Large Circular Dichroism Ellipticities for N-Templated Helical Polypeptides Are Inconsistent with Currently Accepted Helicity Algorithms

An unprecedented , high degree of helicity as judged by CD spectroscopy is observed in N-templated model peptides of the type AcHel-(Ala₄Lys)_nAla₂-NH₂ (AcHel-Ala peptide pictured; AcHel is an N-terminal helix-inducing template for polypeptides). These results raise concern over the current methods for determining 100 % helicity.     

Fluorescence Detected Circular Dichroism for Highly Sensitive Observation of Exciton Coupled CD

Summary. The fluorescence detected circular dichroism (FDCD) appearance of some exciton coupling compounds is reviewed by a newly developed ellipsoidal mirror system (FDCD465). The FDCD465 achieves both the complete elimination of the polarization artifact and a dramatic enhancement of detection sensitivity expanding the applicability of exciton coupling fluorophores. A variety of chromophores that possess large extinction coefficients and high fluorescence quantum yields are available for exciton coupled FDCD, regardless of the degree of fluorescence polarization. Different types of FDCD devices can regulate FDCD polarization artifacts and the photoselection effect, enabling effective surveys of FDCD potential.     

Stereodynamic Coordination Complexes. Dependence of Exciton Coupled Circular Dichroism Spectra on Molecular Conformation and Shape

Summary. Chiral amino alcohols, amino ethers, and amino thioethers were converted to tetradentate ligands by alkylation with a variety of chromophore-bearing alkyl heterocycles. Copper(II) complexes of the ligands display conformational diastereomerism in solution. The resultant propeller-shaped compounds were characterized by a variety of techniques, including exciton coupled circular dichroism (ECCD). Three X-ray crystal structures of complexes are described that partially support structural assignments, but also suggest that solid state structures are not always indicative of solution species. Thus, ECCD may in some cases provide data that is strongly complimentary to X-ray crystallography. These results are presented in a larger context of coordination complexes that display dynamic behavior that can be studied by chiroptical spectroscopy.     

脱羰青蒿素和青蒿素的圆二色谱

含有过氧和内酯基团的青蒿素1是一个结构独特的倍半萜。它的圆二色谱也较特殊,在258nm 和228nm 处分别显示一极大值和一极小值。对此梁晓天提出258nm 处的极大值是一因转折而引起的假象。最近我们以光化学氧化反应高产率地制备了脱羰青蒿素2,     

Molecular Exciton Approach to Anisotropic Absorption and Circular Dichroism II. The Partial Optic Axis and its Application in Molecular Exciton Theory

Summary. The first part of the present communication develops the general theory of the partial optic axis, which is an excitation specific structural feature first proposed as an aid to the choice of chromophoric reference points in a molecular exciton approach to optical rotatory power, and discusses its applicability to symmetry analyses in the light of the distinction between the three categories for anisotropic optical rotatory power, i.e. chiral, achiral optically active, and achiral optically inactive molecular structures. The second part of the communication discusses the special role played by the concept of the partial optic axis in the evaluation of the anisotropic chromophoric intensity contributions in a molecular exciton approach, in particular in relation to the use of the chromophoric symmetry for the systematic selection and characterization of the intensity contributions.Permanent address: Holmebjerg 5, DK-2950 Vedbæk, Denmark     

苯甲酰基修饰环糊精衍生物的圆二色性与分子构象

环糊精(简称ＣＤ)分子的独特结构特点和性能使其成为超分子化学中重要的主体模型之一[1,2].ＣＤ及其衍生物在分子识别,模拟酶等领域的研究和应用受到广泛重视[3].光活性组分的超分子体系可通过光诱导能量传递进行选择性光化学反应[4].我们与日本Ｉｎｏｕｅ等以单(6ｏ苯甲酰基)βＣＤ为光增感剂,研究了顺式环辛烯的光异构化反应[5],但对于反应过程中ＣＤ衍生物构象有何变化,反应发生在ＣＤ空腔内还是空腔外等反应机理尚不清楚.为了研究环辛烯光异构化反应机理并提供新的光增感剂,我们合成并报导了一系列含发色团的ＣＤ衍生物[6,7].通过研究客…     

自旋标记鬼桕类化合物圆二色谱的研究

给已知抗癌药物分子中引入稳定氮氧自由基,合成较母体药物高效低毒的抗癌药物已引起国内外学者的重视.我们在具有抗癌活性的鬼桕毒分子中引入稳定氮氧自由基,药理试验表明其活性保持或增加,而毒性却显著地降低.     

Observation of Photoelectron Circular Dichroism Using a Nanosecond Laser

Photoelectron circular dichroism (PECD) is a fascinating phenomenon both from a fundamental science aspect but also due to its emerging role as a highly sensitive analytic tool for chiral recognition in the gas phase. PECD has been studied with single-photon as well as multi-photon ionization. The latter has been investigated in the short pulse limit with femtosecond laser pulses, where ionization can be thought of as an instantaneous process. In this contribution, we demonstrate that multi-photon PECD still can be observed when using an ultra-violet nanosecond pulse to ionize chiral showcase fenchone molecules. Compared to femtosecond ionization, the magnitude of PECD is similar, but the lifetime of intermediate molecular states imprints itself in the photoelectron spectra. Being able to use an industrial nanosecond laser to investigate PECD furthermore reduces the technical requirements to apply PECD in analytical chemistry.     

Vibrational Excitons in CH-Stretching Fundamental and Overtone Vibrational Circular Dichroism Spectra

Summary. A set of vibrational circular dichroism (VCD) spectra in the CH-stretching fundamental region for about twenty compounds belonging to the class of essential oils was empirically analyzed by the use of a sort of vibrational exciton mechanism, involving three centers. Through a general formula applicable to many coupled dipole oscillators, the rotational strengths of the previously identified vibrational excitons are evaluated. The results are then critically reviewed by the use of recent ab initio methodology, as applied to selected molecules of the original set. Further insight is gained by model calculations adding up the contribution of the coupling between electric dipole moments associated with normal mode behavior and that of the polarizability from polarizable groups. The former part is responsible for the excitonic behavior of the VCD spectra. For the same selected molecules we have also investigated whether some excitonic behavior is taking place in the second overtone region, and have concluded that this is not the case.     

罂粟花粉中阿拉伯半乳聚糖的结构及圆二色性的研究

从罂粟花粉中纯化得到由等量阿啦伯和半乳糖基组成的阿拉伯半乳聚糖ＰＳ（１），经ＮＭＲ和甲基化分析，证明ＰＳ（１）分子中含有β（１→３），β（１→６），β（１→３，６）半乳吡喃糖苷键和α（１→５），β（１→３）阿拉伯呋喃糖苷键。在２００ｎｍ处，ＰＳ（１）具有正圆二色性。     

圆二色光谱分析蛋白质构象的方法及研究进展

介绍了远紫外圆二色光谱数据计算蛋白质二级结构,辨认蛋白质三级结构类型的原理、拟合方法、实验技术。对近紫外圆二色作为光谱探针,研究蛋白质中芳香氨基酸残基、二硫键微环境的变化作了简单介绍。     

Redox-Switched Exciton-Coupled Circular Dichroism: A Novel Strategy for Binary Molecular Switching

Intense chiroptical properties and efficient reversibility of the chemical redox cycle support the adaptability of the Cu^I/Cu^II complex system 1 / 2 for the development of a binary molecular device. Reduction proceeds with ascorbic acid, and oxidation with ammonium persulfate.     

一种简易长光程可见紫外、圆二色薄层光谱电化学池

设计了一种简易实用的长光程薄层光谱电化学池,该池以10mm石英比色槽为光学窗,2块5×10×50mm的聚四氟乙烯块为电极支承体,玻碳片为工作电极,且薄层厚度可调。以K_3Fe(CN)_6和L-酪氨酸为工作体系进行光谱和电化学实验表明,该池具有良好的薄层电化学特征。对于水溶液,在大于220nm波长内给出良好的光谱和较快的光谱电化学响应,可方便地用于圆二色谱-电化学研究。     

Near-Ultraviolet Circular Dichroism and Two-Dimensional Spectroscopy of Polypeptides

A fully quantitative theory of the relationship between protein conformation and optical spectroscopy would facilitate deeper insights into biophysical and simulation studies of protein dynamics and folding. In contrast to intense bands in the far-ultraviolet, near-UV bands are much weaker and have been challenging to compute theoretically. We report some advances in the accuracy of calculations in the near-UV, which were realised through the consideration of the vibrational structure of the electronic transitions of aromatic side chains.     

Interplay of Exciton Coupling and Large‐Amplitude Motions in the Vibrational Circular Dichroism Spectrum of Dehydroquinidine

A detailed analysis of the computed structure, energies, vibrational absorption (VA) and circular dichroism (VCD) spectra of 30 low‐energy conformers of dehydroquinidine reveals the existence of families of pseudo‐conformers, the structures of which differ mostly in the orientation of a single O?H bond. The pseudo‐conformers in a family are separated by very small energy barriers (i.e., 1.0 kcal mol^?1 or smaller) and have very different VCD spectra. First, we demonstrate the unreliable character of the Boltzmann factors predicted with DFT. Then, we show that the large differences observed between the VCD spectra of the pseudo‐conformers in a family are caused by large‐amplitude motions involving the O?H bond, which trigger the appearance/disappearance of strong VCD exciton‐coupling bands in the fingerprint region. This interplay between exciton coupling and large‐amplitude‐motion phenomena demonstrates that when dealing with flexible molecules with polar bonds, vibrational averaging of VCD spectra should not be neglected. In this regard, the dehydroquinidine molecule considered here is expected to be a typical example and not the exception to the rule.     

Circular Dichroism and Optical Rotatory Dispersion — Principles and Application to the Investigation of the Stereochemistry of Natural Products

The principles of circular dichroism and of optical rotatory dispersion are described. Examples are given to illustrate the use of these methods for the determination of the configurations and conformations of natural and synthetic products. Some limitations of the octant rule are mentioned.     

Circular Dichroism and Absolute Conformation: Application of Qualitative MO Theory to Chiroptical Phenomena

Although chiroptical methods have been used in chemistry for some 150 years, and the basic theory was developed by Rosenfeld 50 years ago, their success in determining absolute conformation rests mainly on empirical rules. Some of these can be proved theoretically; however, in the case of more complex molecules this is not yet always possible. In what follows, an attempt is made to bridge the wide gap between theory and empiricism: “qualitative MO-theory”, which has already been successfully used to explain reaction mechanisms, the shape of molecules, or photoelectron spectra, can also be applied to circular dichroism (CD). In general we shall have to be content with a reasonable interpretation of the experimentally determined sign correlation, but even this will be of great value since it can yield general relations between the geometry of molecules and the sign of CD. Such relations become particularly important when only a few examples of a given chromophore are known. If first and second sphere of a molecule are achiral then sector rules can be put forward, and in other cases helicity or chirality rules.