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.     

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…     

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

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 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.     

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.     

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.     

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.     

Magnetic Circular Dichroism and Electronic Spectra of Tropothione

The magnetic circular dichroism (MCD) and electronic absorption spectra of tropothione have been measured. The circular-dichroism (CD) spectrum of the β-cyclodextrin complex with tropothione is also reported. The absorption bands of tropothione are assigned.     

Molecular Design Guidelines for Large Magnetic Circular Dichroism Intensities in Lanthanide Complexes

Magneto optical devices based on the Faraday effects of lanthanide ion have attracted much attention. Recently, large Faraday effects were found in nano‐sized multinuclear lanthanide complexes. In this study, the Faraday rotation intensities were estimated for lanthanide nitrates [Ln^III(NO₃)₃?n H₂O: Ln=Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm) and Eu^III complexes with β‐diketone ligands, using magnetic circular dichroism. Eu ions exhibit the largest Faraday rotation intensity for ⁷F₀→⁵D₁ transitions, and high‐symmetry fields around the Eu ions induce larger Faraday effects. The molecular design for the enhancement of Faraday effects in lanthanide complexes is discussed.     

Induced Circular Dichroism Spectra of β- and γ-Cyclodextrin Complexes with Indazolinone and Related Compounds

The magnetic circular dichroism (MCD) spectra of indazolinone, isatine, and 3-iminoisoindolinone and the induced circular dichroism (ICD) spectra of the inclusion complexes with β- or γ-cyclodextrins (CDs) have been measured. The spectra are interpreted by results of the ZINDO calculations. In the presence of cyclodextrin, the OH indazolinone tautomer is main structure that is consistent with that in the DMSO solution. The structures of the inclusion complexes are very different because of the scale of the cavity of cyclodextrin or position of quinone in molecules.     

Conformation Analysis of Ile-Ala-Val-Pro Peptide and Its Derivatives by Circular Dichroism

Ile-Ala-Val-Pro as a hypocholesterolemic peptide was isolated from soybean protein. We have synthesized four peptides, Ile-Ala-Val-Pro-Gly-Glu-Val-Ala, Leu-Ile-Ala-Val-Pro-Gly-Glu-Val-Ala, Ile-Ala-Val-Pro-Thr-Gly-Val-Ala, Leu-Ile-Ala-Val-Pro-Thr-Gly-Val-Ala, with a conserved Ile-Ala-Val-Pro amino acid sequence, for circular dichroism investigations. These four peptide sequences were also found in the amino acid sequence in soybean protein, which was defined from the genomic sequence. Additionally for a detailed analysis of conformation features of these peptides, the Ile-Ala-Val-Pro and Leu-Ile-Ala-Val-Pro were also synthesized. All peptides were prepared using standard fluorenylmethyloxycarbonyl methodology and the peptide yields ranged from 90 to 95% of the theoretical yields with purity after purification above 99%.     

Circular Dichroism Spectra of 5- and 7-Bromo-6-ketosteroids

Circulardichroismspectra of 5- and 7-bromo-6-ketosteroidsof the cholestane and stigmastane serieswere studied. A negative Cotton effect corresponding to the n-* -transition of the ketone was observed in the CD spectra of 5-bromo-6-ketosteroids at 300 nm. The analogous Cotton effect was positive in CD spectra of 7-bromo-6-ketosteroids.     

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.