) spectroscopy and imaging technology. Taking ibuprofen as an example, based on the THz absorption spectra of the enantiomers RS-ibuprofen, (R)-(-)-ibuprofen, and (S)-(+)-ibuprofen, their characteristic peak frequencies, peak amplitude differences and peak area differences were extracted to qualitatively and quantitatively distinguish and identify the three substances. THz spectral imaging provides more intuitive results than those obtained from previous methods. In quantitative identification, the stability and detection accuracy of THz spectroscopy are much greater than those of Raman spectroscopy (88.8-99.8% vs. 21.42-94.62%, respectively). The qualitative recognition accuracy was 100%, and the quantitative recognition standard deviation was less than 0.01, and it is also a non-destructive testing method. Furthermore, the above method combined with principal component analysis (PCA) and the support vector machine (SVM) neural network classification algorithm was applied to the analysis of other chiral drugs. These results are significant for the rapid, accurate and non-destructive identification of chiral drugs.

NMR-Based Quantum Mechanical Analysis Builds Trust and Orthogonality in Structural Analysis: The Case of a Bisdesmosidic Triglycoside as Withania somnifera Aerial Parts Marker

The present study demonstrates the relationship between conventional and quantum mechanical (QM) NMR spectroscopic analyses, shown here to assist in building a convincingly orthogonal platform for the solution and documentation of demanding structures. Kaempferol-3-O-robinoside-7-O-glucoside, a bisdesmosidic flavonol triglycoside and botanical marker for the aerial parts of Withania somnifera, served as an exemplary case. As demonstrated, QM-based 1H iterative full spin analysis (HiFSA) advances the understanding of both individual nuclear resonance spin patterns and the entire 1H NMR spectrum of a molecule and establishes structurally determinant, numerical HiFSA profiles. The combination of HiFSA with regular 1D 1H NMR spectra allows for simplified yet specific identification tests via comparison of high-quality experimental with QM-calculated spectra. HiFSA accounts for all features encountered in 1H NMR spectra: nonlinear high-order effects, complex multiplets, and their usually overlapped signals. As HiFSA replicates spectrum patterns from field-independent parameters with high accuracy, this methodology can be ported to low-field NMR instruments (40-100 MHz). With its reliance on experimental NMR evidence, the QM approach builds up confidence in structural characterization and potentially reduces identity analyses to simple 1D 1H NMR experiments. This approach may lead to efficient implementation of conclusive identification tests in pharmacopeial and regulatory analyses: from simple organics to complex natural products.

Helicity-Preserving Optical Cavity Modes for Enhanced Sensing of Chiral Molecules

Researchers routinely sense molecules by their infrared vibrational "fingerprint" absorption resonances. In addition, the dominant handedness of chiral molecules can be detected by circular dichroism (CD), the normalized difference between their optical response to incident left- and right- handed circularly polarized light. Here, we introduce a cavity composed of two parallel arrays of helicity-preserving silicon disks that allows one to enhance the CD signal by more than 2 orders of magnitude for a given molecule concentration and given thickness of the cell containing the molecules. The underlying principle is first-order diffraction into helicity-preserving modes with large transverse momentum and long lifetimes. In sharp contrast, in a conventional Fabry-Perot cavity, each reflection flips the handedness of light, leading to large intensity enhancements inside the cavity, yet to smaller CD signals than without the cavity.

Chemometric analysis of enantioselective Raman spectroscopy data enables enantiomeric ratio determination

In-line determination of the enantiomeric ratio is still a challenge in process analytical technology (PAT). This study combines enantioselective Raman (esR) spectroscopy with partial least-squares regression (PLSR) to determine the enantiomeric fraction of the chiral molecule (5,6)-diphenyl-morpholin-2-one diluted in dimethyl sulfoxide (DMSO) as a proof-of-concept. Morpholinone derivates are potential candidates for pharmaceutical applications. The PLS weights were carefully analyzed in order to avoid misleading regression results, e.g. caused by sample impurities. A suitable PLSR model was found with two components and it was validated by a leave-one-out cross-validation. The enantiomeric fraction ef₍₊₎ could be calculated with deviations from the prepared ef₍₊₎ in the range of -0.031 and +0.052 from the esR spectra recorded at a half-wave retarder angle of 30.0°.

Principal component analysis to enhance enantioselective Raman spectroscopy

Enantioselective Raman (esR) spectroscopy is an innovative technique with a high potential for online process monitoring in chiral media, e.g. in the pharmaceutical industry. A prerequisite for an effective application is to combine the experimental approach with suitable concepts for data analysis. In this work, we present a chemometric approach to analyze the esR spectra recorded in an automatized polarization-resolved Raman set-up. It is demonstrated that the proposed method is capable of distinguishing between the enantiomers of the chiral alcohol 4-methylpentan-2-ol in a fully unsupervised fashion. Furthermore, it is shown that the difficulty of facing only small intensity differences between the esR spectra of the enantiomers can be overcome by feeding difference spectra between the pure enantiomers and the racemate into the principal component analysis (PCA) algorithm. The enantiomers are clearly discriminable along the first principal component.

Applications of OR/ECD/VCD to the structure elucidation of natural products

OR, ECD and VCD are powerful methods to determine the absolute configuration of natural products either applied independently or in combination.

One-Dimensional 13C NMR Is a Simple and Highly Quantitative Method for Enantiodiscrimination

The discrimination of enantiomers of mandelonitrile by means of 1D ¹³C NMR and with the aid of the chiral solvating agent (S)-(+)-1-(9-anthryl)-2,2,2-trifluoroethanol (TFAE) is presented. ¹H NMR fails for this specific compound because proton signals either overlap with the signals of the chiral solvating agent or do not show separation between the (S)-enantiomer and the (R)-enantiomer. The ¹³C NMR method is validated by preparing artificial mixtures of the (R)-enantiomer and the racemate, and it is shown that with only 4 mg of mandelonitrile a detection limit of the minor enantiomer of 0.5% is obtained, corresponding to an enantiomeric excess value of 99%. Furthermore, the method shows high linearity, and has a small relative standard deviation of only 0.3% for the minor enantiomer when the relative abundance of this enantiomer is 20%. Therefore, the ¹³C NMR method is highly suitable for quantitative enantiodiscrimination. It is discussed that ¹³C NMR is preferred over ¹H NMR in many situations, not only in molecules with more than one chiral center, resulting in complex mixtures of many stereoisomers, but also in the case of molecules with overlapping multiplets in the ¹H NMR spectrum, and in the case of molecules with many quaternary carbon atoms, and therefore less abundant protons.

Enantioselective Raman spectroscopy (esR) for distinguishing between the enantiomers of 2-butanol

The first experimental application of enantioselective Raman spectroscopy (esR) is demonstrated using the example of the chiral alcohol 2-butanol. Samples of the neat enantiomers and the racemic mixture were analyzed in a self-built Raman set-up. The Raman spectrum allows the discrimination of the chemical species. It is shown that the optical rotation of a Raman peak with a small depolarization ratio can be measured. In addition, without any sample modification, e.g. chiral solvent, the enantiomers are distinguishable at a suitable half-wave retarder angle detecting only the vertically polarized component of the Raman signal.