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Dichroism
Kinds of Dichroism Terms modified by Dichroism Selected AbstractsMagnetic Materials: X-Ray Magnetic Circular Dichroism Picks out Single-Molecule Magnets Suitable for Nanodevices (Adv. Mater.ADVANCED MATERIALS, Issue 2 20092/2009) The surface sensitivity of X-ray magnetic circular dichroism in extreme conditions has been exploited to investigate the first layers of bulk single-molecule magnets (SMMs), as reported by Roberta Sessoli and co-workers on p. 167. Striking differences have emerged between two classes of SMM having different structural constraints, thus highlighting the importance of molecular design in the realization of molecular spintronic devices. [source] X-Ray Magnetic Circular Dichroism Picks out Single-Molecule Magnets Suitable for NanodevicesADVANCED MATERIALS, Issue 2 2009Matteo Mannini The surface sensitivity of X-ray Magnetic Circular Dichroism in extreme conditions is exploited to investigate the first layers of bulk single-molecule magnets (SMM). Striking differences emerge between two classes of SMM with different structural constraints, thus highlighting the importance of molecular design in the realization of molecular spintronic devices [source] Circular Dichroism of the Photoreceptor Pigment OxyblepharisminPHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 6 2005Osvaldo Pieroni ABSTRACT Circular dichroism (CD) was used to study the structure of oxyblepharismin (OxyBP), the photoreceptor chromophore for the photophobic response of the blue form of Blepharisma japonicum. Both the chromophore associated to its native protein and the free chromophore in ethanol solution were investigated. CD spectra in the far-UV range indicate that OxyBP induces a slight increase in the ,-helix content of the protein matrix. CD spectra in the near-UV and visible region of the spectrum show that OxyBP adopts a chiral conformation with a preferential geometry not only when associated to its protein matrix, but also when isolated and dissolved in ethanol. This experimental result is related to the existence of a high-energy interconversion barrier between two enantiomeric structures of the molecule and discussed on the basis of an asymmetric biosynthesis of its precursor, blepharismin. [source] Circular Dichroism of Designed Peptide Helices and ,-Hairpins: Analysis of Trp- and Tyr-Rich PeptidesCHEMBIOCHEM, Issue 12 2005Radhakrishnan Mahalakshmi VCD versus ECD spectroscopy. Peptides rich in aromatic residues yield anomalous far-UV electronic circular dichroism (ECD) spectra that preclude secondary structure assignment. The utility of vibrational circular dichroism (VCD) in conformation analysis is demonstrated by using a set of well-defined peptide helices and hairpins containing proximal aromatic residues. [source] Determination of the Absolute Configurations of Natural Products via Density Functional Theory Calculations of Vibrational Circular Dichroism, Electronic Circular Dichroism, and Optical Rotation: The Iridoids Plumericin and Isoplumericin.CHEMINFORM, Issue 37 2007P. J. Stephens Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source] Magnetic Circular Dichroism and Absorption Spectra of Phosphinidene in Noble-Gas MatricesCHEMINFORM, Issue 17 2005Jeremy J. Harrison Abstract For Abstract see ChemInform Abstract in Full Text. [source] One-Pot Synthesis of Helical Aromatics: Stereoselectivity, Stability Against Racemization, and Assignment of Absolute Configuration Assisted by Experimental and Theoretical Circular Dichroism.CHEMINFORM, Issue 12 2005Masashi Watanabe Abstract For Abstract see ChemInform Abstract in Full Text. [source] Electronic Structure, Spectra, and Magnetic Circular Dichroism of Cyclohexa-, Cyclohepta-, and CyclooctapyrroleCHEMISTRY - A EUROPEAN JOURNAL, Issue 14 2005Alexander Gorski Abstract Three recently obtained expanded porphyrins represent nice examples of compounds for which the electronic and spectral properties can be predicted from symmetry considerations alone. Perimeter-model-based theoretical analysis of the electronic structure of doubly protonated cyclo[6], cyclo[7], and cyclo[8]pyrrole leads to the anticipation of qualitatively the same electronic absorption and magnetic circular dichroism patterns for all three compounds. These predictions are fully confirmed by experiments, as well as DFT and INDO/S calculations. Due to a characteristic pattern of frontier molecular orbitals, a degenerate HOMO and a strongly split LUMO pair, the three cyclopyrroles show comparable absorption intensity in the Q and Soret regions. Magnetic circular dichroism spectra reveal both A and B Faraday terms, of which the signs and magnitudes are in remarkably good agreement with theoretical expectations. The values of the magnetic moments of the two lowest degenerate excited states have also been obtained. [source] Linear Dichroism in the XANES of Partially Oriented Samples: Theory and Application to the Photosynthetic Manganese Complex,CHEMPHYSCHEM, Issue 6 2010Peter Liebisch Dr. Abstract For molecular systems which are partially ordered with respect to one macroscopic axis, for example, the sample,surface normal, X-ray absorption spectroscopy (XAS) with linearly polarized synchrotron radiation can provide information on structure and orientation of the X-ray absorbing site (polarized or linear-dichroism XAS). Examples for such partially oriented systems are protein-carrying membrane particles deposited in the form of multilayers on surfaces, layered minerals, surface films or imperfect 2D crystals, planar electrodes or catalytic surfaces. For electric dipole transitions, equations are derived describing how partial unidirectional orientation determines the linear dichroism (LD). The approach presented facilitates description of 1) LD in multiple-scattering contributions of the extended X-ray absorption fine-structure (EXAFS) and 2) of LD in the X-ray absorption near-edge structure (LD-XANES). Structural and orientation information becomes accessible by combination with ab initio XANES calculations. The LD-XANES approach is applied to the water-oxidizing Mn complex of photosystem II. The results suggest that the (,-O)-(,-O) vectors of the Mn-(,-O)2 -Mn units are at an average angle to the membrane normal of 30,45°. The best-fit structure in connection with crystallographic data suggests a specific oxidation-state assignment: Mn(1)IIIMn(2)IIIMn(3)IVMn(4)IV. However, currently this assignment remains uncertain. In future studies, the LD-XANES analysis should play an important role in construction of unequivocal atomic-resolution model of the photosynthetic Mn complex. [source] Conformational Effects on Circular Dichroism in the Photoelectron Angular DistributionCHEMPHYSCHEM, Issue 4 2006Devis Di Tommaso Dr. Abstract The B-spline density-functional method has been applied to the conformers of the (1R,,2R)-1,2-dibromo-1,2-dichloro-1,2-difluoroethane molecule. The cross section, asymmetry, and dichroic parameters relative to core and valence orbitals, which do not change their nature along the conformational curve, have been systematically studied. While the cross section and the asymmetry parameter are weakly affected, the dichroic parameter appears to be rather sensitive to the particular conformer of the molecule, suggesting that this dynamical property could be a useful tool for conformational analysis. The computational method has also been applied to methyl rotation in methyloxirane. Unexpected and dramatic sensitivity of the dichroic-parameter profile to the methyl rotation, both in the core and valence states, has been found. Boltzmann averaging over the conformers reproduces quite closely the profiles previously obtained for the minimum-energy conformation, which is in good agreement with the experimental results. [source] 11th International Conference on Circular DichroismCHIRALITY, Issue 9 2008Professor Ben L. Feringa Guest Editor No abstract is available for this article. [source] Sensitive Electrochemical Detection of Native and Aggregated ,-Synuclein Protein Involved in Parkinson's DiseaseELECTROANALYSIS, Issue 13-14 2004Michal Masa Abstract The aggregation of ,-synuclein, a 14,kDa protein, is involved in several human neurodegenerative disorders, including Parkinson's disease. We studied native and in vitro aggregated ,-synuclein by circular dichroism (CD), atomic force microscopy (AFM) and electrochemical methods. We used constant current chronopotentiometric stripping analysis (CPSA) to measure hydrogen evolution catalyzed by ,-synuclein (peak H) at hanging mercury drop electrodes (HMDE) and square-wave stripping voltammetry (SWSV) to monitor tyrosine oxidation at carbon paste electrodes (CPE). To decrease the volume of the analyte, most of the electrochemical measurements were performed by adsorptive transfer (medium exchange) from 3,6,,L drops of ,-synuclein samples. With both CPE and HMDE we observed changes in electrochemical responses of ,-synuclein corresponding to protein fibrillization detectable by CD, fluorescence and AFM. Aggregation-induced changes in peak H at HMDE were relatively large in strongly aggregated samples, suggesting that this electrochemical signal may find use in the analysis of early stages of ,-synuclein aggregation. This assumption was documented by marked changes in the peak H potential and height in samples withdrawn at the end of the lag and the beginning of the elongation phase. Native ,-synuclein can be detected down to subnanomolar concentrations by CPSA. [source] Separation and aquatic toxicity of enantiomers of the pyrethroid insecticide lambda-cyhalothrin,ENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 1 2008Chao Xu Abstract Chiral pollutants are receiving growing environmental concern due to differential biological activities of their enantio-mers. In the present study, enantiomeric separation of the pyrethroid insecticide lambda-cyhalothrin (LCT) was investigated by high-performance liquid chromatography (HPLC) using the columns of Chiralpak AD (amylase tris[3,5-dimethyl-phenylcarbamate]), Chiralpak AS (amylase tris[(S)-1-phenyl carbamate]), Chiralcel OD (cellulose tris[3,5-dimethylphenyl carbamate]), and Chiralcel OJ (cellulose tris[4-methyl benzoate]) with different chiral stationary phases. The differential toxicities of the enantiomers in aquatic systems were evaluated using the acute zebrafish (Danio rerio) toxicity test and the zebrafish embryo test. The enantiomers of LCT were separated completely on all the columns tested and detected by circular dichroism at 236 nm. Better separations were achieved at lower temperatures (e.g., 20°C) and lower levels of polar modifiers (,5%) in mobile phase. Ethanol was found to be a good modifier of the mobile phase for all the columns, although isopropanol acted better for the Chiralcel OD column. The (,)-enantiomer was >162 times more toxic than its antipode to zebrafish in the acute test. The embryo test indicated that the exposure to LCT enantioselectively induced crooked body, yolk sac edema, and pericardial edema and that the (,)-enantiomer was 7.2 times stronger than the (+)-enantiomer in 96-h mortality. The malformations were induced by the racemate and its (,)-enantiomer at lower concentrations tested (e.g., 50 ,g L,1), whereas the (+)-enantiomer induced malformations at relatively higher concentrations (,100 ,g L,1). These results suggest that the toxicological effects of chiral pesticides must be evaluated using their individual enantiomers. [source] Molecular Interaction between a Gadolinium,Polyoxometalate and Human Serum AlbuminEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 34 2009Li Zheng Abstract Polyoxometalates (POMs) show promising antibacterial, antiviral (particularly anti-HIV), antitumor, and anticancer activities, but the mechanism of these potential therapeutic effects remains to be elucidated at the molecular level. The interaction between the Gd-containing tungstosilicate [Gd(,2 -SiW11O39)2]13, and human serum albumin (HSA) was studied by several techniques. Fluorescence spectroscopy showed an energy transfer between the single tryptophan residue of HSA and the POM. Circular dichroism led to the conclusion that the POM significantly altered the secondary structure of HSA. Isothermal titration calorimetry revealed an enthalpy-driven binding reaction between HSA and the POM, resulting in the formation of a 1:1 complex.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] Cobalt(III) Complexes of a Tripodal Ligand Containing Three Imidazole Groups: Properties and Structures of Racemic and Optically Active SpeciesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 8 2008Hirofumi Nakamura Abstract The complex [Co(H3L)](ClO4)3·H2O (1), where H3L {tris[2-(4-imidazoylmethylideneamino)ethyl]amine} is a tripodal ligand obtained by condensation of tris(2-aminoethyl)amine and 4-formylimidazole in a 1:3 molar ratio, was synthesized and optically resolved by fractional crystallization of the diastereomeric salt with [Sb2{(R,R)-tart}2]2, [(R,R)-tart = (2R,3R)-tartrate(4,) ion]. From the less soluble part, ,-[Co(H2L)][Sb2{(R,R)-tart}2]·4H2O (2) was isolated. Starting from 2, two optically active complexes, ,-[Co(H3L)](ClO4)3·1.5H2O (,- 1) and ,-[Co(L)] (,- 3), were obtained. The crystal structures of these complexes are compared with those of the racemic structures. ,- 1 shows an unusually strong circular dichroism (, = 488 nm, ,, = ,7.74 M,1,cm,1) in the first d,d absorption band region. The effects of deprotonation,reprotonation of the uncoordinated imidazole NH groups of ,-[Co(H3L)]3+ on the UV/Vis and CD spectra and on the cyclic voltammograms were studied in methanol. Although the deprotonation,reprotonation reactions are reversible, the redox couple for the completely deprotonated species [CoIII/II(L)]0/, is not observed. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] A Crystalline Phase Transition and Optical Properties in a CoIICuII Oxamato-Bridged Ferrimagnetic ChainEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 24 2005Cynthia L. M. Pereira Abstract The compound [CoCu(opba)(DMSO)3] (1) [opba = ortho -phenylenebis(oxamato)] has been synthesized and characterized. Its crystal structure has been analyzed by X-ray diffraction techniques at 100 and 298 K. A structural phase-transition has been detected at around 150 K. An orthorhombic crystalline system is found at both temperatures, with very similar unit-cell dimensions. At room temperature 1 crystallizes in the Pnam space group (, -1 phase), with a = 7.6712(2), b = 14.8003(3), c = 21.0028(5) Ĺ, and Z = 4, whereas at low temperature it crystallizes in the Pna21 space group (, -1 phase), with a = 7.3530(2), b = 14.5928(4), c = 21.0510(7) Ĺ, and Z = 4. Both crystalline phases consist of linearly ordered bimetallic chains with the [Cu(opba)]2, units tied by CoII ions to form a one-dimensional system. The DMSO molecules in , -1, which are coordinated to either CuII or CoII, are disordered. At low temperature, a small reorganization of the CuII and CoII environments is observed. The origin of this phase transition, which is completely reversible, is the modification of the crystalline packing with the temperature. Linear birefringence measurements were done on single crystals in the 100,300 K temperature range. Around 150 K, the linear birefringence curve shows an inflexion that is interpreted as being related to the conversion of ,-1 into , -1. Both dc and ac magnetic measurements were performed on the polycrystalline sample. The results reveal a one-dimensional ferrimagnetic behavior. Single crystal optical characterization at room temperature shows that 1 presents a very strong dichroism superposed on the linear birefringence. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Biomimetic Self-Assembly of Tetrapeptides into Fibrillar Networks and OrganogelsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 27 2008Sajid Iqbal Abstract The self-assembly features of a family of tetrapeptides with a silk-inspired structure are presented. An exhaustive study of the influence of the terminal alkyl chain length in this process is undertaken. Scanning electron microscopy (SEM), wide-angle X-ray diffraction (WAXD), FTIR spectroscopy, and circular dichroism were used for structural analysis. These compounds, as in the natural model, self-assemble into antiparallel ,-sheet structures that further organize to form fibrillar aggregates. Furthermore, some of them arecapable of forming a crowded network that entraps thesolvent leading to physical gels with different microscopic morphologies. A model for the assembly process is proposed.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Synthesis, Solution Structure and Biological Activity of Val-Val-Pro-Gln,a Bioactive Elastin PeptideEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 8 2005Caterina Spezzacatena Abstract Val-Val-Pro-Gln (valyl-valyl-prolyl-glutamine) is a small but highly conserved sequence present in all elastins. We describe its synthesis by mixed anhydride solution chemistry as an alternative to solid-phase peptide synthesis (SPPS). The molecular structure of the tetrapeptide in solution was investigated by classical spectroscopy, such as circular dichroism (CD), nuclear magnetic resonance (NMR) and Fourier Transform Infrared Spectroscopy (FTIR). The biological activity of Val-Val-Pro-Gln was evaluated by a bromodeoxyuridine (BrdU) incorporation assay with normal human dermal fibroblasts. This small peptide may play a critical role in control of matrix metabolism through its release from the elastin polypeptide chain during periods of tissue breakdown and remodelling. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Enantioselective Recognition of Aliphatic Amino Acids by ,-Cyclodextrin Derivatives Bearing Aromatic Organoselenium Moieties on the Primary or Secondary SideEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 8 2003Yu Liu Abstract Spectrophotometric titrations have been performed in order to determine the stability constants of inclusion complexation of some aliphatic amino acids with four structurally related organoselenium-modified ,-cyclodextrins: mono(6-phenylseleno-6-deoxy)-,-cyclodextrin (1a), mono[6-(p -methoxyphenylseleno)-6-deoxy]-,-cyclodextrin (1b), mono(2-phenylseleno-2-deoxy)-,-cyclodextrin (2a), and mono[2-(p -methoxyphenylseleno)-2-deoxy]-,-cyclodextrin (2b). Conformation analysis by circular dichroism and 2D NMR spectroscopic studies revealed that the aryl-substituted ,-cyclodextrins gave self-inclusion intramolecular complexes in aqueous solution, while the extent of penetration depended both on the positions and on the structures of substituents. Quantitative investigation on the binding ability of the hosts with amino acids showed that they were able to recognize the size and the shape of guests, affording supramolecular complexes with quite small stability constants ranging from 24 to 355 M,1. The molecular recognition abilities are discussed from the viewpoints of induced-fitting mechanisms, geometric complementary, and cooperative binding processes. Furthermore, these ,-cyclodextrin derivatives displayed considerable enantioselectivity towards L/D -amino acid isomers, giving the highest L -enantioselectivity (up to 8.4) for inclusion complexation between leucine and 2a. The binding modes of L/D -leucine with 1b were elucidated from NOESY studies and the chiral recognition phenomena were interpreted accordingly. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] Application of Synchrotron Radiation Techniques for Model Validation of Advanced Structural Materials,ADVANCED ENGINEERING MATERIALS, Issue 6 2009Annick Froideval Abstract Synchrotron radiation techniques represent powerful tools to characterize materials down to the nanometer level. This paper presents a survey of the state-of-the-art synchrotron-based techniques which are particularly well-suited for investigating materials properties. Complementary X-ray absorption techniques such as extended X-ray absorption fine structure (EXAFS), X-ray magnetic circular dichroism (XMCD), photoemission electron microscopy (PEEM) are used to address the individual local atomic structure and magnetic moments in Fe,Cr model systems. The formation of atomic clusters/precipitates in such systems is also investigated by means of scanning transmission X-ray microscopy (STXM). Such advanced analytical techniques can not only offer valuable structural and magnetic information on such systems, they can also serve for validating computational calculations performed at different time and length scales which can help improve materials lifetime predictions. [source] The optimization of protein secondary structure determination with infrared and circular dichroism spectraFEBS JOURNAL, Issue 14 2004Keith A. Oberg We have used the circular dichroism and infrared spectra of a specially designed 50 protein database [Oberg, K.A., Ruysschaert, J.M. & Goormaghtigh, E. (2003) Protein Sci. 12, 2015,2031] in order to optimize the accuracy of spectroscopic protein secondary structure determination using multivariate statistical analysis methods. The results demonstrate that when the proteins are carefully selected for the diversity in their structure, no smaller subset of the database contains the necessary information to describe the entire set. One conclusion of the paper is therefore that large protein databases, observing stringent selection criteria, are necessary for the prediction of unknown proteins. A second important conclusion is that only the comparison of analyses run on circular dichroism and infrared spectra independently is able to identify failed solutions in the absence of known structure. Interestingly, it was also found in the course of this study that the amide II band has high information content and could be used alone for secondary structure prediction in place of amide I. [source] The conformational stability of the Streptomyces coelicolor histidine-phosphocarrier proteinFEBS JOURNAL, Issue 11 2004Characterization of cold denaturation, protein interactions Thermodynamic parameters describing the conformational stability of the histidine-containing phosphocarrier protein from Streptomyces coelicolor, scHPr, have been determined by steady-state fluorescence measurements of isothermal urea-denaturations, differential scanning calorimetry at different guanidinium hydrochloride concentrations and, independently, by far-UV circular dichroism measurements of isothermal urea-denaturations, and thermal denaturations at fixed urea concentrations. The equilibrium unfolding transitions are described adequately by the two-state model and they validate the linear free-energy extrapolation model, over the large temperature range explored, and the urea concentrations used. At moderate urea concentrations (from 2 to 3 m), scHPr undergoes both high- and low-temperature unfolding. The free-energy stability curves have been obtained for the whole temperature range and values of the thermodynamic parameters governing the heat- and cold-denaturation processes have been obtained. Cold-denaturation of the protein is the result of the combination of an unusually high heat capacity change (1.4 ± 0.3 kcal·mol,1·K,1, at 0 m urea, being the average of the fluorescence, circular dichroism and differential scanning calorimetry measurements) and a fairly low enthalpy change upon unfolding at the midpoint temperature of heat-denaturation (59 ± 4 kcal·mol,1, the average of the fluorescence, circular dichroism and differential scanning calorimetry measurements). The changes in enthalpy (m,Hi), entropy (m,Si) and heat capacity (m,Cpi), which occur upon preferential urea binding to the unfolded state vs. the folded state of the protein, have also been determined. The m,Hi and the m,Si are negative at low temperatures, but as the temperature is increased, m,Hi makes a less favourable contribution than m,Si to the change in free energy upon urea binding. The m,Cpi is larger than those observed for other proteins; however, its contribution to the global heat capacity change upon unfolding is small. [source] The histidine-phosphocarrier protein of Streptomyces coelicolor folds by a partially folded species at low pHFEBS JOURNAL, Issue 10 2003Gregorio Fernández-Ballester The folding of a 93-residue protein, the histidine-phosphocarrier protein of Streptomyces coelicolor, HPr, has been studied using several biophysical techniques, namely fluorescence, 8-anilinonaphthalene-1-sulfate binding, circular dichroism, Fourier transform infrared spectroscopy, gel filtration chromatography and differential scanning calorimetry. The chemical-denaturation behaviour of HPr, followed by fluorescence, CD and gel filtration, at pH 7.5 and 25 °C, is described as a two-state process, which does not involve the accumulation of thermodynamically stable intermediates. Its conformational stability under those conditions is ,G = 4.0 ± 0.2 kcal·mol,1 (1 kcal = 4.18 kJ), which makes the HPr from S. coelicolor the most unstable member of the HPr family described so far. The stability of the protein does not change significantly from pH 7,9, as concluded from the differential scanning calorimetry and thermal CD experiments. Conformational studies at low pH (pH 2.5,4) suggest that, in the absence of cosmotropic agents, HPr does not unfold completely; rather, it accumulates partially folded species. The transition from those species to other states with native-like secondary and tertiary structure, occurs with a pKa = 3.3 ± 0.3, as measured by the averaged measurements obtained by CD and fluorescence. However, this transition does not agree either with: (a) that measured by burial of hydrophobic patches (8-anilinonaphthalene-1-sulfate binding experiments); or (b) that measured by acquisition of native-like compactness (gel-filtration studies). It seems that acquisition of native-like features occurs in a wide pH range and it cannot be ascribed to a unique side-chain titration. These series of intermediates have not been reported previously in any member of the HPr family. [source] The refolding of type II shikimate kinase from Erwinia chrysanthemi after denaturation in ureaFEBS JOURNAL, Issue 8 2002Eleonora Cerasoli Shikimate kinase was chosen as a convenient representative example of the subclass of ,/, proteins with which to examine the mechanism of protein folding. In this paper we report on the refolding of the enzyme after denaturation in urea. As shown by the changes in secondary and tertiary structure monitored by far UV circular dichroism (CD) and fluorescence, respectively, the enzyme was fully unfolded in 4 m urea. From an analysis of the unfolding curve in terms of the two-state model, the stability of the folded state could be estimated as 17 kJ·mol,1. Approximately 95% of the enzyme activity could be recovered on dilution of the urea from 4 to 0.36 m. The results of spectroscopic studies indicated that refolding occurred in at least four kinetic phases, the slowest of which (k = 0.009 s,1) corresponded with the regain of shikimate binding and of enzyme activity. The two most rapid phases were associated with a substantial increase in the binding of 8-anilino-1-naphthalenesulfonic acid with only modest changes in the far UV CD, indicating that a collapsed intermediate with only partial native secondary structure was formed rapidly. The relevance of the results to the folding of other ,/, domain proteins is discussed. [source] Design, synthesis and properties of synthetic chlorophyll proteinsFEBS JOURNAL, Issue 11 2001Harald K. Rau A chemoselective method is described for coupling chlorophyll derivatives with an aldehyde group to synthetic peptides or proteins modified with an aminoxyacetyl group at the ,-amino group of a lysine residue. Three template-assembled antiparallel four-helix bundles were synthesized for the ligation of one or two chlorophylls. This was achieved by coupling unprotected peptides to cysteine residues of a cyclic decapeptide by thioether formation. The amphiphilic helices were designed to form a hydrophobic pocket for the chlorophyll derivatives. Chlorophyll derivatives Zn-methylpheophorbide b and Zn-methyl-pyropheophorbide d were used. The aldehyde group of these chlorophyll derivatives was ligated to the modified lysine group to form an oxime bond. The peptide,chlorophyll conjugates were characterized by electrospray mass spectrometry, analytical HPLC, and UV/visible spectroscopy. Two four-helix bundle chlorophyll conjugates were further characterized by size-exclusion chromatography, circular dichroism, and resonance Raman spectroscopy. [source] Physico-chemical requirements for cellular uptake of pAntp peptideFEBS JOURNAL, Issue 5 2001Role of lipid-binding affinity The pAntp peptide, corresponding to the third helix of the Antennapedia homeodomain, is internalized by a receptor-independent process into eucaryotic cells. The precise mechanism of entry remains unclear but the interaction between the phospholipids of plasma membrane and pAntp is probably involved in the translocation process. In order to define the role of peptide,lipid interaction in this mechanism and the physico-chemical properties that are necessary for an efficient cellular uptake, we have carried out an Ala-Scan mapping. The peptides were labeled with a fluorescent group (7-nitrobenz-2-oxo-1,3-diazol-4-yl-; NBD) and their cell association was measured by flow cytometry. Furthermore, we determined the fraction of internalized peptide by using a dithionite treatment. Comparison between cell association and cell uptake suggests that the affinity of pAntp for the plasma membrane is required for the import process. To further investigate which are the physico-chemical requirements for phospholipid-binding of pAntp, we have determined the surface partition coefficient of peptides by titrating them with phospholipid vesicles having different compositions. In addition, we estimated by circular dichroism the conformation adopted by these peptides in a membrane-mimetic environment. We show that the phospholipid binding of pAntp depends on its helical amphipathicity, especially when the negative surface charge density of phospholipid vesicles is low. The cell uptake of pAntp, related to lipid-binding affinity, requires a minimal hydrophobicity and net charge. As pAntp does not seem to translocate through an artificial phospholipid bilayer, this might indicate that it could interact with other cell surface components or enters into cells by a nonelucidated biological mechanism. [source] Structure,activity relationship of the p55 TNF receptor death domain and its lymphoproliferation mutantsFEBS JOURNAL, Issue 5 2001Gert De Wilde Upon stimulation with tumor necrosis factor (TNF), the TNF receptor (TNFR55) mediates a multitude of effects both in normal and in tumor cells. Clustering of the intracellular domain of the receptor, the so-called death domain (DD), is responsible for both the initiation of cell killing and the activation of gene expression. To characterize this domain further, TNFR55 DD was expressed and purified as a thioredoxin fusion protein in Escherichia coli. Circular dichroism, steady-state and time-resolved fluorescence spectroscopy were used to compare TNFR55 DD with DDs of the Fas antigen (Fas), the Fas-associating protein with DD (FADD) and p75 nerve growth factor receptor, for which the 3-dimensional structure are already known. The structural information derived from the measurements strongly suggests that TNFR55 DD adopts a similar fold in solution. This prompted a homology modeling of the TNFR DD 3-D structure using FADD as a template. In vivo studies revealed a difference between the two lymphoproliferation (lpr) mutations. Biophysical techniques were used to analyze the effect of changing Leu351 to Ala and Leu351 to Asn on the global structure and its impact on the overall stability of TNFR55 DD. The results obtained from these experiments in combination with the modeled structure offer an explanation for the in vivo observed difference. [source] B,Z DNA Transition Triggered by a Cationic Comb-Type CopolymerADVANCED FUNCTIONAL MATERIALS, Issue 22 2009Naohiko Shimada Abstract The conformational transition from right-handed B,DNA to left-handed Z,DNA,the B,Z transition,has received increased attention recently because of its potential roles in biological systems and its applicability to bionanotechnology. Though the B,Z transition of poly(dG,dC),·,poly(dG,dC) is inducible under high salt concentration conditions (over 4,M NaCl) or by addition of multivalent cations, such as hexaamminecobalt(III), no cationic polymer were known to induce the transition. In this study, it is shown by circular dichroism and UV spectroscopy that the cationic comb-type copolymer, poly(L -lysine)- graft -dextran, but not poly(L -lysine) homopolymer or a basic peptide, induces the B,Z transition of poly(dG,dC),·,poly(dG,dC). At a cationic amino group concentration of 10,4,M the copolymer stabilizes Z,DNA. The transition pathway from the B to the Z form is different to that observed previously. We speculate that the cationic backbone of the copolymer, which reduces electrostatic repulsion among DNA phosphate groups, and the hydrophilic dextran chains, which reduce activity of water, cooperate to induce the B,Z transition. The copolymer specifically modified the micro-environment around DNA molecules to induce Z,DNA formation through stable and spontaneous inter-polyelectrolyte complex formation. [source] Proton Transport from Dendritic Helical-Pore-Incorporated PolymersomesADVANCED FUNCTIONAL MATERIALS, Issue 18 2009Anthony J. Kim Abstract The ability to add synthetic channels to polymersome (polymer vesicle) membranes could lead to novel membrane composites with unique selectivity and permeability. Proton transport through two different synthetic pores, self-assembled from either a dendritic dipeptide, (6Nf-3,4-3,5)12G2-CH2 -Boc-L-Tyr-L-Ala-OMe, or a dendritic ester, (R)-4Bp-3,4-dm8G1-COOMe, incorporated into polymersome membranes are studied. Polymersomes provide an excellent platform for studying such transport processes due to their robustness and mechanical and chemical stability compared to liposomes. It is found that the incorporated dendritic dipeptide and dendritic ester assemble into stable helical pores in the poly(ethylene oxide)-polybutadiene (PEO-PBD) polymersomes but not in the poly(2-methyloxazoline)-poly(dimethylsiloxane)-poly(2-methyl oxazoline) (PMOX-PDMS-PMOX) polymersomes. The incorporation is confirmed by circular dichroism (CD), changes in purely synthetic mechanical strength (e.g., areal expansion modulus) as assessed by micropipette aspiration, and cryo-TEM. In addition to the structural analyses, a transport measurement shows the incorporated dendritic helical pores allow facile transport of protons across the polymersome membranes after up to one month of storage. This integration of synthetic porous channels with polymersome substrates could provide a valuable tool for studying active transport processes in a composite membrane. These composites will ultimately expand the family of biologically inspired porous-membrane mimics. [source] Magnetic and Structural Investigation of ZnSe Semiconductor Nanoparticles Doped With Isolated and Core-Concentrated Mn2+ IonsADVANCED FUNCTIONAL MATERIALS, Issue 15 2009Christina Graf X-Ray magnetic circular dichroism (XMCD) experiments on diluted magnetic semiconductor nanocrystals (2,7,nm) are reported in order to study their local electronic structure and magnetic properties. ZnSe nanoparticles containing either single manganese ions (Mn2+) distributed in the lattice of the entire particle or a MnSe core in the center are prepared using high temperature approaches. The Mn2+ concentration is varied between less than one to several tens of manganese ions per nanocrystal. For all samples it is shown that the Mn2+ is exclusively present in the bulk of ZnSe nanoparticles with no evidence for oxidation to higher Mn-oxidation states. The magnetic ions are highly polarized inside the nanocrystals reaching about 80% of the theoretical value of a pure d5 state under identical conditions for the case of isolated manganese ions. Nanocrystals with a MnSe core ZnSe shell structure reach <50% of this value. Thus, their polarization is significantly more hindered, which is due to the significantly enhanced Mn,Mn interactions and a more distorted crystalline lattice. In contrast, no coupling between the manganese centers is observed in the nanoparticles doped samples with low concentrations of Mn2+, indicating that these ions are isolated in the bulk of the nanoparticles. [source] | |||||||||||||||||||||||||||||||||||||||||||