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Resonance Raman optical activity (RROA) is commonly measured as the difference in intensity of Raman scattered right and left circularly polarized light, IR -IL , when a randomly polarized light is in resonance with a chiral molecule. Strong and sometimes mono-signate experimental RROA spectra of several chiral solutes were reported previously, although their signs and relative intensities could not be reproduced theoretically. By examining multiple light-matter interaction events which can occur simultaneously under resonance, we show that a new form of chiral Raman spectroscopy, eCP-Raman, a combination of electronic circular dichroism and circularly polarized Raman, prevails. By incorporating the finite-lifetime approach for resonance, the experimental patterns of the model chiral solutes are captured theoretically by eCP-Raman, without any RROA contribution. The results open opportunity for applications of eCP-Raman spectroscopy and for extracting true RROA experimentally.

Amyloidal protein fibrils occur in many biological events, but their formation and structural variability are understood rather poorly. We systematically explore fibril polymorphism for polyglutamic acid (PGA), insulin and hen egg white lysozyme. The fibrils were grown in the presence of "seeds", that is fibrils of the same or different protein. The seeds in concentrations higher than about 5 % of the total protein amount fully determined the structure of the final fibrils. Fibril structure was monitored by vibrational circular dichroism (VCD) spectroscopy and other techniques. The VCD shapes significantly differ for different fibril samples. Infrared (IR) and VCD spectra of PGA were also simulated using density functional theory (DFT) and a periodic model. The simulation provides excellent basis for data interpretation and reveals that the spectral shapes and signs depend both on fibril length and twist. The understanding of fibril formation and interactions may facilitate medical treatment of protein misfolding diseases in the future.

Change of molecular properties with pressure is an attracting means to regulate molecular reactivity or biological activity. However, the effect is usually small and so far explored rather scarcely. To obtain a deeper insight and estimate the sensitivity of vibrational optical activity spectra to pressure-induced conformational changes, we investigate small model molecules. The Ala-Ala dipeptide, isomaltose disaccharide and adenine-uracil dinucleotide were chosen to represent three different biomolecular classes. The pressure effects were modeled by molecular dynamics and density functional theory simulations. The dinucleotide was found to be the most sensitive to the pressure, whereas for the disaccharide the smallest changes are predicted. Pressure-induced relative intensity changes in vibrational circular dichroism and Raman optical activity spectra are predicted to be 2-3-times larger than for non-polarized IR and Raman techniques.

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.

Helical α-aminoisobutyric acid (Aib) foldamers show great potential as devices for the communication of conformational information across phospholipid bilayers, but determining their conformation in bilayers remains a challenge. In the present study, Raman, Raman optical activity (ROA), infrared (IR) and vibrational circular dichroism (VCD) spectroscopies have been used to analyze the conformational preferences of Aib foldamers in solution and when interacting with bilayers. A 310 -helix marker band at 1665-1668 cm-1 in Raman spectra was used to show that net helical content increased strongly with oligomer length. ROA and VCD spectra of chiral Aib foldamers provided the chiroptical signature for both left- and right-handed 310 -helices in organic solvents, with VCD establishing that foldamer screw-sense was preserved when the foldamers became embedded within bilayers. However, the population distribution between different secondary structures was perturbed by the chiral phospholipid. These studies indicate that ROA and VCD spectroscopies are valuable tools for the study of biomimetic structures, such as artificial signal transduction molecules, in phospholipid bilayers.

Lanthanide complexes are used as convenient spectroscopic probes for many biomolecules. Their binding to proteins is believed to be enhanced by the presence of histidine, but the strength of the interaction significantly varies across different systems. To understand the role of peptide length and sequence, short histidine-containing peptides have been synthesized (His-Gly, His-Gly-Gly, His-Gly-Gly-Gly, Gly-His, Gly-His-Gly, His-His, and Gly-Gly-His) and circularly polarized luminescence (CPL) induced at the [Eu(dpa)3 ]3- complex has been measured by means of a Raman optical activity (ROA) spectrometer. The obtained data indicate relatively weak binding of the histidine residue to the complex, with a strong participation of other parts of the peptide. Longer peptides, low pH, and a histidine residue close to the N-peptide terminus favor the binding. The binding strengths are approximately proportional to the CPL intensity and roughly correlate with predictions based on molecular dynamics (MD) simulations. The specificity of lanthanide binding to the peptide structure and its intense luminescence and high optical activity make the ROA/CPL technique suitable for probing secondary and tertiary structures of peptides and proteins.

Optical activity of regular molecular assemblies, such as protein fibrils or nucleic acid condensates, is often significantly stronger than for isolated molecules. Previous modeling suggested that this may be caused by the ordered quasi-periodic structure and a long-order synchronization of chromophore excitations. In the present study, we briefly review this phenomenon and investigate some aspects on simple models related to protein vibrational optical activity. The transition dipole coupling (TDC) model is used to generate vibrational circular dichroism (VCD) and Raman optical activity (ROA) spectra. While a linear arrangement of chromophores produced relatively simple couplet intensity patterns, a richer band structure was predicted for planar geometries. A stacking of β-sheet planes has been identified as another powerful source of the enhancement. The results do not completely reproduce experimental observations but are consistent with them and confirm that chiroptical methods may be extremely useful to study aggregation of chiral molecules.

Structure and flexibility of natural compounds determine their biological activity. In the present study, electronic circular dichroism (ECD), vibrational circular dichroism (VCD), and Raman optical activity (ROA) spectra of cocaine hydrochloride in aqueous solutions were measured and related to the structure with the aid of density functional theory (DFT) computations. Additional measurements in deuterated environment made assignment of vibrational bands easier. The results suggest that the prevalent cocaine conformation in solution differs from that adopted in hydrochloride crystal. The spectroscopic results and computational analysis are consistent with X-ray structures of known cocaine-receptor complexes, in which the compound adopts a variety of conformations. All three kinds of chiroptical spectra exhibited significantly greater conformational sensitivity than unpolarized absorption or Raman scattering. The ROA technique provided the largest number of well-resolved bands, bearing rich structural information.

Raman spectroscopy based on the 1064-nm laser excitation was suggested as a handy non-invasive technique allowing to quickly determine sugar content in honey and similar food products. In the present study, the green 532-nm laser radiation is explored instead as it provides higher-quality spectra in a shorter time. The sample fluorescence was quenched by purification with activated carbon. For control mixture decomposition of Raman spectra to standard subspectra led to a typical error of the sugar content of 3%. Raman optical activity (ROA) spectra that could be measured at the shorter excitation wavelength as well provided a lower accuracy (~8%) than the Raman spectra because of instrumental sensitivity and noise limitations. The results show that Raman spectroscopy provides elegant and reliable means for fast analyses of sugar-based food products. Copyright © 2016 John Wiley & Sons, Ltd.

The detailed electronic structures of fluorescent chromophores are important for their use in imaging of living cells. A series of green fluorescent protein chromophore derivatives is examined by magnetic circular dichroism (MCD) spectroscopy, which allows the resolution of more bands than plain absorption and fluorescence. Observed spectral patterns are rationalized with the aid of time-dependent density functional theory (TDDFT) computations and the sum-over-state (SOS) formalism, which also reveals a significant dependence of MCD intensities on chromophore conformation. The combination of organic and theoretical chemistry with spectroscopic techniques also appears useful in the rational design of fluorescence labels and understanding of the chromophore's properties. For example, the absorption threshold can be heavily affected by substitution on the phenyl ring but not much on the five-member ring, and methoxy groups can be used to further tune the electronic levels.

Monensin is a natural antibiotic that exhibits high affinity to certain metal ions. In order to explore its potential in coordination chemistry, circular dichroism (CD) spectra of monensic acid A (MonH) and its derivatives containing monovalent cations (Li(+) , Na(+) , K(+) , Rb(+) , Ag(+) , and Et4 N(+) ) in methanolic solutions were measured and compared to computational models. Whereas the conventional CD spectroscopy allowed recording of the transitions down to 192 nm, synchrotron radiation circular dichroism (SRCD) revealed other bands in the 178-192 nm wavelength range. CD signs and intensities significantly varied in the studied compounds, in spite of their similar crystal structure. Computational modeling based on the Density Functional Theory (DFT) and continuum solvent model suggests that the solid state monensin structure is largely conserved in the solutions as well. Time-dependent Density Functional Theory (TDDFT) simulations did not allow band-to-band comparison with experimental spectra due to their limited precision, but indicated that the spectral changes were caused by a combination of minor conformational changes upon the monovalent cation binding and a direct involvement of the metal electrons in monensin electronic transitions. Both the experiment and simulations thus show that the CD spectra of monensin complexes are very sensitive to the captured ions and can be used for their discrimination. Chirality 28:420-428, 2016. © 2016 Wiley Periodicals, Inc.

Magnetic Raman optical activity of gases provides unique information about their electric and magnetic properties. Magnetic Raman optical activity has recently been observed in a paramagnetic gas (Angew. Chem. Int. Ed. 2012, 51, 11058; Angew. Chem. 2012, 124, 11220). In diamagnetic molecules, it has been considered too weak to be measurable. However, in chlorine, bromine and iodine vapors, we could detect a significant signal as well. Zeeman splitting of electronic ground-state energy levels cannot rationalize the observed circular intensity difference (CID) values of about 10(-4). These are explicable by participation of paramagnetic excited electronic states. Then a simple model including one electronic excited state provides reasonable spectral intensities. The results suggest that this kind of scattering by diamagnetic molecules is a general event observable under resonance conditions. The phenomenon sheds new light on the role of excited states in the Raman scattering, and may be used to probe molecular geometry and electronic structure.

The geometry and the electronic structure of chiral lanthanide(III) complexes are traditionally probed by electronic methods, such as circularly polarised luminescence (CPL) and electronic circular dichroism (ECD) spectroscopy. The vibrational phenomena are much weaker. In the present study, however, significant enhancements of vibrational circular dichroism (VCD) and Raman optical activity (ROA) spectral intensities were observed during the formation of a chiral bipyridine-Eu(III) complex. The ten-fold enhancement of the vibrational absorption and VCD intensities was explained by a charge-transfer process and the dominant effect of the nitrate ion on the spectra. A much larger enhancement of the ROA and Raman intensities and a hundred-fold increase of the circular intensity difference (CID) ratio were explained by the resonance of the λ = 532 nm laser light with the (7)F0 → (5)D0 transitions. This phenomenon is combined with a chirality transfer, and mixing of the Raman and luminescence effects involving low-energy (7)F states of europium. The results thus indicate that the vibrational optical activity (VOA) may be a very sensitive tool for chirality detection and probing of the electronic structure of Eu(III) and other coordination compounds.

Although electronic and magnetic circular dichroism (ECD, MCD) spectra reveal valuable details about molecular geometry and electronic structure, quantum-chemical simulations significantly facilitate their interpretation. However, the simulated results may depend on the choice of coordinate origin. Previously (Štěpánek and Bouř, J. Comput. Chem. 2013, 34, 1531), the sum-over-states (SOS) methodology was found useful for efficient MCD computations. Approximate wave functions were "resolved" using time-dependent density functional theory, and the origin-dependence was avoided by placing the origin to the center of mass of the investigated molecule. In this study, a more elegant way is proposed, based on the localized orbital/local origin (LORG) formalism, and a similar approach is also applied to generate ECD intensities. The LORG-like approach yields fully origin-independent ECD and MCD spectra. The results thus indicate that the computationally relatively cheap SOS simulations open a new way of modeling molecular properties, including those involving the origin-dependent magnetic dipole moment operator.

Lanthanide complexes exhibit interesting spectroscopic properties yielding many applications as imaging probes, natural chirality amplifiers, and therapeutic agents. However, many properties are not fully understood yet. Therefore, we applied magnetic circular dichroism (MCD) spectroscopy, which provides enhanced information about the underlying electronic structure to a series of lanthanide compounds. The metals in the M(3+) state included Y, La, Eu, Tb, Dy, Ho, Er, Tm, Yb, and Lu; the spectra were collected for selected tetraphenylporphin (TPP) and octaethylporphin (OEP) complexes in chloroform. While the MCD and UV-VIS absorption spectra were dominated by the porphyrin signal, metal binding significantly modulated them. MCD spectroscopy was found to be better suited to discriminate between various species than absorption spectroscopy alone. The main features and trends in the lanthanide series observed in MCD and absorption spectra of the complexes could be interpreted at the Density Functional Theory (DFT) level, with effective core potentials on metal nuclei. The sum over state (SOS) method was used for simulation of the MCD intensities. The combination of the spectroscopy and quantum-chemical computations is important for understanding the interactions of the metals with the organic compounds.

Magnetic coordination complexes based on Schiff bases are promising new molecular materials for electronics. Two μ-oxo FeIII dimeric complexes of enantiomers of Schiff base ligands (N,N′-(1R,2R)-1,2-diphenylethylenebis(salicylideneimine) (H2salphen-R, 1 a) and N,N′-(1S,2S)-1,2-diphenylethylenebis(salicylideneimine) (H2salphen-S, 1 b)) were synthesized; further reaction with 4-salicylideneamino-1,2,4-triazole (Hsaltrz) led to enantiomers of two one-dimensional (1D) FeIII coordination complexes. The structures of these complexes were determined by X-ray diffraction. Magnetic susceptibility measurements revealed that μ-oxo dimeric complexes displayed strong antiferromagnetic coupling, whereas the 1D complexes exhibited paramagnetic behavior. The chirality of the Schiff bases transferred to macromolecular chirality of the complexes, which could be monitored by both vibrational and electronic circular dichroism (CD) spectroscopic methods. The macroscopic handedness was manifested in CD signals attributed to exciton coupling between the ligands. Thus, the chiral spectroscopies were useful to probe the chirality of the complexes, their structure, and the polymerization degree.

Induced resonance Raman optical activity (IRROA) proved to be a very sensitive method to detect molecular chirality. It is exhibited, for example, by complexes of lanthanides with chiral alcohols or ketones. So far, the phenomenon has not been understood at a quantitative level. To elucidate its mechanisms and to correctly relate the spectra to the structure, a transition polarizability model (TPM) is developed and applied to a camphor-europium complex. The model well reproduces the high ROA/Raman intensity ratio of the IRROA observed experimentally. The results additionally indicate a fundamental role of the nonchiral fod ligand in the Eu(fod)3 compound for the chirality enhancement. The TPM model thus serves as a guidance for both experimental and theoretical studies to come. © 2013 Wiley Periodicals, Inc.

Clusters of a solute and a few solvent molecules obtained from molecular dynamics (MD) are a powerful tool to study solvation effects by advanced quantum chemical (QC) methods. For spectroscopic properties strongly dependent on the solvation, however, a large number of clusters are needed for a good convergence. In this work, a parallel variable selection (PVS) method is proposed that in some cases efficiently reduces the number of clusters needed for the averaging. The mass, charge, or atomic density MD distributions are used as a secondary variable to preselect the most probable cluster geometries used for averaging of solute spectral properties. When applied to nuclear magnetic resonance chemical shift of a model alcohol, the method allowed one to significantly reduce the total computational time, by a factor of 10. Even larger savings were achieved for the modeling of Raman and Raman optical activity spectra of (S)-lactamide molecule dissolved in water. The results thus suggest that the PVS method can be generally used for simulations of spectroscopic properties of solvated molecules and makes multiscale MD/QC computations more affordable. V C 2012 Wiley Periodicals, Inc.

Molecular pincers or tweezers are designed to hold and release the target molecule. Potential applications involve drug distribution in medicine, environment technologies, or microindustrial techniques. Typically, the binding is dominated by van der Waals forces. Modeling of such complexes can significantly enhance their design; yet obtaining accurate complexation energies by theory is difficult. In this study, density functional theory (DFT) computations combined with dielectric continuum solvent model are compared with the potential of mean force approach using umbrella sampling and the weighted histogram analysis method (WHAM) with molecular dynamics (MD) simulations. For DFT, functional and basis set effects are discussed. The computed results are compared to experimental data based on NMR spectroscopic measurements of five synthesized tweezers based on the Tröger's basis. Whereas the DFT computations correctly provided the observed trends in complex stability, they failed to produce realistic magnitudes of complexation energies. Typically, the binding was overestimated by DFT if compared to experiment. The simpler semiempirical PM6-DH2X scheme proposed lately yielded better magnitudes of the binding energies than DFT but not the right order. The MD-WHAM simulations provided the most realistic Gibbs binding energies, although the approximate MD force fields were not able to reproduce completely the ordering of relative stabilities of model complexes found by NMR. Yet the modeling provides interesting insight into the complex geometry and flexibility and appears as a useful tool in the tweezers' design.

Novel copper(II) coordination compounds with chiral macrocyclic imine ligands derived from R-/S-camphor were asymmetrically synthesized and characterized with the aid of chiroptical spectroscopies. Crystal structures of both enantiomers were determined by single crystal X-ray diffraction analysis. Circular dichroism (CD) spectra were analyzed using a simplified exciton model as well as quantum chemical computations. The absolute configuration of the copper(II) coordination compounds determined from CD was found consistent with the crystal data. The copper(II) complexes were further investigated by vibrational CD (VCD) measurement combined with density functional theory calculation. The complex formation was evidenced by spectral shifts of the characteristic bands in the CD and VCD spectra. Chirality 24:451–458, 2012. © 2012 Wiley Periodicals, Inc.

J coupling in NMR spectroscopy is conventionally associated with covalent bonds. A noncovalent contribution often called through-space coupling (TSC) has been observed for heavy atoms. In this study, the TSC was detected and analyzed for the more common (1)H-(1)H coupling as well. In synthesized model molecules the hydrogen positions could be well controlled. For several coupling constants the through-space mechanism was even found to be the predominant factor. The nature and magnitude of the phenomenon were also analyzed by density functional computations. Calculated carbon- and hydrogen-coupling maps and perturbed electronic densities suggest that the aromatic system strongly participates in the noncovalent contribution. Unlike covalent coupling, which is usually governed by the Fermi contact, TSC is dominated by the diamagnetic term comprising interactions of nuclei with the electron orbital angular momentum. The computations further revealed a strong distance and conformational dependence of TSC. This suggests that the through-space coupling can be explored in molecular structural studies in the same way as the covalent one.

Aromatic nucleophilic substitution on the C9 carbon of acridine plays an important role in multiple biological and medicinal applications. The rate of the key reaction is strongly dependent on the environment and acridine substituents. In this study, the factors influencing the reaction mechanism were studied theoretically and verified experimentally for simplified systems. Density functional theory was used for the computations. The activation energy of a model derivative was determined experimentally from a kinetic study. Also, the relative reactivities of selected compounds were verified by a competition experiment. The theoretical predictions correlate well with the observations. The computed reaction path confirms that the thiol group attacks the aromatic core activated by the nitrogen heteroatom and replaces halogen or amino substituents via a well-defined Meisenheimer transition state. The reaction barrier is strongly influenced by both the solvent and substituents on the aromatic system. A multistep mechanism is proposed for the reaction of aminoacridine in which the aqueous solvent participates in the reaction. Strong correlations between reaction energies and geometrical parameters were observed and can be used to rationalize the design of acridine drugs as well as to avoid lengthy computations of the transition states.

Raman optical activity (ROA) spectroscopy is used to investigate the backbone conformation of valinomycin in methanol and dioxane solution. Experimental Raman and ROA spectral differences are interpreted by using density functional calculations, molecular dynamics, and Cartesian tensor transfer. Of the several conformers with different numbers of intramolecular hydrogen bonds which were preselected by calculations of relative energies, the dominant ones are identified on the basis of ROA. To separate the backbone signal from that of the side chains, conformational search for the isopropyl residues is performed for each backbone conformer. In dioxane, the most populated conformer does not exhibit C(3) symmetry, but adopts a distorted "bracelet" structure, similar to a crystal structure. This complements previous NMR spectroscopic results that could not distinguish the nonsymmetric structures. In methanol, a different, "propeller" conformer is indicated by ROA, which has three loops resembling a standard β-turn peptide motif. Molecular dynamics simulations suggest that the propeller structure is very flexible in methanol. Spectra simulated for geometries not having the β-turn do not agree with experiment. On the basis of these results, a distinct +/- ROA couplet at ∼1335/1317 cm(-1) observed in the extended amide III region is assigned to a turn in the valinomycin backbone.

The solution structures of (3R,4S)- and (3S,4R)-4-(4-fluorophenyl)-3-hy- droxylmethyl-1-methylpiperidine, which are intermediates in the synthesis of the two pharmaceuticals paroxetine and femoxetine, were studied by vibrational circular di- chroism (VCD) spectroscopy. In addition, six derivatives with different substituents attached to the C3 atom were prepared and their VCD and absorption spectra discussed with the aid of ab initio simulations. The VCD spectra were found to be sensitive to the geometry changes. In addition, a subtle variation caused by intermolecular aggregation was apparent in the spectra. The VCD technique can be applied for structural analysis of chiral pharmaceuticals in solutions. © 2002 Wiley Periodicals, Inc. Biopolymers (Biospec- troscopy) 67: 298 -301, 2002