Characteristic Cotton Effects Research Articles

Characteristic chromanone acids from Calophyllum membranaceum: Determination of C-3 configuration and anti-inflammatory activity

One undescribed homologous furanochromanone (1) featuring a 6/6/5/3 tetracyclic skeleton and four highly oxidized pyranochromanones (2−5), along with a set of four pyranochromanone stereoisomers [(±)-6a and (±)-6b], were isolated from the leaves of Calophyllum membranaceum Gardn. Et Champ. Their structures were elucidated by using spectroscopic data, Snatzke's method, quantum-chemical calculations, and X-ray crystallographic analysis. The correlation of characteristic Cotton effects and specific chemical shifts with C-3 configuration provided a convenient approach to assign the C-3 configuration of 2,3-dimethylchromanones. The stereochemical assignments of 3-OH substituted pyranochromanones by quantum-based NMR methods following single/double MTPA derivatization were consistent with the ECD/NMR prediction, which verified the feasibility and reliability of the proposed empirical rule. The underlying mechanism was further clarified by conformational and molecular orbital analyses. Moreover, biological evaluation and binding assays demonstrated that compound 3 (KD = 0.45 μM) tightly binds to the TLR4–MD2 target, thereby inhibiting the TLR4/MyD88-dependent and -independent signal pathways. This study provides the first evidence that Calophyllum chromanones are a novel structural type of TLR4 inhibitors, exerting their anti-inflammatory effects by disrupting the binding between TLR4 and MD2.

Cyclohelminthol CPs: Scope and Limitations of Density Functional Theory-Based Structural Elucidation of Natural Products.

The effectiveness and limitations of density functional theory (DFT) calculations in the structural determination of complexed and conformationally flexible natural products were demonstrated using the cyclohelminthols CP-1 (1) CP-2 (2), CP-3 (3), and CP-4 (4) newly isolated from Helminthosporium velutinum yone96. Prior to DFT calculations, the structures were tentatively assigned using conventional spectroscopic analyses. The structures were verified with reference to DFT-derived 13C and 1H NMR chemical shifts, 3JHH and nJCH values, and electronic circular dichroism (ECD) spectra. The 13C chemical shift calculations were very effective for verifying the ring-structure moieties but less effective for verifying the geometry of the side chain in which the juncture asymmetric carbon (C-16) was apart from the ring-structure moiety. However, 1H chemical shift calculations compensated for the imperfection of the latter. ECD spectral calculations were used to determine the absolute configurations. Calculations for virtual simple model molecules enabled us to evaluate the reliability of the ECD spectral calculation and derive the chiral torsion responsible for the characteristic Cotton effects.

Biomimetic Chirality Sensing with Pyridoxal-5'-phosphate.

Pyridoxal-5'-phosphate (PLP) is introduced to a biomimetic indicator displacement assay for simultaneous determination of the absolute configuration, enantiomeric composition and concentration of unprotected amino acids, amino alcohols and amines. The chiroptical assay is based on fast imine metathesis with a PLP aryl imine probe to capture the target compound for circular dichroism and fluorescence sensing analysis. The substrate binding yields characteristic Cotton effects that provide information about the target compound ee and the synchronous release of the indicator results in a nonenantioselective off-on fluorescence response that is independent of the enantiomeric sample composition and readily correlated to the total analyte concentration.

Alleno-acetyllenic scaffolding for the construction of axially chiral C60 dimers

The preparation of enantiopure dumbbell-type dimeric fullerenes consisting of two C60 units connected by axially chiral alleno-acetylenic spacers is reported for the first time. As a key step, the attachment of the terminal alkyne moiety of the spacers to C60 was efficiently accomplished by employing an in situ C60-ethynylation methodology. In addition to spectral analyses, single-crystal X-ray crystallographic studies allowed for the unambiguous structural assignment of two C60–alleno-acetylene conjugates. Circular dichroism measurements showed that the axial chirality of the allene moieties linked to the fullerene sphere is able to perturb the intrinsic symmetry of the fullerene π-system. Large characteristic Cotton effects were observed for two bisfullerenes in the 200–350 nm spectral region. UV/Vis absorption spectroscopic studies showed improved molar absorptivity of these dimeric fullerenes, but no strong evidence for a significant through-space electronic communication between the two C60 spheres; electrochemical investigations further confirmed this conclusion.

Chiral Glutathione-protected Ag Triangular Nanoplates Synthesized by Protectant-substitution Reaction: Chiroptical and Surface Structure Analysis

Abstract Chiral Ag:SG (glutathione) triangular nanoplates were synthesized by substitution reaction of achiral Ag:poly(vinylpyrrolidone) (PVP) nanoplates with glutathione free GSH molecules. Analysis of the chiroptical properties of the synthesized nanoplates using a circular dichroism spectrometer revealed characteristic Cotton effects with wavelengths that corresponded well to those of plasmon resonances for Ag nanoplates. X-ray photoelectron spectrometry analysis of their chemical compositions revealed that complete substitution of PVP molecules with GSH molecules proceeds on the nanoplates.

(R)‐(+)‐[VCD(–)984]‐4‐Ethyl‐4‐methyloctane: A Cryptochiral Hydrocarbon with a Quaternary Chiral Center. (2) Vibrational CD Spectra of Both Enantiomers and Absolute Configurational Assignment

AbstractTo characterize precisely the chiroptical properties of 4‐ethyl‐4‐methyloctane (1), one of the so‐called cryptochiral compounds, the enantiomer (S)‐(–)‐[VCD(+)984]‐1 was synthesized in an enantiopure form. The observed specific rotation values at 365 nm of both enantiomers agreed well, but were opposite in sign, confirming their enantiomeric relationship, although their absolute values were very small, reflecting their cryptochirality. The vibrational circular dichroism (VCD) spectra of both enantiomers were measured neat and showed characteristic Cotton effects around 1150–900 cm–1; the VCD spectral curves were mirror images of each other. Based on the observed positive VCD band at 984 cm–1 for (S)‐(–)‐1, its absolute configuration was fully designated as (S)‐(–)‐[VCD(+)984]‐1. The VCD spectrum was quantum chemically calculated by the DFT MO method at the B3PW91/6‐31G(d,p) level, and the simulated VCD curve for (S)‐1 agreed very well with the observed VCD spectrum of (S)‐(–)‐1, confirming the absolute configuration.

Host-guest complexation-triggered chiroptical change of poly(phenylacetylene)s bearing binaphthocrown ether moieties on the main chain

Diacetylene monomers with respective lengths of the oxyethylene chains were cyclopolymerized with a rhodium catalyst to produce novel poly(phenylacetylene)s bearing a different cavity size of the chiral crown ether in the repeating units (2a–c). In the circular dichroism spectra of the resulting polymers, characteristic Cotton effects were observed in the range from 350 to 500 nm corresponding to the absorption of the conjugated polymer backbone, indicating that the polymers possessed a helical structure with an excess single screw sense induced by the covalently bonded binaphthyl units. The host–guest complexation of 2a–c with achiral guests produced a chiroptical change based on the fluctuation in the main chain conformation. The behavior of the complexation-induced chiroptical change was essentially dictated by the cavity size of the binaphthocrown ether units. Additionally, a chirality-responsive helicity change was observed in the case of the complexation of 2a–c with chiral guests, which also depended on the crown ether size. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1197–1206, 2010

Copper(ii) and zinc(ii) complexes with C-glycoside-pendant dipicolylamine (DPA)-amino acid conjugates

A series of dipicolylamine (DPA)-functionalized amino acid ligands with a pendant C-glycosyl amide has been synthesized and complexed with Cu(II) and Zn(II). Amino acid derivatives equipped with a DPA unit were conjugated with C-glycosylamine prepared from acetobromoglucose in 4 steps. These ligands bind to metal ions as a N(3) or N(3)O chelate utilizing three DPA nitrogens and carbamoyl oxygen atoms. In the crystal structures of copper(II) and zinc(II) complexes of AcL1 (2-(N',N'-bis(2'''-pyridylmethyl)amino)-N-(2'-(2'',3'',4'',6''-tetra-O-acetyl-beta-D-glucopyranosyl)ethyl)acetamide, carbamoyl oxygen binds to the metal centre, however, NMR analysis reveals that AcL1 acts as a tridentate ligand in solution. The CD spectrum of the copper(II) complex with glycine-based AcL1 at the d-d transition region indicates that the chirality at the sugar moiety does not induce an asymmetric metal coordination environment. In contrast, the chirality at the amino acid moiety for alanine and phenylalanine derivatives, AcL2 and AcL3, respectively, induces characteristic Cotton effect in the 500-800 nm region depending on the nature of the alpha-carbon substituent of the amino acid moiety.

Synthesis and property of helical poly(phenylacetylene)s bearing chiral ruthenium complexes and real space imaging of meso‐ and nanoscopic structures by atomic force microscopy

AbstractNovel sets of helical poly(phenylacetylene)s bearing a chiral ruthenium (Ru) complex with opposite chirality (Δ and Λ forms) as a bulky pendant (poly‐1 and poly‐2) were synthesized through the polymerization of the corresponding optically pure phenylacetylenes with a rhodium catalyst, and their structures in solution and morphology on solid substrates were investigated with NMR, ultraviolet–visible, and circular dichroism (CD) spectroscopies and with atomic force microscopy (AFM), respectively. The obtained cis–transoidal polymers (poly‐1 and poly‐2) showed characteristic Cotton effects in the region of metal‐to‐ligand charge transfer of the chiral Ru pendants. Poly‐1 and poly‐2 were thought to have a predominantly one‐handed helical conformation induced by the chiral pendants. However, the apparent Cotton effects derived from the helically twisted π‐conjugated polymer backbone could not be observed, probably because of the strong chiral chromophoric pendants. However, in the AFM images, the helical polymers adsorbed on mica could be easily discerned as isolated strands, and the visualization and discrimination of the right‐ and left‐handed helical structures of the chiral polymers were achieved by high‐resolution AFM imaging. On the basis of the AFM observations together with the CD measurements and computational calculation results, possible structures of poly‐1 and poly‐2 were examined. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4621–4640, 2004

Application of fluorescamine XII. Chiroptical properties of fluorescamine condensation compounds with some less common α-amino acids

Fluorescamine (FLURAM ®) reacts efficiently with less common chiral α-amino acids to form pyrrolinone-type chromophores. The characteristic Cotton effects at 324-300 and at 290-263 nm can be related to the absolute configuration of the parent α-amino acid without isolation of the products. These CD bands are negative and positive, respectively, for the derivatives of the L-α-amino acids.

Chiroptical properties of fluorescamine condensation compounds with amino acid esters [formula omitted]. Pyrrolinone chirality rule

Chiral α-amino acid esters react readily with fluorescamine (Fluram ®) to form pyrrolinone-type chromophores. The characteristic Cotton effects given by these chromophoric derivatives provide a means for the determination of the absolute configuration of the parent amino acid ester. The strong CD band in the 330-320 nm region of the L-amino acid ester derivative is always negative, while it is positive for the D-amino acid ester derivatives. A pyrrolinone chirality rule is proposed to explain the sign of this Cotton effect.

Homologous copper(1)-(thiolate) 2-chromophores in yeast copper thionein

Improved isolation techniques were devised and successfully applied for the convenient isolation of Cu-thionein from Saccharomyces cerevisiae. Homogeneous copper-thioneins containing eight, two and one Cu per 10 000 g of thionein were obtained. No inorganic sulphide was determined and a constant stoichiometry of copper : cysteine-sulphur of 1 : 2 was measured regardless whatever Cu-thionein was examined. This 1 : 2 stoichiometry was supported by electron absorption spectrometry, circular dichroism and X-ray photoelectron spectrometry. A linear correlation of both ε and θ values with the copper content was seen. The characteristic Cotton effects at 245, 283, 302, 328 and 359 nm disappeared irreversibly when Cu-thionein was oxidized with H 2O 2. Titration with p-chloromercuribenzoate affected these Cotton extrema in a similar way and the above observed stoichiometry of 1 : 2 was confirmed. The binding energy values of the copper 2p 3/2 core electrons at 932.3 eV and the absence of a satellite structure suggested the assignment of Cu(1) in the Cu-thioneins. The sulphur 2p-values at 162.7 eV were attributed to thiolate sulphur. Treatment with H 2O 2 gave rise to the appearance of Cu(II) 2p 3/2 satellites at 940.0 and 942.4 eV as well as a shifted main signal to 933.9 eV. At the end of the oxidation all sulphur was oxidized to RSO 3- (S 2p = 167.9 eV). The chemical and physico-chemical data allow the conclusion that homologous Cu(I)-(thiolate) 2-chromophores are present in these microbial Cu-thioneins.

Chiroptical Properties of Fluorescamine Derivatives of Chiral Primary Aminesin situ

Abstract Primary amines react efficiently with fluorescamine (FLURAM®) to form pyrrolinone-type chromophores with long wavelength absorption maxima at 380–410 nm. When the primary amine is chiral, these chromophoric derivatives exhibit characteristic Cotton effects. The chiroptical properties of the reaction mixtures can be related to the absolute configuration of the amines without isolation of the products.

The effect of substrate analogs on the circular dichroic spectra of thymidylate synthetase from Lactobacillus casei.

Circular dichroism studies from 290 to 400 nm with the thymidylate synthetase from Lactobacillus casei revealed characteristic Cotton effects in the presence of various folate analogs plus 5-fluoro-2'-deoxyuridylate. Omission of either substrate analog prevented the appearance of the Cotton effects. When 5-fluoro-2'-deoxyuridylate and (+/-)-5,10-methylenetetrahydrofolate are mixed with the synthetase, a ternary complex results which yields distinctive minor negative ellipicity bands at 285 and 332 nm and a major negative ellipticity bands at 285 and 332 nm and a major positive band at 305 nm. Similar results were obtained with the ternary complex containing (+)-5,10-methylenetetrahydrofolate, but the enzymically inactive (-) diastereoisomer induced only the positive band at 305 nm. More intense Cotton effects were elicited by (+/-)-5,11-methylenetetrahydrohomofolate with a major positive ellipticity band at 308 nm and a minor negative band at 335 nm. A ternary complex was also formed with dihydrofolate, which provided a major circular dichroic band at 305 nm and a broad minor negative band in the region of 335 nm. Deoxyuridylate and thymidylate also formed ternary complexes with dihydrofolate, but their ellipicity bands were much less intense. Other folate analogs that formed ternary complexes with 5-fluoro-2'-deoxyuridylate to provide characteristic circular dichroic spectra were tetrahydrofolate, tetrahydrohomofolate, 10-methyltetrahydrofolate, and a 2-amino-4-hydroxyquinazoline derivative. By measuring the increment in ellipticity at 305 nm on addition of specific ligands to enzyme solutions, it was determined that the L. casei thymidylate synthetase contains two binding sites for 5-fluoro-2'-deoxyuridylate and for each of the diastereoisomers of 5,10-methylenetetrahydrofolate. An improved procedure is presented for the large-scale purification and crystallization of L. casei thymidylate synthetase.

A simple method for determining the absolute configuration of α-amino acids

Chiral α-amino acids react with 2-methoxy - 2,4 - diphenyl - 3 (2 H) - furanone 1 to afford N-substituted 3,5 - diphenyl - 5 - hydroxy - 2 - pyrrolin - 4 - ones 2. The characteristic cotton effects given by these chromophoric derivatives provide a means for the determination of the absolute configuration of the parent amino acids. The longest wavelength(first) extremum in the chiroptical spectra (ORD and CD) of the l-amino acid derivatives is always positive, while it is negative for the d-amino acid derivatives.

Optical rotatory dispersion. Part XLVIII. Some 3,3-diarylprop-1-enes and related quinones: the absolute configuration of latifolin

The optical rotatory dispersion (o.r.d.) curves of latifolin and its derivatives, which are substituted 3,3-diarylprop-1-enes, have been compared in the short wavelength region with those of related quinol diacetates of known absolute configuration; the characteristic Cotton effects at 235 mµ show that the absolute configuration of latifolin is R. The o.r.d. curves of the related quinones of the dalbergione series confirm this assignment.

Hydrogen Ion Equilibria of Human Carbonic Anhydrases B and C

Human erythrocytes contain three distinct forms of carbonic anhydrase (carbonate hydro-lyase, EC 4.2.1.1) which have been denoted as A, B, and C (l-3). In a previous study (4) one of us has investigated the hydrogen ion equilibria of carbonic anhydrase B at ionic strength 0.15. Here we report similar studies on enzyme C at two diierent ionic strengths, and further studies of enzyme B at low ionic strength (0.05). The two enzymes differ markedly in catalytic activity, the specific activity of enzyme C being much higher than that of enzyme B (5). In some respects they also differ markedly in amino acid composition. They are indeed closely alike in their content of histidine, arginine, and aromatic amino acids; each contains only one cysteine residue, and there are no disullide bonds in either (1, 6, 7).’ However, enzyme C contains distinctly more lysine than B, and also its content of ionizable carboxyl groups is somewhat higher. Correspondingly, the isoelectric point of enzyme C (pH 7.3) is higher than that of B, which is 5.7 (1). The minor component, enzyme A, which will not concern us further here, is almost indistinguishable from enzyme B in amino acid composition, although its isoelectric point appears to be slightly lower (1, 6). In optical rotatory dispersion studies both enzymes B and C show characteristic Cotton effects between 260 and 300 mp, which disappear on denaturation. The dispersion patterns for the two native enzymes show a certain broad general similarity but differ in detail (8). The previous titration study of enzyme B (1) showed that most of the histidyl and tyrosyl residues are masked in the native protein and become reactive only after denaturation in acid or alkali (4). Studies of difference spectra between the native and the acid-denatured enzymes B and C suggest that most or all of the tryptophanyl residues in the native protein are in the int.erior of the native molecules, and come into contact with the

Conformational studies on the interaction of chymotrypsin with substrates and inhibitors

Optical rotatory dispersion studies of chymotrypsin gave linear Drude plots for native chymotrypsin in the presence and absence of substrates. Chymotrypsin preparations in which the serine at the active site was sulfonylated by sulfonic esters of various sizes all gave non-linear Drude plots and characteristic Cotton effect troughs at 233 mμ. Chymotrypsin in which the serine at the active site was converted to dehydroalanine gave linear Drude plots and a marked Cotton effect trough at 233 mμ. The Moffitt and Blout plots paralleled the Drude results, i.e. were linear for native chymotrypsin and non-linear for the serine-modifled enzymes. The individual constants, the shape of the plots and the Cotton effects all indicate that conformational changes are induced by substrates and inhibitors of chymotrypsin.

ANOMALOUS ROTATORY DISPERSION OF ENZYME COMPLEXES, II. THE ASYMMETRIC BINDING OF COENZYMES AND INHIBITORS TO LIVER ALCOHOL DEHYDROGENASE

The inhibition of the yeast and liver alcohol dehydrogenases' by chelating agents is accompanied by progressive changes in optical rotation.2' 3 Such alterations of the optical rotatory properties of yeast alcohol dehydrogenase are due to the binding of chelating' agents to the asymmetric zinc site of the enyme resulting in a characteristic Cotton effect. The rotatory power of this Cotton effect, when superimposed upon the background dispersion curve of the native protein, accounts for the rotational changes which appear during the course of enzymatic inhibition, and these changes can be differentiated from those due to alterations in protein conformation.4 Such observations suggested that changes in optical rotatory dispersion might be a frequent concomitant of the binding of small molecules to proteins and might, thereby, be used to identify the interactions with enzymes of optically active, chromophoric substrates, coenzymes, inhibitors, and other site specific reagents.4 To this end we have investigated the alcohol dehydrogenase of horse liver. This zinc containing enzyme5' 6 interacts stoichiometrically with two moles of coenzyme, DPN(H), to form the enzymatically holoenzyme7 and is inhibited by a number of chelating agents.' Zinc is not removed during chelation nor does the enzyme dissociate into subunits by electrophoresis or ultracentrifugation. 1,10phenanthroline, OP, reversibly inhibits the enzyme by chelation of the zinc atom forming a mixed complex: LADH-Zn OP.9 The coenzyme binds in a specific though as yet unknown manner at or near the zinc site of the enzyme. A variety of approaches indicate that zinc is a component of the active center of liver alcohol dehydrogenase.'I The present report demonstrates that the binding of reduced diphosphopyridine nucleotide to liver alcohol dehydrogenase is accompanied by the appearance of a pronounced Cotton effect at 327 m,. The magnitude of this Cotton effect is a direct function of the molar ratio of the interactants, thereby permitting a quantitative assessment of coenzyme binding. Moreover, characteristic changes in the optical rotation of the enzyme-coenzyme complex identify its interaction with substrate and inhibitors. Similar effects are observed when the analogues of DPNH bind to liver alcohol dehydrogenase. The considerations pertinent to the choice of metalloenzymes for the exploration of the molecular basis of enzyme action have been discussed3 and preliminary accounts of the present studies have been reported.2' 13 Methods and Materials.-Crystalline alcohol dehydrogenase of horse liver was obtained from C. F. Boehringer and Soehne, Mannheim, W. Germany. Prior to use, the enzyme was dialyzed for 5 days against 0.1M phosphate buffer, pH 7.5 at 4? to remove low molecular weight impurities which absorb radiation at 280