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Linear Dichroism (linear + dichroism)
Selected AbstractsLinear 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] Polarization control of ultrashort mid-IR laser pulses for transient vibrational circular dichroism measurements,CHIRALITY, Issue 1E 2009Mathias Bonmarin Abstract Linear dichroism and birefringence artifacts are a major source of concern in transient circular dichroism measurements. They mainly arise from interaction of an imperfectly circular polarized probe beam with a nonisotropic sample. We present in this article a procedure to generate mid-IR pulses of highly symmetric left and right handed circular or elliptical polarization states for transient VCD measurements. An infrared femtosecond laser source is synchronized to the natural frequency of a photo elastic modulator. Residual static birefringence of the modulator and the sample cell can be largely compensated by carefully controlling the arrival time of the ultrashort probe pulses at the modulator. Chirality 21:E298,E306, 2009. © 2009 Wiley-Liss, Inc. [source] A multiple-scattering theory of circular and linear dichroism for photoemission and photoabsorptionJOURNAL OF SYNCHROTRON RADIATION, Issue 1 2002S. Di Matteo A unified treatment of circular dichroism, both natural and magnetic, in (spin-resolved) photoemission and photoabsorption from core levels is given, valid in the many-body case and for extended systems, together with an extension of the formalism to treat linear dichroism. The reduction of this scheme to a one-electron picture in the framework of multiple scattering theory is briefly discussed and shows the intimate connection of the two spectroscopies via a generalized optical theorem. Plausibility arguments are given that in correlated d -band systems screening and relaxation effects are not so drastic as in other cases, due to the autoscreening action of the excited photoelectron, so that the final density of states is much like the initial unperturbed one. It is shown how to exploit this point of view to obtain in favorables cases separated orbital and spin moment radial (surface) distribution maps from dichroic magnetic EXAFS spectra (photoelectron diffraction patterns) related to the ground state. Dichroic natural spectra, both in photoemission and absorption, are shown to be sensitive only to atoms in chiral geometry. [source] Chiroptical Properties of Terthiophene Chromophores Dispersed in Oriented and Unoriented Polyethylene FilmsMACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 6 2004Andrea Pucci Abstract Summary: Two new chiral terthiophene chromophores II and III were prepared with 99% enantiomeric excess. Chiroptical properties of these dyes dispersed in ultra high molecular weight polyethylene (UHMWPE) films were determined and compared with the same properties in solution. In the solid state, the optical activity strongly depends on the interaction mechanisms within small crystalline aggregates of chromophores. The film morphology and chromophore dispersion were also investigated by scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). The good correlation between chromophore aggregation and chiroptical activity of the binary films promotes circular dichroism (CD) as an effective technique for monitoring the phase dispersion behaviour of dichroic dyes into polymer matrices. By mechanical stretching of the film a linearly dichroic orientation of the chromophores is obtained which results in a high degree of linear dichroism. The influence of the uniaxial orientation of terthiophene molecules along the drawing direction of UHMWPE on the chiroptical properties of the films, and the possible application of the oriented devices as linear polarizers are discussed. Absorption and CD spectra of unoriented UHMWPEII film at different rotation angles ,. [source] Effect of Digitonin on the Rhodopsin Meta I-Meta II Equilibrium,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 4 2005Istvan Szundi ABSTRACT Absorbance difference spectra were recorded from 10 ,s to 540 ms after photoexcitation of sonicated suspensions of hypotonically washed bovine rod outer segments with varying amounts of the detergent digitonin added (0 to 2%) at 20°C. Metarhodopsin I480 and metarhodopsin II displayed the expected anomalous pH dependence at pH 6 and 8 (i.e. opposite to that expected from direct protonation of the chromophore Schiff base). However, increasing levels of digitonin eliminated the pH dependence of the equilibrium, and at 2% digitonin the pH 6 and pH 8 data were both similar to the data collected at pH 8 without digitonin. Addition of 0.5% azolectin restored approximately 50% of the anomalous pH dependence at pH 6 in the 2% digitonin sample. The possibility that digitonin induced large-scale aggregation of rhodopsin in the disk membrane that could be reversed by azolectin was tested using time-resolved linear dichroism. Those results showed that even 0.3% digitonin disrupted the membrane, and no large aggregates were detectable under any conditions. Thus, digitonin reduces the activity of a component of the disk membrane required for metarhodopsin II formation, and that deficiency can be compensated for by azolectin. [source] Magnetisation dynamics of Fe nanoclusters exchange-coupled to magnetic substratesPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 15 2004C. Binns Abstract Synchrotron radiation was used to study exposed Fe nanoclusters with controlled sizes in the size range 140,270 atoms (1.5,1.8 nm) deposited in situ from a gas aggregation source on to magnetic vitrovac amorphous ribbons. Magnetic linear dichroism in the angular distribution (MLDAD) of the Fe 3p core level photoemission was used to show that the clusters are exchange-coupled to the substrate with an interface molecular field of about 20 T. The switching dynamics was studied on the nanosecond timescale by time-resolved spin-polarised photoemission. At low coverages, below the percolation threshold, when the film consists of separated islands the switching dynamics of the cluster islands is identical to the clean substrate. At around the percolation threshold, however, the data indicates that the magnetic reversal propagates faster in the cluster film than in the substrate. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Photochromic composites based on porous stretched polyethylene filled by nematic liquid crystal mixturesPOLYMERS FOR ADVANCED TECHNOLOGIES, Issue 2 2010Alexey Bobrovsky Abstract A number of the novel photochromic polyethylene (PE)-based liquid crystal composites were prepared and studied. The oriented stretched porous polyethylene films were used as the polymer matrices. Commercial liquid crystals doped with new photochromic compounds were introduced into PE films and photo-optical properties of the obtained composites were investigated. It was shown that a director of nematic liquid crystals is highly oriented along the stretching axis of PE films resulting in noticeable linear dichroism of the PE composite films. New approaches for reversible or irreversible image recording on PE LC composites by UV irradiation were demonstrated. Copyright © 2009 John Wiley & Sons, Ltd. [source] Supramolecular order following binding of the dichroic birefringent sulfonic dye Ponceau SS to collagen fibersBIOPOLYMERS, Issue 3 2005B. C. Vidal Abstract The optical anisotropies (linear dichroism or LD and birefringence) of crystalline aggregates of the sulfonic azo-dye Ponceau SS and of dye complexed with chicken tendon collagen fibers were investigated in order to assess their polarizing properties and similarity to liquid crystals. In some experiments, the staining was preceded by treatment with picric acid. Crystalline fibrous aggregates of the dye had a negative LD, and their electronic transitions were oriented perpendicular to the filamentary structures. The binding of Ponceau SS molecules to the collagen fibers altered the LD signal, with variations in the fiber orientation affecting the resulting dichroic ratios. The long axis of the rod-like dye molecule was assumed to be bound in register, parallel to the collagen fiber. Picric acid did not affect the oriented binding of the azo dye to collagen fibers. There were differences in the optical anisotropy of Ponceau SS-stained tendons from 21-day-old and 41-day-old chickens, indicating that Ponceau SS was able to distinguish between different ordered states of macromolecular aggregation in chicken tendon collagen fibers. In the presence of dichroic rod-like azo-dye molecules such as Ponceau SS, collagen also formed structures with a much higher degree of orientation. The presence of LD in the Ponceau SS-collagen complex even in unpolarized light indicated that this complex can act as a polarizer. © 2005 Wiley Periodicals, Inc. Biopolymers 78: 121,128, 2005 This article was originally published online as an accepted preprint. The "Published Online" date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com [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] A new method for fibrous protein analysis illustrated by application to tubulin microtubule polymerisation and depolymerisation,CHIRALITY, Issue 9 2006Rachel Marrington Abstract A thermostatted micro volume Couette cell has been designed to enable linear dichroism (LD) data to be collected at a range of temperatures. The cell is a development of the traditional Couette flow LD cell and includes the recent development of micro-volume LD (20,40 ,L) coupled with the addition of a heating element, temperature probe and controller. This new micro volume Couette LD cell opens the way not only to the LD analysis of systems where sample volume is critical, but also for the LD analysis of temperature sensitive samples. The polymerization of the microtubule protein tubulin has been followed in a range of different conditions using the thermostatted micro volume Couette LD cell. The focusing lenses on the cell, which are required for the microvolume cell, have the side benefit of significantly reducing the light-scattering artifacts caused by the large size of tubulin microtubules. It is now possible to monitor real-time polymerization and depolymerization kinetics, and any structural rearrangements of chromophores within the polymer. In the case of tubulin, the LD spectra revealed a greater change in the orientation of tryptophan residues at ,290 nm during polymerization compared to other contributing chromophores,guanine, phenylalanine, and tyrosine. The improvements in instrumental design have also allowed LD spectra of tubulin to be collected down to ,230 nm (previous data have only been available from the near UV region), which means that some indication of protein backbone-orientation changes are now available. It was observed during this work that apparent LD intensity maxima are in fact artifacts when the high-tension voltage is high. The onset of such artifacts has been observed at much lower voltages with light-scattering fibrous proteins (including tubulin) than with nonscattering samples. Therefore, caution must be used when interpreting LD data collected with medium to high photomultiplier tube voltages. Chirality, 2006. © 2006 Wiley-Liss, Inc. [source] Measurement of circular birefringence and circular dichroism of the single crystals of ,-(+)589 - and ,-(,)589 -tris(ethylenediamine)cobalt(III) triiodide monohydrate by the extended HAUP methodCHIRALITY, Issue 5 2004Ryo Matsuki Abstract We have achieved measuring four optical parameters simultaneously, namely, linear birefringence (LB), circular birefringence (CB), linear dichroism (LD), and circular dichroism (CD), of single crystals of ,-(+)589 - and ,-(,)589 -tris(ethylenediamine) cobalt(III) triiodide monohydrate (1) along the <001> plane at the fixed wavelength (514.5 nm). Such measurements are possible only when the High Accuracy Universal Polarimeter (HAUP) is employed; it is called the extended HAUP method. Our experimental results showed that both LB and LD of the ,-(+)589 -(1) crystal have the same magnitude as those of the ,-(,)589 -(1) crystal. It was also revealed for the first time that the CB data of crystals of ,-(+)589 -(1) and ,-(,)589 -(1) are almost of the same magnitude, but are of opposite sign, reflecting their opposite absolute configurations. On the other hand, although the CD data obtained for ,-(+)589 -(1) is almost three times larger than that for ,-(,)589 -(1,) these CD data are also opposite in sign, as expected from the opposite chirality of crystals. Chirality 16:286,293, 2004. © 2004 Wiley-Liss, Inc. [source] | |||||||||||||