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.     

Fluorescence Detected Exciton Coupled Circular Dichroism: Development of New Fluorescent Reporter Groups for Structural Studies

Summary. In collaboration with Jasco Corporation we have recently developed an FDCD (fluorescence detected circular dichroic) instrument J-465, which eliminates the photoselection artifacts and efficiently collects the emitted light from the sample solution based on the ideal ellipsoidal mirror principle. Using the J-465 we have investigated a variety of fluorophores with/without polarization for exciton-coupled FDCD stereochemical analysis. The following three cases extend the FDCD methodology to new challenging areas beyond the limits of conventional CD: (1) substrates containing C=C double bonds, (2) molecules with sterically hindered hydroxyls, and (3) substrates bearing remote stereogenic centers. The pico- to nano-level FDCD analysis described in this paper leads to promising opportunities for the stereochemical analysis of a wide range of natural products with minuscule amounts of sample available.     

Selective Monitoring of Stern-Volmer Quenching of Chiral Molecules Via Fluorescence Detected Circular Dichroism

Abstract

The use of fluorescence detected circular dichroism (FDCD) for monitoring the quenching of chiral molecules is described. This technique combines the specificity for chiral centers of circular dichroism (CD), the sensitivity and selectivity of the fluorescence measurement with the additional chemical information provided by fluorescence quenching. Spectra are acquired using a multidimensional FDCD spectrometer which obtains a FDCD matrix of spectral data as a function of multiple excitation wavelengths (CD information) and multiple fluorescence emission wavelengths. This combination of FDCD and fluorescence quenching produces a very selective monitor for chiral fluorophores.     

The Exciton Model and the Circular Dichroism of Polypeptides

Summary. Exciton coupling of the 190nm ^* transition is an important factor in the CD spectrum of peptides and proteins. The CD spectrum of the -helix is dominated by the exciton effect. The spectrum is sensitive to the direction of the ^* transition dipole moment, especially for short helices. Exciton theory is much less successful in accounting for the CD spectrum of the poly(proline)II (PPII) conformation, an important conformer in collagen and in unordered peptides. Mixing of the ^* transition with high-energy transitions in the peptide backbone and in side chains must be considered to explain the strong negative CD band near 200nm of PPII.     

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

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.     

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.     

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.     

高灵敏检测葡萄糖的新型荧光纳米传感器

构建了一种用于高灵敏检测葡萄糖的新型荧光纳米传感器.在辣根过氧化物酶(HRP)的催化下,H2O2氧化3,3′,5,5′-四甲基联苯胺(TMB),生成具有强吸光性质的TMB多聚体,导致1-氧-1H-非那烯-2,3-二腈(1-Oxo-1H-phenalene-2,3-dicarbonitrile, OPD)分子的荧光发生淬灭,基于此实现H2O2的定量检测,线性范围分别为0.05~0.80 μmol/L和1~10 μmol/L,检出限(3σ)为0.02 μmol/L.由于葡萄糖氧化酶(Gox)可催化葡萄糖分解产生H2O2,基于此可以实现葡萄糖分子的定量检测,线性范围分别为0.1~3.0 μmol/L和4.0~30 μmol/L, 检出限(3σ)为0.02 μmol/L.将本方法用于实际血清样品中葡萄糖的定量检测,结果与临床检测结果相符.     

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

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.     

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     

Induced Circular Dichroism and Magnetic Circular Dichroism Spectra of Maleimide and Related Molecules

The induced circular dichroism (ICD) spectra of the inclusion complexes of maleimide, phthalimide, and naphthalene‐2,3‐dicarboximide with β‐ or γ‐cyclodextrin (CDx) have been measured. The structure of the CDx inclusion complexes are interpreted by the signs and shapes of ICD spectra compared with the results of PPP calculations. In maleimide and naphthalene‐2,3‐dicarboximide, the ICD spectra of the β‐CDx inclusion complex are very similar to those of the γ‐CDx inclusion complex in spite of the differences in dimensions between the cavity of β‐CDx and that of γ‐CDx. In phthalimide, the ICD spectra of the β‐CDx inclusion complex are very different from those of the γ‐CDx inclusion complex. The split‐type ICD bands at 220–235 nm show that the dimer of phthalimide is formed in the presence of β‐CDx.     

Magnetic Circular Dichroism and Induced Circular Dichroism Spectra of N‐Bromophthalimide

The magnetic circular dichroism (MCD), induced circular dichroism (ICD) spectra, and the absorption spectra of N‐bromophthalimide have been measured. The ICD spectrum of the β‐cyclodextrin complex with N‐bromophthalimide is also reported. The absorption bands of N‐bromophthalimide are assigned.     

A New One-Pot Fluorescence Derivatization Strategy for Highly Sensitive MicroRNA Analysis

MicroRNAs (miRNAs) modulate the expression of over 30 % of mammalian genes during development and apoptosis, and abnormal expression of miRNAs may lead to a range of human pathologies. Therefore, analysis of miRNAs is valuable for disease diagnostics. In this work, a novel one-pot fluorescence derivatization strategy was developed for miRNA analysis. The mechanism of the derivatization reaction was explored by using instrumental methods, including liquid chromatography, fluorescence spectroscopy, and mass spectrometry. Highly fluorescent N⁶-ethenoadenine (ϵ-adenine) was formed and detached from the miRNA sequence through the reaction of adenine in nucleic acids with 2-chloroacetaldehyde (CAA) at 100 °C. This is the first experimental evidence that the cooperation of formed ϵ-adenine and water-mediated hydrogen-bond interaction between the proton at the 2′- and the oxyanion at 3′-positions stabilized the oxocarbenium significantly, which makes the depurination and derivatization of miRNA highly effective. Based on this derivatization strategy, a facile and sensitive high-performance liquid chromatography method was developed for quantitative assay of miRNAs. In combination with magnetic solid-phase extraction (MSPE), the HPLC method was shown to be useful for the determination of microRNAs at sub-picomolar level in serum samples.     

新型高灵敏电荷耦合器件阵列荧光检测装置

本报道用一般光源溴钨灯及电荷耦合器件阵列检测器组装的荧光检测装置，对装置的各种操作参数进行了优化及评价，此装置测定荧光素的检出限达到５×１０＾－１０ｍｏｌ／Ｌ，线性范围为１×１０＾－９－２×１０＾－６ｍｏｌ／Ｌ。     

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.     

Carbohydrate Binding and Unfolding of Spatholobus parviflorus Lectin: Fluorescence and Circular Dichroism Spectroscopic Study

Biophysical and carbohydrate binding studies have been carried out on a lectin of Spatholobus parviflorus (SPL) seeds isolated by affinity chromatography on cross-linked guar gum. It agglutinated erythrocytes of all ABO blood groups. SDS-PAGE, both in reducing and non-reducing conditions, showed two bands with MW of 29 and 31 kDa. MALDI TOF analysis revealed two peaks at 60 and 120 kDa, indicating that SPL is a hetero-dimeric tetramer. Temperature and pH stability studies revealed that SPL is a thermostable protein and its lectin activity is unaffected in the temperature range of 0–70 °C. Its activity was maximal in the pH range of 7–8. Unfolding studies with different denaturants like urea and guanidine hydrochloride indicated its globular nature and the presence of tryptophan in the highly hydrophobic area, which could be correlated to the results of fluorescence spectroscopic studies. The effect of carbohydrate binding on the lectin, shown by circular dichroism spectra, indicated the changes in its secondary and tertiary structures. SPL exerted anti-fungal activity against Aspergillus sp.