The induced circular dichroism of helically shaped racemic bilatrienes

Summary The imperfect mirror image relationship of the circular dichroism of (M) and (P) helical biliverdins bound to a homochiral entity (ICD) has been studied using the kinetically stable racemates (M+P)2 and (M+P)3. The phenomena associated with and eventual implications for the interpretation of ICD spectra of naturally occurring kinetically labile biliverdins in terms of chiral recognition are discussed.Dedicated to ProfessorKarl Schlögl on the occasion of his 70^th birthday     

乙基橙与α,β,γ-环糊精包结物诱导圆二色性的研究

本文研究了乙基橙(EO)α、β、γ、环糊精(CD~x)包结物的诱导圆二色性(ICD)。用Kirkwood偶合振动模型探讨了包结物的包结形式都为轴向包结。然后利用Job连续变量作图法确定了其络合比, 即a-CDx/EO=1:1,βCDx(γ-CDx)/EO=2:1、1:1。     

Induction and inversion of chirality in poly‐L‐lysine and methyl orange complex

The mechanism of complexation of poly‐L‐lysine (PLL) with methyl orange (MO) and the appearance of induced circular dichroism (ICD) were investigated as a function of dye concentration and temperature and compared with that of the PLL‐ethyl orange complex. The formation of stoichiometric complexes with uniform size and intensities of ICDs depended on the length of alkyl groups (methyl and ethyl) as determined from quartz crystal microbalance measurement, absorption spectra, and CD spectra data. Furthermore, at constant PLL concentration, a dependence of the inversion of the ICD on the MO concentration (CMO) was observed in the PLL‐MO complexes, which to our knowledge has not been reported for a constant dye concentration system. The positive ICD in the PLL‐MO complexes observed at CMO lower than 2.0 × 10^?5 M showed reversible changes in response to heating and cooling, whereas the negative ICD in PLL‐MO complexes observed at higher CMO inverted to positive ICD values in response to a decrease in temperature. These results demonstrate the alkyl group substituent in the dye, dye concentration, and temperature play important roles in the formation of PLL‐azo dye complex and ICD appearance. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

火菇素的圆二色性与溶液二级结构

为了弄清火菇素蛋白的结构与功能的关系并揭示抗癌机理,使其更好地发挥临床作用,测定了火菇素的圆二色性,并用蛋白质二级结构解析程序分析了火菇素的溶液二级结构。火菇素远紫外圆二色性的研究表明,其水溶液在208nm处表现为宽大负峰,最大平均残基摩尔椭圆度[θ]~2~0~8=-6574deg·cm^2·dmol^-^1,在223nm处为肩,经二级结构解析程序计算分析,火菇素的二级结构和二硫键和芳香氨基酸对火菇素圆二色性的贡献分别为77.4%和22.6%,二级结构的组成为：α-螺旋19.7%,β-折叠和β-转角50.1%,无规卷曲和γ-转角30.2%。火菇素二级结构对pH,SDS和乙醇有一定的稳定性,在pH4.6～9.4范围内,火菇素的结构几乎不发生变化,但在碱性太强的环境中火菇素发生不可逆变性,火菇素对热变性很敏感。     

Determination of dipole moment in the ground and excited state by experimental and theoretical methods of N-nonyl acridine orange

The absorption and fluorescence spectra of N-nonyl acridine orange are determined at room temperature (298 K) in cyclohexane, benzene, carbon tetrachloride, chloroform, chlorobenzene and dichloromethane. The ground state of dipole moment was obtained by impedance measurements using Guggenheim-Debeye's method. The experimental excited state dipole moment of N-nonyl acridine orange was determined using Bakhshiev's and Kawski-Chamma-Viallet's formulae and solvent polarity parameter proposed by Reichardt. These experimental results were completed with theoretical results using quantum chemical methods. The experimental (muexp=10.76 D) and theoretical (mucal=9.9 D) dipole moments in the ground and excited state (muexp*=14.56 D) were compared.     

Determination of molecular stereochemistry using vibrational circular dichroism spectroscopy: absolute configuration and solution conformation of 5-formyl-cis,cis-1,3,5-trimethyl-3-hydroxymethylcyclohexane-1-carboxylic acid lactone

The determination of the absolute configuration of chiral molecules is an important aspect of molecular stereochemistry. Vibrational circular dichroism (VCD) is the extension of electronic CD into the infrared region where fundamental vibrational transitions occur. VCD has a number of advantages over all previous methods of absolute configuration assignment. The absolute configuration and predominant solution-state conformation in CDCl(3) of the chiral lactone, 5-formyl-cis,cis-1,3,5-trimethyl-3-hydroxymethylcyclohexane-1-carboxylic acid lactone, 1, obtained by the comparison of measured and calculated VCD spectra, are reported. It is found that (-)-1 corresponds to the absolute configuration (1S,3S,5R)-1.     

The inversion phenomenon of the helical twist sense in antiferroelectric liquid crystal phase from electronic and vibrational circular dichroism

The investigation of the helical pitch and the helical twist sense for several liquid crystal compounds in antiferroelectric phase have been performed. Electronic circular dichroic (ECD) and vibrational circular dichroic (VCD) spectroscopies have proved the existence of unwound helical structure in antiferroelctric phase. Obtained results may confirm the assumption connected via the inversion phenomena in liquid crystalline chiral phase with the change of the concentration of different conformers promoting opposite handedness. Two examples of such conformers, obtained by conformational analysis, have been proposed.     

Relationships between the stability,circular dichroism spectrum and type of substitution in inclusion complexes formed between phenophthalein and different cyclodextrins

The perturbation of the three-site interaction of phenolphthalein on complexation with different cyclodextrins (CDs) has been investigated as a function of the type and degree of substitution of the CD. The UV visible spectra are practically identical, while the circular dichroism spectra change dramatically both in intensities and signs. Parallel to this, the stability constants are influenced not only by the size of the cavity but also by the substitution of its rims, particularly by that of the primary hydroxyls. The two phenomena can be related to allow conclusions concerning the mode of inclusion.     

Phthalimide as a chromophoric tag in the circular dichroism determination of absolute configuration of α-aminoacid amides and dipeptides. A case of a dipeptide isostructurality

Both N-phthaloyl aminoacid amides and methyl amides as well as N-phthaloyl dipeptide methyl esters give Cotton effects of opposite signs at around 220 and 240 nm. The signs of these Cotton effects are directly correlated with the absolute configuration of the N-phthaloyl substituted stereogenic center: for -configuration the Cotton effect at 240 nm is positive and the Cotton effect at 220 is negative. The X-ray analysis shows that the diastereomeric , and , N-phthaloyl alanylvaline methyl esters form isostructural orthorombic crystals due to the conformational flexibility of the molecules and a similar space requirements of the methoxycarbonyl and isopropyl groups.     

Solvatochromic effects in the absorption and fluorescence spectra of indazole and its amino derivatives

Absorption and fluorescence spectra of Indazole (In) in five solvents and those of 5-amino, 6-amino, and 7-aminoindazoles (5-AI 6-AI, 7-Al) in thirteen solvents have been studied. Using the multiparametric approach of Taft et al., spectral characteristics of these molecules have been analysed on the basis of electrostatic effects, hydrogen bond donor ability and hydrogen bond accepting ability of the solvents. In the excited singlet state, all three effects are nearly equally prominent, whereas in the ground state 5-AI acts as a better hydrogen bond acceptor than the other amines. Stokes shifts and the difference between the radiative decay constants determined experimentally and using Strickler and Berg's equation indicate different geometries of the amines in the S₀ and S₁ states. A decrease in the non-radiative decay constants of 5-AI and 6-AI with an increase in the polarity of the solvents predict better planar geometry in the S₁ state in comparison to that in the S₀ state. Semi-empirical quantum mechanical calculations have been used to find the nature of transitions, total atomic charges at the basic centres and dipole moments of all the aromatic amines in the S₀ and S₁ states. Results so obtained are compared and discussed.     

Induced circular dichroism spectra of the cyclodextrin complexes with 2-Thioxo-1,3-benzodithiole and 2-Selenoxo-1,3-benzodithiole

The induced circular dichroism (ICD) spectra of -cyclodextrin (-CD) complexes with 2-thioxo-1,3-benzodithiole and 2-selenoxo-1,3-benzodithiole have been measured. From the signs of the ICD spectra of both complexes, the polarization directions for the excited states have been investigated.     

Molecular recognition studies on supramolecular systems (XXIV)——Conformations and complexation abilities of chemically modified cyclodextrins in aqueous solution

Circular dichroism spectral and fluorescence decay methods have been employed to determine the conformations of mono[6-(p-tolylseleno)-6-deoxy]-p-CD(1), mono(6-anilino-6-deoxy) β -CD (2) and mono[6-(L-tryptophan)-6-deoxy]-β-CD (3) in phosphate buffer solution (pH 7.2, 0.1 mol dm-3) at 298.15 K. The results indicate that compounds 2 and 3 formed self-inclusion complexes in aqueous buffer solution, while the substituent of compound 1 was not included into cyclodextrin cavity at all. Furthermore, the complex stability constant (logKs) and Gibbs free energy change (-ΔG° ) of these three cylcodextrin derivatives with several cycloalkanols have been determined by circular dichroism spectral titration in phosphate buffer solution at 298.15 K. It is found that the location of the substituent affects the stability of host-guest complex in aqueous solution.     

The modification of the triple helical structure of gelatin in aqueous solution 3. The influence of cationic surfactants

The interaction between cationic surfactants (hexadecyl and dodecyl trimethyl ammonium bromide) and gelatin was characterized by measuring the circular dichroism. The interaction between the cationic surfactants and gelatin is weak in comparison to that of anionic surfactants. When the concentration of cationic surfactants is sufficiently low, refolding of the gelatin-strands to the triple helical structure by rechilling the solution from 298 K to 283 K is complete. The triple helical content of the solution is affected more strongly by the cationic surfactants in acidic solution than at pHs 7 or 10. The interaction depends on the apolar group of the surfactant and is found to be stronger for DTAB than for CTAB at 298 K. Coagulation of the hydrophobic gelatin-cationic surfactant complexes does not comprise that pan of gelatin which is able to refold the triple helical structure. Therefore, the gelatin-strands of lower molecular weights are thought to react favorably with the surfactant ions.     

Tight β-turns in peptides. DFT-based study of infrared absorption and vibrational circular dichroism for various conformers including solvent effects

Vibrational circular dichroism (VCD) has had a large impact on configurational studies of organic molecules largely due to the theoretical advances made by Philip Stephens and co-workers. For peptides, the structural issue is not one of the configuration, but of conformation, and the flexibility of the oligomeric structure raises major computational challenges. Turns are a vital aspect of peptide and protein conformation that allow such structures to fold into a compact unit. However, unlike helices and sheets, they are not extended repeating structures, but each residue has a different local conformation. Also, when turns are part of larger peptides their termini are connected to completely different structural elements. We have done extended comparative density functional theory (DFT) computations to characterize the expected spectral contributions of selected turn structures to the amide IR and VCD spectra of peptides. The isolated vacuum results for tri-amide turns (Ac-X-Y-NH₂) of a few different sequences are compared with calculations involving correction for solvation effects. In particular, we looked at the sequence variation in spectra and structure between Ala-Ala, Aib-Gly and D-Pro-Gly for the turn-specific X–Y residues. The nature of some turn-associated, amide originating, spectral transitions are developed and tested.     

Circular dichroism and fluorescence spectroscopy studies of the effect of cyclodextrins on the thermal stability of chicken egg white lysozyme in aqueous solution

Circular dichroism spectroscopy revealed that the thermal stability of chicken egg white lysozyme in an aqueous buffer solution is significantly lowered by the addition of 6-O-α-d-glucosyl-β-cyclodextrin (G1-β-CD), whereas it is raised by the addition of methyl α-d-glucopyranoside. The α- and γ-cyclodextrin also lowered the thermal stability, although the effects were less prominent than that of G1-β-CD. Fluorescence spectroscopy suggested that cyclodextrins include the side chains of tryptophan residues within their internal cavities to lower the thermal stability of lysozyme. The fluorescence intensity of a sample, re-cooled to 25 °C after thermal denaturation at 75 °C in the presence of G1-β-CD, was stronger than that observed for native lysozyme. The fact that the fluorescence intensity of the re-cooling sample was stronger than that of the native one indicates that G1-β-CD persists in binding to the side chains of tryptophan residues of the re-cooled lysozyme.     

Thermal diffusion with stepwise association equilibria in aqueous solutions of the ionic dyes methylene blue and methyl orange

Soret coefficients are measured conductimetrically for dilute aqueous solutions of the stepwise-associating ionic dyes methylene blue (chloride salt) and methyl orange (sodium salt) at 25°C. In contrast to the behavior of other dilute aqueous electrolytes, the Soret coefficients of the dyes increase with concentration. An approximate treatment of thermal diffusion of stepwise-associating solutes is developed to interpret the results. The analysis is used to estimate the intrinsic enthalpies of transport of the monomeric and the associated forms of the dyes. The enthalpy of association makes a large reactive contribution to the enthalpy of transport of methyl orange.     

New insights into the metal ion–peptide hydroxamate interactions: Metal complexes of primary hydroxamic acid derivatives of common dipeptides in aqueous solution

Complex formation of primary dipeptide hydroxamic acids, L-Ala-L-AlaNHOH and L-Ala-L-SerNHOH, as well as the corresponding Z-protected ones, Z-L-Ala-L-AlaNHOH and Z-L-Ala-L-SerNHOH (Z = benzyloxycarbonyl), with iron(III), aluminium(III), nickel(II), copper(II) and zinc(II) was studied in aqueous solution by pH-potentiometric and spectroscopic (UV–Vis, EPR, CD, ¹H NMR) methods. The exclusive formation of [O,O] chelated hydroxamate complexes was found with iron(III) and aluminium(III) with all the ligands. Formation of linkage isomers with the involvement of either [O,O] hydroxamate or [NH₂,CO] chelates was detected both in the zinc(II)-L-Ala-L-AlaNHOH and -L-Ala-L-SerNHOH systems. Upon increasing the pH, none of these chelating sets are capable of preventing the hydrolysis of the metal ion. The formation of stable complexes was found in the nickel(II) and copper(II) systems above pH ∼ 6 with a [NH₂, N_amide, N_hydrox.] binding mode after deprotonation and coordination of the peptide amide and the hydroxamate group. With an excess of copper(II), the formation of trinuclear [Cu₃H_xL₂]^x+4 type (x = −4 to −6) complexes as the major species was also detected. Blocking the terminal amino group in the Z-protected ligands results in a dramatic decrease of the nickel(II) and zinc(II) binding strengths, and insoluble complexes with copper(II). No indication was found for the role of the hydroxyl group of the serine moiety in metal ion binding.     

Complex formation of hexakis(2,3,6-tri-O-methyl)-α-cyclodextrin with substituted benzenes in aqueous solution

The complex formation of hexakis (2,3,6-tri-O-methyl)--cyclodextrin with substituted benzenes has been investigated by circular dichroism (CD) spectroscopy. The sign and shape of the CD spectra markedly differ from the spectra of corresponding -cyclodextrin complexes because of the distorted conformation of the host molecule and/or the difference in the geometry of the host-guest interaction. Enthalpy and entropy changes of the complex formation are determined by using the CD band intensities measured at various temperatures and host concentrations. Negative values of H and S indicate that the hydrophobic interaction is not the major driving force for the complex formation. The guest molecule is suggested to be tightly bound within the host cavity through the induced-fit conformational change of the host molecule.