Incident circularly polarized Raman optical activity spectrometer based on circularity conversion method

A simple incident circular polarization Raman optical activity (ICP ROA) spectrometer was constructed by applying the method of circularity conversion. The circular polarization of the incident laser light was modulated between right and left by the insertion of a half‐wave plate and not by using a Pockels cell which is usually used in ICP ROA instruments. On the basis of the concept of the virtual enantiomer (Hug, W. Applied Spectroscopy, 2003, 57, 1), circularity converters were inserted in the optical train, which could effectively compensate the systematic offset. The new instrument successfully attained photon shot‐noise‐limited conditions for all bands except for the very strongly polarized Raman band. The ROA spectra of some standard chiral samples were measured to demonstrate the performance of the spectrometer. Copyright © 2010 John Wiley & Sons, Ltd.     

Raman optical activity of an achiral element in a chiral environment

Raman optical activity (ROA) is a relatively new technique used to determine the structure of chiral molecules and is proving useful in the study of biological molecules such as proteins and DNA/RNA. Here, for the first time, we demonstrate the applicability of ROA as a technique to study achiral groups in chiral environments, detecting the induced chirality of N‐(fluorenyl‐9‐methoxycarbonyl) (Fmoc) in a chiral self‐assembled structure of Fmoc‐dipeptides. This technique is therefore of interest to those studying self‐assembled systems that adopt a chiral structure. Copyright © 2009 John Wiley & Sons, Ltd.     

Two-dimensional correlation analysis of Raman optical activity data on the α-helix-to-β-sheet transition in poly(L-lysine)

Raman optical activity (ROA) has evolved into an incisive probe of structure and conformational transitions in polypeptides and proteins revealing many signal patterns characteristic of specific secondary structural elements. In order to further facilitate analysis of ROA spectral intensity variations, two-dimensional correlation methods are applied to ROA and Raman spectra monitoring the α-helix-to-β-sheet transition in poly(L-lysine) as a function of temperature. Pretreatment of data using background subtraction, normalization and gentle smoothing is essential for the successful generation of 2D ROA correlations, 2D Raman correlations and 2D Raman/ROA heterocorrelations. The pseudoscalar nature of ROA spectra results in detailed 2D correlation analyses providing extensive interpretation of spectral intensity variations. Synchronous plots indicate band assignments consistent with established assignments in poly(L-lysine) together with possible new assignments. Corresponding asynchronous plots probe the temporal sequence of the conformational transition indicating distinct temporal phases while monitoring aggregation through a small amount of β-structure present at the start of the experiment ahead of α-helix unfolding. This study demonstrates the potential of 2D correlation analysis as a valuable technique for the extraction of detailed information about aggregation and conformational transitions in polypeptides and proteins from associated ROA and Raman spectra. Results indicate that aggregation of poly(L-lysine) monomers precedes intramolecular conversion of α-helix to β-sheet, which is then followed by fibril formation.     

The aqueous Raman optical activity spectra of 4(R)‐hydroxyproline: theory and experiment

Vibrational Raman optical activity (ROA) spectra have been measured for aqueous solutions of 4(R)‐hydroxyproline at three different pH values and are compared with theoretical results calculated for several conformations of anionic, cationic and zwitterionic 4(R)‐hydroxyproline using density functional theory (DFT) and the polarizable continuum model (PCM). The experimental ROA bands have been ascribed to the normal modes by comparison of the experimental and calculated vibrational frequencies and ROA intensities. Overall, using PCM for geometry optimization and force field calculations gives simulated Raman and ROA spectra that agree with the main features of the experimental spectra, whereas using PCM also in the calculations of optical tensors seems more problematic. Copyright © 2010 John Wiley & Sons, Ltd.     

傅立叶喇曼旋光实验技术

所谓的喇曼光性(ROA)是指手性分子对于左右圆偏振光,具有很小差别喇曼散射截面(10~(-4))的现象。测量这种微小的差别的实验技术目前已有所发展,近年傅立叶喇曼光谱技术迅速发展提示我们思考傅立叶喇曼旋光实验的可能性,在此,我们将应用傅立叶振动圆二色性中的有关概念来探讨实现傅立叶喇曼旋光实验技术的可能性。     

Raman optical activity spectra: basis set and electron correlation effects

Raman and vibrational Raman optical activity (ROA) spectra have been calculated for three molecules: H₂O₂, CHDTF and CHDTOH. The effects of electron correlation on the ROA parameters of these relatively small systems have been investigated by means of the multiconfigurational self-consistent field (SCF) approach. A range of correlation-consistent basis sets has been used in the calculations. The basis set convergence of the calculated ROA parameters is fairly well achieved when sets of at least d-aug-cc-pVTZ quality are used. On the other hand, the aug-cc-pVDZ set seems to be sufficient for a qualitative analysis. Diffuse functions, possibly a double set, are essential to ensure a good quality of the calculated ROA spectrum. In most cases the sign and approximate magnitude of the ROA are correctly predicted at the SCF level, and the corresponding ROA spectra should be considered qualitatively accurate.     

Simultaneous acquisition of all four forms of circular polarization Raman optical activity: results for α‐pinene and lysozyme

A new circularly polarized (CP) Raman spectrometer is described that demonstrates simultaneous acquisition of all four forms of circular polarization Raman optical activity (ROA). The instrument is a design extension of a commercially available back scattering circular polarization (SCP) ROA spectrometer. Circular polarization of the incident beam is introduced with a quarter‐wave plate, and a half‐wave plate alternately positioned in and out of the beam controls the modulation between right circular polarization (RCP) or left circular polarization (LCP) states. Combining this modulation with the simultaneous detection of RCP and LCP scattered Raman radiation allows the measurement of incident circular polarization (ICP), SCP, in‐phase dual circular polarization(DCP_I) and out‐of‐phase DCP_II‐ROA. In addition, three different forms of backscattered Raman spectra, namely unpolarized, highly polarized, and depolarized Raman spectra, as well as a degree of circularity spectrum are obtained. The performance of the new all‐CP ROA spectrometer is evaluated with neat α‐pinene and aqueous hen lysozyme solution. Copyright © 2011 John Wiley & Sons, Ltd.     

Solving chemical classification problems using polarized Raman data

When solving chemical classification problems, multivariate analysis has proven to be an important mathematical tool. Unpolarized spectroscopic data, IR, NIR, and UV‐Visible absorption data and unpolarized vibrational Raman data, are typically analyzed by two‐way chemometric methods, e.g. principal component analysis (PCA). When the unpolarized spectra of the different molecules are almost identical, the PCA results in low recognition ratios or even fails. In contrast to absorption processes, Raman scattering can provide polarized data. It is shown, by using mathematical simulations, that the outcome of the PCA can be improved considerably by using the polarized, vibrational Raman data instead of the unpolarized data. The improvement stems from the increased amount of molecular information, which is now available for the PCA of the vibrational data, because the polarization properties of the scattering, expressed through the depolarization ratio (DPR), is very sensitive to small changes in distinct molecular properties and insensitive to sample and experimental variations. For molecules possessing some symmetry, a change of the DPR may be induced by a decrease in symmetry and for highly symmetric molecules non‐dispersive Raman modes typically become dispersive. For dispersive modes, a wavenumber‐dependant change of the DPR may also result from a small energy shift of an allowed electronic transition. We show that the increased information content inherently present in the polarized data, opens up new possibilities for combining the solution of classification problems with an unveiling of details of the different properties and processes in bio‐physic due to various perturbations and changes of the structure of the bio‐molecules. It is also demonstrated that the increased access to molecular information enables in vitro detection of molecular changes often encountered when analyzing biological functions, which are reflected in changes in the excited electronic states. Copyright © 2010 John Wiley & Sons, Ltd.     

Chiral asymmetry of anti-symmetric coordinates studied by the Raman differential bond polarizability of S-phenylethylamine

<正>The Raman optical activity(ROA) study on 5-phenylethylamine is presented by the intensity analyses via bond polarizability and differential bond polarizability.Ample information concerning the physical picture of this chiral system is obtained,and its ROA mechanism is constructed.Especially,we propose that the asymmetric modes and/or the off-diagonal elements of the electronic polarizability tensor are the potential keys to the exploration of ROA.     

Use of Raman and Raman optical activity for the structural characterization of a therapeutic monoclonal antibody formulation subjected to heat stress

Structural complexity of biological drug products presents an analytical challenge in terms of early detection of aggregation and/or degradation. In the present study, Raman and Raman optical activity (ROA) were evaluated for their sensitivity to detect heat‐induced molecular instability in an Immunoglobulin G4 subclass therapeutic monoclonal antibody present in its formulation matrix. The therapeutic antibody was subjected to heat stress at 50 °C and was analyzed at various time points up to 1 month. The current results suggest that Raman and ROA are sensitive to early‐stage detection of heat‐induced instability of the antibody, in which significant changes could be observed at 1 week of stress. ROA could provide early detection of the subtle differences at the tertiary structure level in a heat‐stressed monoclonal antibody and Raman/ROA spectra could provide early detection in secondary structural changes as well. Copyright © 2015 John Wiley & Sons, Ltd.     

基于折线形手征负折射介质结构的设计与仿真

分别设计和仿真了一种新型的工作在微波段和光波段的折线形手征介质结构.利用仿真得到的透射系数和反射系数反演计算了该结构的旋光角、椭偏度、相对介电常数、相对磁导率、手征参数以及折射率等参数.结果表明该结构在这两种波段下都显示出极大的旋光角和椭偏度,且因具有大的手征参数,而不再需要介电常数和磁导率同时为负就可实现左旋圆极化波(LCP)和右旋圆极化波(RCP)的负折射率. 关键词： 手征介质 旋光角 椭偏度 负折射率     

[MnHg(SCN)4(H2O)2]·2C4H9NO(MMTWD)晶体的光谱分析

根据商群对称性分析法对MMTWD晶体的振动模作了理论计算.计算结果显示,通过实验可观察到的拉曼散射峰和红外反射带应分别不超过297和148个.运用实验的手段,分别测定晶体的拉曼光谱和红外光谱,给出了MMTWD晶体在50～3 000 cm-1范围内的拉曼光谱图和400～4 000 cm-1范围里的红外光谱图,并对光谱中的谱线作了指认.给出了MMTWD晶体的分子结构示意图,最终确定MMTWD晶体结构为三维网状结构,说明在众多新型光功能材料当中MMTWD晶体是一种更适合生长成较大尺寸且稳定性较好的非线性光学晶体,在光电子领域中具有良好的应用前景.     

木榄醇手性光谱的密度泛函研究

手性光学理论研究有助于解释手性分子的旋光机理和设计新的手性药物.采用B3LYP方法,计算了木榄醇A—C的旋光度和圆二色谱.从分子结构、正则振动和电子结构方面,探索了分子手性微观起源,分析了旋光度和电子圆二色谱的溶剂效应.表明OH的引入可调节分子的手性,甲基和苯环加强了分子的手性.发生在手性骨架上的振动和电子跃迁,加强了分子的手性.溶剂效应减小旋光度、削弱电子圆二色谱. 关键词： 木榄醇 光学活性 密度泛函理论 圆二色谱     

手性分子2, 3-丁二醇的旋光拉曼光谱研究

本文从拉曼峰和旋光拉曼峰出发,通过键极化率和微分键极化率分析研究(2R, 3R)-2, 3-丁二醇. 通过分子C₁和C₂两种点群的优化结构,获得不依赖于这两种结构的结果 和有关这个手性系统物理图像的丰富信息.对分子拉曼键极化率分析,得出在拉曼弛豫过程中, 电荷主要从外围流向骨架结构.对分子微分键极化率的分析,显示在不对称C原子和与其相联系的H原子 两侧化学键, C-O和C-CH₃的微分键极化率的符号正好相反,意味着这个分子具有相当好的手性 不对称性质.对比对称和反对称的键极化率、微分键极化率,本文得到这样的结论: 对于(特别是键伸缩的)键极化率,(大体上是)对称的大于反对称的; 而对于微分键极化率则是反对称的大于对称的. 关键词： 旋光拉曼 键极化率 微分键极化率 2,3-丁二醇     

Vibrational analysis of cinchona alkaloids in the solid state and aqueous solutions

Cinchona alkaloids are well‐known antimalarial compounds also used in asymmetric synthesis in organic chemistry. In this work, vibrational spectra of quinine, quinidine, cinchonine, and cinchonidine were acquired and interpreted on the basis of theoretical calculations. Normal Raman spectra of the alkaloids in solution exhibit similar patterns and cannot be used for differentiation between the derivatives (e.g. quinine and cinchonidine) and corresponding pseudoenantiomers (e.g. quinine and quinidine). Thus, Raman Optical Activity (ROA) method was applied to show distinct differences related to the configuration of chiral atoms. ROA allowed unequivocal identification of the pseudoenantiomers based on the sign of the characteristic bands from a single measurement. The experiments were supported by the theoretical approach including conformational study followed by wavenumber calculations and Potential Energy Distribution (PED) analysis. For quinine, vibrational spectroscopy was additionally used to show its structural changes in aqueous solutions at various pH and its distribution in a pharmaceutical product. Spatial distribution of quinine in a drug was observed by the FT‐Raman mapping technique. Copyright © 2015 John Wiley & Sons, Ltd.     

Controllable and switchable chiral near-fields in symmetric graphene metasurfaces

A strong chiral near-field plays significant roles in the detection, separation and sensing of chiral molecules. In this paper, a simple and symmetric metasurface is proposed to generate strong chiral near-fields with both circularly polarized light and linearly polarized light illuminations in the mid-infrared region. Owing to the near-field interaction between plasmonic resonant modes of two nanosheets excited by circularly polarized light, there is a strong single-handed chiral near-field in the gap between the two graphene nanosheets and the maximum enhancement of the optical chirality could reach two orders of magnitude. As expected, the intensity and the response wavelength of the chiral near-fields could be controlled by the Fermi level and geometrical parameters of the graphene nanosheets, as well as the permittivity of the substrate. Meanwhile, based on the interaction between the incident field and scattered field, the one-handed chiral near-field in the gap also could be generated by the linearly polarized light excitation. For the two cases, the handedness of the chiral near-field could be switched by the polarized direction of the incident light. These results have potential opportunities for applications in molecular detection and sensing.     

Hydrogen bonding in homochiral dimers of hydroxyesters studied by Raman optical activity spectroscopy

Spectroscopic analysis of homochiral dimerization is important for the understanding of the homochirality of life and enantioselective catalysis. In this paper, (S)‐methyl lactate and related molecules were studied to provide detailed structural information on hydrogen bonding in homochiral dimers of chiral α‐hydroxyesters through the experimental and theoretical study of Raman optical activity. Different homochiral dimers can be distinguished by comparing their simulated Raman optical activity spectra with the experimental results. Hydrogen bonding motions are decoded with the aid of vibrational motion analysis, which are apparently involved in vibrational motions below 800 cm^–1. A common feature related to the chain‐bending mode also indicates the absolute configuration of methyl lactate and related molecules. The differing behavior of electric dipole–electric quadrupole invariants (β(A)²) compared with the electric dipole–magnetic dipole invariant (β(G′)²), suggests that the intermolecular hydrogen bonding motion behaves differently from the intramolecular one in the asymmetric molecular electric and magnetic fields. These results may help understand hydrogen‐bonded self‐recognition and other dynamical features in chiral recognition. Copyright © 2011 John Wiley & Sons, Ltd.     

Bisignate resonance Raman optical activity: a pseudo breakdown of the single electronic state model of RROA?

Raman optical activity (ROA) spectra were calculated for a set of conformers of astaxanthin, which is a non‐rigid molecule exhibiting strong resonance enhancement in the visible range. Single electronic state theory of the Resonance ROA (RROA) predicts the spectrum to be monosigned. For astaxanthin, it appeared that some of the conformers exhibit different sign of the bands than the other conformers. As a result, the conformer population averaged spectrum of astaxanthin can exhibit both signs of the bands, or be monosigned depending on which conformers are dominating, that reflects a departure from the single electronic state approximation. Moreover, use of different basis sets and/or density functional theory (DFT) functionals results in different conformer populations, thus yielding again either monosigned ROA spectrum or with bands of both signs. Consequences of these two findings for the astaxanthin RROA spectrum are discussed. Copyright © 2014 John Wiley & Sons, Ltd.     

In situ analysis of chiral components of pichtae essential oil by means of ROA spectroscopy: experimental and theoretical Raman and ROA spectra of bornyl acetate

In this paper, a novel approach to analyze in situ (−)‐bornyl acetate (BA) in pichtae essential oil (Siberian fir needle oil, Abies sibirica oil) by means of Raman optical activity (ROA) is reported. As part of this approach, a conformational study in the gas phase of (+)‐ and (−)‐BA has been carried out, predicting the presence of three conformers for each enantiomer at 298.15 K. The structures of these conformers were optimized with density functional theory with the Becke 3LYP functional and 6–311 + + g** basis set. Subsequently, the Raman and ROA spectra were simulated in order to compare them with the experimentally measured spectra of the neat enantiomers of BA. Finally, the combination of Raman and ROA spectroscopy as well as DFT calculations was successfully applied not only for the detection of BA but also for the determination of the specific enantiomer of BA present in the investigated pichtae essential oil samples. Thus, the ROA technique described here has the potential to be used as a fast and easy commercial method to control the quality of essential oils. Copyright © 2011 John Wiley & Sons, Ltd.     

双折射光纤受激拉曼散射偏光特性的实验研究

对椭圆芯光纤受激拉曼散射偏光特性进行了系统的实验研究。实验巾观察到8级斯托克斯线和2级反斯托克斯线,对不同偏振态的抽运光激励下各级斯托克斯线的偏振特性、拉曼频移等参量进行了分析,并给出了经验公式。其结果与实验数据符合良好。实验表明,双折射光纤受激拉曼谱的各级斯托克斯线的偏振状态不但与拉曼介质有关还与抽运光的偏振态有关,入射抽运光偏振态对低阶次的斯托克斯线拉曼频移的影响较小,而对高级斯托克斯线拉曼频移影响较大。