Surface enhanced Raman optical activity of molecules on orientationally averaged substrates: theory of electromagnetic effects

We present a model for electromagnetic enhancements in surface enhanced Raman optical activity (SEROA) spectroscopy. The model extends previous treatments of SEROA to substrates, such as metal nanoparticles in solution, that are orientationally averaged with respect to the laboratory frame. Our theoretical treatment combines analytical expressions for unenhanced Raman optical activity with molecular polarizability tensors that are dressed by the substrate's electromagnetic enhancements. We evaluate enhancements from model substrates to determine preliminary scaling laws and selection rules for SEROA. We find that dipolar substrates enhance Raman optical activity (ROA) scattering less than Raman scattering. Evanescent gradient contributions to orientationally averaged ROA scale to first or higher orders in the gradient of the incident plane-wave field. These evanescent gradient contributions may be large for substrates with quadrupolar responses to the plane-wave field gradient. Some substrates may also show a ROA contribution that depends only on the molecular electric dipole-electric dipole polarizability. These conclusions are illustrated via numerical calculations of surface enhanced Raman and ROA spectra from (R)-(-)-bromochlorofluoromethane on various model substrates.     

Use of a hydrogel polymer for reproducible surface enhanced Raman optical activity (SEROA)

We present surface enhanced Raman optical activity (SEROA), as well as Raman, SERS and ROA, spectra of D- and L-ribose. By employing a gel forming polyacrylic acid to control colloid aggregation and associated birefringent artefacts we observe the first definitive proof of SEROA through measurement of mirror image bands for the two enantiomers.     

Calculation of Raman optical activity spectra of methyl-β-D-glucose incorporating a full molecular dynamics simulation of hydration effects

We report calculations of the Raman and Raman optical activity (ROA) spectra of methyl-β-D-glucose utilizing density functional theory combined with molecular dynamics (MD) simulations to provide an explicit hydration environment. This is the first report of such combination of MD simulations with ROA ab initio calculations. We achieve a significant improvement in accuracy over the more commonly used gas phase and polarizable continuum model (PCM) approaches, resulting in an excellent level of agreement with the experimental spectrum. Modeling the ROA spectra of carbohydrates has until now proven a notoriously difficult challenge due to their sensitivity to the effects of hydration on the molecular vibrations involving each of the chiral centers. The details of the ROA spectrum of methyl-β-D-glucose are found to be highly sensitive to solvation effects, and these are correctly predicted for the first time including those originating from the highly sensitive low frequency vibrational modes. This work shows that a thorough consideration of the role of water is pivotal for understanding the vibrational structure of carbohydrates and presents a new and powerful tool for characterizing carbohydrate structure and conformational dynamics in solution.     

The Influence of the Amino Acid Side Chains on the Raman Optical Activity Spectra of Proteins

The Raman optical activity (ROA) spectra of proteins show distinct patterns arising from the secondary structure. It is generally believed that the spectral contributions of the side-chains largely cancel out because of their flexibility and the occurrence of many side-chains with different conformations. Yet, the influence of the side-chains on the ROA patterns assigned to different secondary structures is unknown. Here, the first systematic study of the influence of all amino acid side-chains on the ROA patterns is presented based on density functional theory (DFT) calculations of an extensive collection of peptide models that include many different side-chain and secondary structure conformations. It was shown that the contributions of the side-chains to a large extent average out with conformational flexibility. However, specific side-chain conformations can have significant contributions to the ROA patterns. It was also shown that α-helical structure is very sensitive to both the exact backbone conformation and the side-chain conformation. Side-chains with χ₁≈−60° generate ROA patterns alike those in experiment. Aromatic side-chains strongly influence the amide III ROA patterns. Because of the huge structural sensitivity of ROA, the spectral patterns of proteins arise from extensive conformational averaging of both the backbone and the side-chains. The averaging results in the fine spectral details and relative intensity differences observed in experimental spectra.     

Conformational analyses of peptides and proteins by vibrational Raman optical activity

Recent developments of vibrational Raman optical activity (ROA) spectroscopy enabled the detailed analyses of the backbone and side chain conformations of peptides and proteins in solution phases. ROA can be used as a powerful analytical technique for determining not only the structures of conformers, but also their populations even for systems in fast conformational equilibria where NMR spectroscopy is difficult to be applied. ROA enabled the monitoring of the secondary structures of denatured or unfolded proteins, such as an amyloid fibril and its prefibril intermediates.     

Can Raman Optical Activity Separate Axial from Local Chirality? A Theoretical Study of Helical Deca‐Alanine

Motivated by experimental work on the distinction of protein secondary structure motifs by Raman optical activity (ROA) spectroscopy, we demonstrate using density functional theory that axial chirality in structures with different local chirality can be filtered out by ROA spectroscopy. To this purpose, two diastereomers of right-handed helical deca-alanine, the (all-S) and the (R,S,R,S,R,S,R,S,R,S) form, are compared. Furthermore, we suggest to interpret calculated ROA spectra of large molecules in terms of vibrational bands rather than individual peaks. This is due to the non-homogeneous effect of the harmonic approximation as well as of the chosen electronic structure method onto the vibrational frequencies, which in a dense region of many vibrations will strongly determine the shape of the spectrum. In addition, the calculated ROA spectrum of (all-S)-deca-alanine is compared to the experimental spectrum of poly-(L)-alanine in solution.     

Raman optical activity: a powerful technique to investigate essential oil components

The theory and some applications of Raman Optical Activity (ROA) towards terpene analysis are presented. With this technique, vibrational optical activity from chiral molecules can be measured providing their absolute configuration. This short review provides data obtained for pinene, verbenone, menthol, camphor, carenes and related molecules. The ROA technique seems to be a powerful tool which permits correlation between the properties of biocompounds and their structure.     

Prediction of Raman Optical Activity Spectra of Chiral 3‐Acetylcamphorato‐Cobalt Complexes

We examine calculated vibrational Raman optical activity (ROA) spectra of octahedral cobalt complexes containing different combinations of acetylacetonato and 3‐acetylcamphorato ligands. Starting from the Δ‐tris(acetylacetonato)cobalt(III) complex, the ROA spectra of isomers generated by successive replacement of acetylacetonato ligands by chiral (+)‐ or (?)‐3‐acetylcamphorato ligands are investigated. In this way, it is possible to assess the influence of the degree of ligand substitution, ligand chirality, and geometrical isomerism on the ROA spectra. In addition, the effect of the Λ‐configuration is studied. It is found that the ROA spectra contain features that make it possible to identify each of the isomers, demonstrating the great sensitivity of ROA spectroscopy to the chiral nature of the various complexes.     

Detection of Circularly Polarized Luminescence of a Cs‐EuIII Complex in Raman Optical Activity Experiments

Circularly polarized luminescence (CPL) spectra are extremely sensitive to molecular structure. However, conventional CPL measurements are difficult and require expensive instrumentation. As an alternative, we explore CPL using Raman scattering and Raman optical activity (ROA) spectroscopy. The cesium tetrakis(3‐heptafluoro‐butylryl‐(+)‐camphorato) europium(III) complex was chosen as a model as it is known to exhibit very large CPL dissymmetry ratio. The fluorescent bands could be discriminated from true Raman signals by comparison of spectra acquired with different laser excitation wavelengths. Furthermore, the ROA technique enables fluorescence identification by measuring the degree of circularity. The CPL dissymmetry ratio was measured as the ROA circular intensity difference of 0.71, the largest one ever reported. The alternative CPL measurement enhances applications of lanthanides in analytical chemistry and chemical imaging of biological objects.     

Two Spectroscopies in One: Interference of Circular Dichroism and Raman Optical Activity

Previously, we and other laboratories have reported an unusual and strong Raman optical activity (ROA) induced in solvents by chiral dyes. Various theories of the phenomenon appeared, but they were not capable of explaining fully the observed ROA band signs and intensities. In this work, an analysis based both on the light scattering theory and dedicated experiments provides a more complete understanding. For example, double-cell magnetic circular dichroism and magnetic ROA experiments with copper-porphyrin complex show that the induced chirality is observed without any contact of the solvents with the complex. The results thus indicate that a combination of electronic circular dichroism (ECD) with the polarized Raman scattering is responsible for the effect. The degree of circularity of solvent vibrational bands is a principal molecular property participating in the event. The insight and the possibility to predict the chirality transfer promise future applications in spectroscopy, chemical analysis and polarized imaging.     

Paving the way to conformationally unravel complex glycopeptide antibiotics by means of Raman optical activity

It is crucial for fundamental physical chemistry techniques to find their application in tackling real-world challenges. Hitherto, Raman optical activity (ROA) spectroscopy is one of the examples where a promising future within the pharmaceutical sector is foreseen, but has not yet been established. Namely, the technique is believed to be able to contribute in investigating the conformational behaviour of drug candidates. We, herein, strive towards the alignment of the ROA analysis outcome and the pharmaceutical expectations by proposing a fresh strategy that ensures a more complete, reliable, and transferable ROA study. The strategy consists of the treatment of the conformational space by means of a principal component analysis (PCA) and a clustering algorithm, succeeded by a thorough ROA spectral analysis and a novel way of estimating the contributions of the different chemical fragments to the total ROA spectral intensities. Here, vancomycin, an antibiotic glycopeptide, has been treated; it is the first antibiotic glycopeptide studied by means of ROA and is a challenging compound in ROA terms. By applying our approach we discover that ROA is capable of independently identifying the correct conformation of vancomycin in aqueous solution. In addition, we have a clear idea of what ROA can and cannot tell us regarding glycopeptides. Finally, the glycopeptide class turns out to be a spectroscopically curious case, as its spectral responses are unlike the typical ROA spectral responses of peptides and carbohydrates. This preludes future ROA studies of this intriguing molecular class.

Raman optical activity tackles the complex conformational space of glycopeptide antibiotics.     

Vibrational Raman optical activity characterization of poly(l-proline) II helix in alanine oligopeptides

A vibrational Raman optical activity (ROA) study of a series of alanine peptides in aqueous solution is presented. The seven-alanine peptide Acetyl-OOAAAAAAAOO-Amide (OAO), recently shown by NMR and UVCD to adopt a predominantly poly(l-proline II) (PPII) helical conformation in aqueous solution, gave an ROA spectrum very similar to that of disordered poly(l-glutamic acid) which has long been considered to adopt the PPII conformation, both being dominated by a strong positive extended amide III ROA band at approximately 1319 cm-1 together with weak positive amide I ROA intensity at approximately 1675 cm-1. A series of alanine peptides Ala2-Ala6 studied in their cationic states in aqueous solution at low pH displayed ROA spectra which steadily evolved toward that of OAO with increasing chain length. As well as confirming that alanine peptides can support the PPII conformation in aqueous solution, our results also confirm the previous ROA band assignments for PPII structure, thereby reinforcing the foundation for ongoing ROA studies of unfolded and partially folded proteins.     

Raman optical activity spectra and conformational elucidation of chiral drugs. The case of the antiangiogenic aeroplysinin-1

We present the determination of the conformational properties of aeroplysinin-1 in aqueous solution by means of a combined experimental and theoretical Raman optical activity (ROA) and vibrational circular dichroism (VCD) study. Aeroplysinin-1 is an antiangiogenic drug extracted from the sponge Aplysina cavernicola which has been proved to be a valuable candidate for the treatment of cancer and other antiangiogenic diseases. Our study shows that this molecule possesses the 1S,6R absolute configuration in aqueous solution, where only two conformers are present to a significant level. We discuss in detail the relationships between the chiro-optical ROA and VCD features, and the structural properties of various energy accessible conformers are described. The present work is one of the first studies in which both ROA and VCD have been used as complementary tools for the determination of absolute configuration and dominant solution-state conformations of an unknown therapeutically significant molecule.     

Theoretical study of the Raman optical activity spectra of 3(10)-helical polypeptides

Raman optical activity (ROA) spectroscopy is a promising analytical method for studying the structure and conformation of polypeptides and proteins in solution. However, the structural information obtained from such vibrational spectra is only indirect and theoretical studies are often necessary to identify how the structure determines the observed spectra. One particular target is the identification and discrimination of different helical secondary structure elements. Herein, a theoretical investigation of the ROA spectra of a series of 3(10)-helical polypeptides is presented. In particular, the effect of chain length, C(α)-substitution pattern, the introduction of larger aliphatic side chains, and the variation of their conformation on the ROA spectra is studied. To extract general principles from these calculations, the positions, intensities, and shapes of the ROA bands are analyzed in terms of localized modes, which makes it possible to identify possible ROA signatures of 3(10) -helical structures, but also provides fundamental insight into the generation of ROA signals in complex polypeptides. Finally, the calculated spectra can be compared to the previously reported ROA spectrum of a specifically designed 3(10) -helical heptapeptide. This allows most of the features in the experimental spectrum to be assigned.     

Demonstration of the ring conformation in polyproline by the Raman optical activity

Raman and Raman optical activity (ROA) spectra of poly-L-proline were recorded in a wide frequency range and analyzed with respect to the proline side chain conformation. The analysis was based on comparison to ab initio simulations of spectral band positions and intensities. The presence of two conformer states of the five-member ring was found, approximately equally populated in the polypeptide. Additionally, Raman and ROA spectral shapes indicated that the peptide adopts the polyproline II helical conformation, in both aqueous and TFE solutions. The helix, however, is perturbed by fluctuations, which affects the vibrational coupling among amino acid residues and broadens the ROA bands. Contributions of the side and main peptide chains to the polyproline ROA intensities have comparable magnitudes. Thus understanding of the origins of both signals is important for determination of the peptide structure by ROA.     

Transfer and Amplification of Chirality Within the “Ring of Fire” Observed in Resonance Raman Optical Activity Experiments

We report extremely strong chirality transfer from a chiral nickel complex to solvent molecules detected as Raman optical activity (ROA). Electronic energies of the complex were in resonance with the excitation‐laser light. The phenomenon was observed for a wide range of achiral and chiral solvents. For chiral 2‐butanol, the induced ROA was even stronger than the natural one. The observations were related to so‐called quantum (molecular) plasmons that enable a strong chiral Rayleigh scattering of the resonating complex. According to a model presented here, the maximal induced ROA intensity occurs at a certain distance from the solute, in a three‐dimensional “ring of fire”, even after rotational averaging. Most experimental ROA signs and relative intensities could be reproduced. The effect might significantly increase the potential of ROA spectroscopy in bioimaging and sensitive detection of chiral molecules.     

Understanding the Signatures of Secondary‐Structure Elements in Proteins with Raman Optical Activity Spectroscopy

A prerequisite for the understanding of functional molecules like proteins is the elucidation of their structure under reaction conditions. Chiral vibrational spectroscopy is one option for this purpose, but provides only indirect access to this structural information. By first‐principles calculations, we investigate how Raman optical activity (ROA) signals in proteins are generated and how signatures of specific secondary‐structure elements arise. As a first target we focus on helical motifs and consider polypeptides consisting of twenty alanine residues to represent α‐helical and 3₁₀‐helical secondary‐structure elements. Although ROA calculations on such large molecules have not been carried out before, our main goal is the stepwise reconstruction of the ROA signals. By analyzing the calculated ROA spectra in terms of rigorously defined localized vibrations, we investigate in detail how total band intensities and band shapes emerge. We find that the total band intensities can be understood in terms of the reconstructed localized vibrations on individual amino acid residues. Two different basic mechanisms determining the total band intensities can be established, and it is explained how structural changes affect the total band intensities. The band shapes can be rationalized in terms of the coupling between the localized vibrations on different residues, and we show how different band shapes arise as a consequence of different coupling patterns. As a result, it is demonstrated for the chiral variant of Raman spectroscopy how collective vibrations in proteins can be understood in terms of well‐defined localized vibrations. Based on our calculations, we extract characteristic ROA signatures of α helices and of 3₁₀‐helices, which our analysis directly relates to differences in secondary structure.     

Unravelling a Direct Role for Polysaccharide β‐Strands in the Higher Order Structure of Physical Hydrogels

The mechanical properties of agarose‐derived hydrogels depend on the scaffolding of the polysaccharide network. To identify and quantify such higher order structure, we applied Raman optical activity (ROA)—a spectroscopic technique that is highly sensitive toward carbohydrates—on native agarose and chemically modified agarose in the gel phase for the first time. By spectral global fitting, we isolated features that change as a function of backbone carboxylation (28, 40, 50, 60, 80, and 93 %) from other features that remain unchanged. We assigned these spectral features by comparison to ROA spectra calculated for different oligomer models. We found a 60:40 ratio of double‐ and single‐stranded α‐helix in the highly rigid hydrogel of native agarose, while the considerably softer hydrogels made from carboxylated agarose use a scaffold of unpaired β‐strands.     

Raman Optical Activity of Glucose and Sorbose in Extended Wavenumber Range

Raman optical activity (ROA) is pursued as a promising method for structural analyses of sugars in aqueous solutions. In the present study, experimental Raman and ROA spectra of glucose and sorbose obtained in an extended range (50–4000 cm⁻¹) are interpreted using molecular dynamics and density functional theory, with the emphasis on CH stretching modes. A reasonable theoretical basis for spectral interpretation was obtained already at the harmonic level. Anharmonic corrections led to minor shifts of band positions (up to 25 cm⁻¹) below 2000 cm⁻¹, while the CH stretching bands shifted more, by ∼180 cm⁻¹, and better reproduced the experiment. However, the anharmonicities could be included on a relatively low approximation level only, and they did not always improve the harmonic band shapes. The dependence on the structure and conformation shows that the CH stretching ROA spectral pattern is a sensitive marker useful in saccharide structure studies.     

Studying the Glycan Moiety of RNase B by Means of Raman and Raman Optical Activity

Raman and Raman optical activity (ROA) spectroscopy are used to study the solution‐phase structure of the glycan moiety of the protein ribonuclease B (RNase B). Spectral data of the intact glycan moiety of RNase B is obtained by subtracting high‐quality spectral data of RNase A, the non‐glycosylated form of the RNase, from the spectra of the glycoprotein. The remaining difference spectra are compared to spectra generated from Raman and ROA data of the constituent disaccharides of the RNase glycan, achieving convincing spectral overlap. The results show that ROA spectroscopy is able to extract detailed spectral data of the glycan moieties of proteins, provided that the non‐glycosylated isoform is available. Furthermore, good comparison between the full glycan spectrum and the regenerated spectra based on the disaccharide data lends great promise to ROA as a tool for the solution‐phase structural analysis of this structurally elusive class of biomolecules.