Quantum Cascade Laser-Based Vibrational Circular Dichroism Imaging for Chiral Biosensing.

Special issue “Advances in Chiroptical Methods”

Vibrational circular dichroism (VCD) offers sensitive structural information at the molecular scale by measuring the differential absorption of left- and right circularly polarized light associated with molecular vibrational transitions, enabling determination of configuration and detection of higher-order asymmetry in biomolecules. With the development of high-brightness quantum cascade lasers (QCLs), opportunities have emerged for advancing instrumentation. However, implementing VCD with QCLs introduces complications due to VCD signals being orders of magnitude weaker than classic absorbance as well as laser fluctuations, amplified further by polarization artifacts arising from highly coherent illumination. In this work, we utilize digitally referenced detection (DRD) that performs per-pulse noise reduction, enabling time-domain acquisition of VCD signals from purely circularly polarized pulse pairs that are, in principle, free of polarization artifacts caused by linear dichroism (LD) and linear birefringence (LB). The method is further extended to simultaneously extract LD signals within the same polarization modulation cycle (∼20 μs), allowing real-time monitoring of molecular orientation information without system modifications. Finally, we compare VCD measurements obtained using the DRD-based approach versus conventional lock-in amplifier (LIA) demodulation, showing a 4-fold spectral signal-to-noise ratio (SNR) improvement when normalized to acquisition time and spectral band size. We further validate the artifact detection capabilities with various levels of LDLB effects through polymer films. These advances establish a robust framework for high-SNR, real-time VCD measurements with built-in detection of LDLB effects, paving the way for accurate analysis of complex chiral systems and enabling future applications in solid-state VCD and chirality imaging.

Enantiomeric excess, the ratio between two enantiomers, is an important process variable in chiral catalysis. To increase the efficiency of such processes, this parameter needs to be monitored as close to real time as possible and ideally without elaborate sample preparation. Vibrational circular dichroism (VCD) provides chiral information in a pretreatment free and nondestructive manner. Since classical VCD suffers from a low time resolution, quantum cascade laser (QCL) based VCD was introduced to enable studying more dynamic processes. This significantly improved time resolution enables the use of EC-QCL VCD for monitoring the change of EE e.g. in a chemical process or a chemical reaction. In such applications, the classical approach of human interpretation of individual VCD spectra is no longer reasonable. Hence, chemometric evaluation of VCD spectral datasets is required. In this work we compare accuracy and stability of common multivariate regression algorithms for predicting EE from EC-QCL VCD spectra. Besides classical partial-least-squares regression, modified multiple linear regressions and models derived from chemical knowledge were investigated. We found that a combination of introducing chemical knowledge via spectral descriptor and a reduction of multicollinearity by a ridge regression model resulted in the most stable prediction. Additionally, least absolute shrinkage and selection operator (Lasso) revealed a potential for sensor design involving dedicated QCL arrays focused on a few relevant wavelengths. In summary, a more comprehensive chemometric perspective on QCL-VCD spectra can yield improvements in predictive performance and the shorter measurement times provided by QCL-VCD aid in acquiring datasets of appropriate size.

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 × 10mm) 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.

Review: 45 refs.

Most of the research in contemporary physical chemistry is devoted to the development of methods that extend our understanding, interpretation, and capacity to predict structural properties and dynamic behavior of molecules. The optical and magnetic spectroscopies, as well as diffraction techniques, are the principal methods for studying properties of molecules, biomolecules, and biopolymers of which the vast majority are chiral. On the other hand, information on molecular configuration can be obtained mainly from optical spectroscopies because other well-established spectroscopic techniques used for structural investigations, such as crystallographic, ESR, and NMR methods, do not allow for registration of signals from an individual conformer owing to intrinsic slow response to structural changes. This is the reason why the optical spectroscopy methods, based on natural chiroptical phenomena, have become so important and their renaissance in the last decade is noticed. Vibrational circular dichroism (VCD) spectroscopy is one such chiroptical technique that sheds new light on many important phenomena studied intensively. We provide an overview of recent theoretical predictions and innovative VCD observations of chirality transfer (called by other authors "induced chirality") from a chiral molecule to an achiral one as a result of hydrogen bond interactions between them. In this tutorial review we search for answers as to whether we can obtain further information about intermolecular interactions using the VCD technique. In our opinion this technique has opened new horizons for both understanding and monitoring intermolecular interactions and it could be used as a relatively new and powerful physicochemical method.

The stereochemistry of all four stereoisomers of tadalafil is determined using vibrational circular dichroism (VCD), electronic circular dichroism (ECD), and optical rotatory dispersion (ORD) spectroscopy. By comparing experimentally obtained VCD spectra to computationally simulated ones, the absolute configuration of the enantiomeric pair (6R, 12aR)/(6S, 12aS) can be confidently assigned without prior knowledge of their relative stereochemistry. IR and NMR spectra are used to aid the assignment of the relative stereochemistry. The IR and VCD difference spectra further confirm the assignment of all stereoisomers. ECD and ORD spectra are used to investigate the complementarity of the three chiroptical techniques. VCD spectroscopy itself is found to have the ability to identify diastereoisomers, and simultaneous use of these chiroptical spectroscopic methods and NMR chemical shifts aids in increasing the reliability of stereochemistry assignment of diastereoisomers.

The behavior of L-cysteine (C3H7NO2S, (2R)-2-amino-3-sulfanylpropanoic acid) in water at different pH values was analyzed both experimentally and theoretically. The behavior was studied at pH values of 5.21 (at this pH, L-cysteine is a zwitterionic species), 1.00 (protonated species), 8.84 (monodeprotonated species), and 13.00 (dideprotonated species). We carried out a vibrational study using nonchiroptical (IR-Raman) and chiroptical (VCD) techniques complemented by quantum chemical calculations. We adopted a dual strategy, as follows. (i) The hybrid density functionals B3LYP and M062X and the ab initio MP2 method were employed, with the same 6-311++G (d,p) basis set, in order to characterize the relative energies and structures of an extensive set of conformers of L-cysteine. The presence of water was included by utilizing the IEF-PCM implicit solvation model. (ii) The vibrational analysis was made using a chirality-sensitive using a chirality-sensitive technique (VCD) and chirality-insensitive techniques (IR, including MIR and FIR, and Raman), especially in aqueous solution. The results obtained theoretically and experimentally were compared in order to deduce the most stable structures at each pH. Moreover, for the first time, the monodeprotonated anion of L-cysteine was detected in aqueous solution by means of IR, Raman and vibrational circular dichroism (VCD). Finally, analysis of the low-frequency region using the IR and Raman techniques was shown to be a very important way to understanding the conformational preference of the zwitterionic species.

Preparation of cruciferous phytoalexin related metabolites, (−)-dioxibrassinin and (−)-3-cyanomethyl-3-hydroxyoxindole, and determination of their absolute configurations by vibrational circular dichroism (VCD)

The ab initio theories of electronic and vibrational circular dichroism are presented in brief. For electronic circular dichroism, emphasis is placed on the derivation of optical rotatory strengths by the perturbative configuration interaction approach as implemented in the program PCI. An application to the chiroptical properties of the disulfide chromophore is described. In the infrared region, the ab initio vibronic coupling theory (VCT) of vibrational circular dichroism (VCD) as implemented in the program VCT90, is presented. The relationship to the ab initio magnetic field perturbation (MFP) formalism and an approximate locally distributed origin-gauge (LDO) model is described. The VCT and MFP formulations are compared in large basis set and electron correlated studies of the model system, NHDT, and the experimentally characterized molecule, 2,3-dideuteriooxirane. The LDO model of VCD is applied, together with NMR and molecular mechanics techniques, to the investigation of the conformations of the anticancer drug, taxol. Coupled oscillator models are introduced. Applications both in the area of electronic and vibrational circular dichroism, especially to the determination of secondary structures of proteins and nucleic acids are remarked upon. © 1994 John Wiley & Sons, Inc.

A new family of homochiral silver complexes based on carbophilic interactions with ortho-phenylene ethynylene (o-OPE) scaffolds containing up to two silver atoms are described. These compounds represent a unique class of complexes with chirality at the metal. Chiral induction is based on the inclusion of chiral sulfoxides, which allow efficient transfer of chirality to the helically folded o-OPE, leading to circularly polarized luminescence (CPL)- and vibrational circular dichroism (VCD)-active compounds. In the presence of silver(I) cations, carbophilic interactions dominate, which promote helical structures with a defined helicity. This is one of the very scarce examples of the use of such interactions in the attractive field of abiotic foldamers. The switching event has been extensively studied by using different chiroptical techniques, including circular dichroism, CPL, and VCD, and represents one of the few CPL switches described in the literature.

Chapter 14 - Vibrational Optical Activity: From Small Chiral Molecules to Protein Pharmaceuticals and Beyond

Of the various chiroptical techniques, one of the newest, vibrational circular dichroism (VCD) promises to yield the greatest amount of structural and stereochemical information. Since the earliest theoretical models [1] and the first successful experimental measurements [2,3], VCD has seen rapid advances in instrumentation and theory. Reliable measurements over most of the vibrational spectrum can be carried out, and reliable theoretical computations of signs and intensities of the fundamental transitions in small to medium sized molecules can be performed. It is not the intention of this article to present a comprehensive review of the development of VCD. This has been accomplished in numerous excellent review articles by the leaders in the field [4, 5, 6, 7, 8, 9, 10, 11]. Instead, we will concentrate rather narrowly on the theoretical developments which have led to successful computer implementation, at an ab initio quantum mechanical level and to review the results of such calculations insofar as they have been reported in the literature to date.

We developed a microscopic scanning for vibrational circular dichroism (VCD) spectroscopy in which a quantum cascade laser is equipped with a highly focused infrared light source to attain a spatial resolution of 100 μm. This system was applied to the forewing of a European hornet to reveal how the protein domains are organised. Two-dimensional patterns were obtained from the VCD signals with steps of 100 μm. We scanned the 1500-1740 cm-1 wavenumber range, which covers amide I and II absorptions. Zone sequenced α-helical and β-sheet domains within an area of 200 μm2 in membranes close to where two veins cross. The sign of the VCD signal at 1650 cm-1 changed from positive to negative when probed along the zone axis, intermediated by the absence of VCD activity. The significance of this zone is discussed from the viewpoint of the mechanical properties required for flying motion. These features are unattainable using conventional FTIR (Fourier transform infrared) or FT-VCD methods with a spatial resolution of ∼10 mm2.

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 β-sheet (in another part of the membrane), and a coil (in a vein).