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At least 703 records · Page 39Linked to original sources
PubMed2005

Transverse mode coupling in an optical resonator.

Small-angle scattering due to mirror surface roughness is shown to couple the optical modes and deform the transmission spectra in a frequency-degenerate optical cavity. A simple model based on a random scattering matrix clearly visualizes the mixing and avoided crossings between multiple transverse modes. These effects are visible only in the frequency-domain spectra; cavity ringdown experiments are unaffected by changes in the spatial coherence, as they probe just the intracavity photon lifetime.

Journal Article↗
PubMed2000

Activation of calcium/calmodulin regulated kinases.

Among numerous protein kinases found in mammalian cell systems there is a distinct subfamily of serine/threonine kinases that are regulated by calmodulin or other related activators in a calcium concentration dependent manner. Members of this family are involved in various cellular processes like cell proliferation and death, cell motility and metabolic pathways. In this contribution we shall review the available structural biology data on five members of this kinase family (calcium/calmodulin dependent kinase, twitchin kinase, titin kinase, phosphorylase kinase, myosin light chain kinase). As a common element, all these kinases contain a regulatory tail, which is C-terminal to their catalytic domain. The available 3D structures of two members, the serine/threonine kinases of the giant muscle proteins twitchin and titin in the autoinhibited conformation, show how this regulatory tail blocks their active sites. The structures suggest that activation of these kinases requires unblocking the active site from the C-terminal extension and conformational rearrangement of the active site loops. Small angle scattering data for myosin light chain kinase indicate a complete release of the C-terminal extension upon calcium/calmodulin binding. In addition, members of this family are regulated by diverse add-on mechanisms, including phosphorylation of residues within the activation segment or the P+1 loop as well as by additional regulatory subunits. The available structural data lead to the hypothesis of two different activation mechanisms upon binding to calcium sensitive proteins. In one model, the regulatory tail is entirely released ("fall-apart"). The alternative model ("looping-out") proposes a two-anchored release mechanism.

Amino Acid Sequence↗
PubMedDate not supplied

Some signal effects in human erythrocytes on specific binding of free doxorubicin and doxorubicin conjugates with magnetite nanoparticles.

Binding of doxorubicin (DOX) immobilized on nanodispersed magnetite (DOX-M conjugates with loading in the range of 0.16-25.1 mg DOX/g carrier) to intact human erythrocytes in concurrence with free DOX was investigated. Two specific binding sites for doxorubicin were revealed at the plasma membrane of human erythrocytes. Changes in the ordering of the DOX-M nanoparticles according to small angle scattering data are consistent with their specific binding at the plasma membrane upon incubation with erythrocytes. Free and conjugated doxorubicin modulated signal transduction in erythrocytes in a similar way. Both up-regulate nitric oxide and cyclic GMP and down-regulate cyclic AMP production and stabilize the membranes of oxidatively damaged erythrocytes.

Animals↗
PubMed1988

The effect of energy window on cardiac ejection fraction.

ECG gated gamma-ray energy spectra from the left ventricle were created each 50 msec during the cardiac cycle. Nine of ten subjects were studied with a nonimaging Nal probe, and the tenth with a high-resolution Germanium detector. Placing multiple energy windows over the energy spectra, EF was found to vary with the energy window selected. Moving a 20% window across the photopeak produced a roughly linear increase in EF with energy (2.3 EF units per 10 keV increase in energy) in eight of the ten subjects. Dividing the photopeak into a low (126-140 keV) and high-energy (140-154 keV) portion gave significantly different EFs (high energy exceeding low energy by 17%). Increasing the width of a narrow window centered about the photopeak produced negligible change in EF. Examining the energy spectra showed that the small-angle scattered radiation (126-139 keV) was proportionately greater at end systole than at end diastole, after normalizing the spectra to the same photopeak area.

Electrocardiography↗
PubMedDate not supplied

[Theoretical and experimental studies on the conformation of the hinge regions in human immunoglobulin G subclasses].

Using the methods of difference adiabatic scanning microcalorimetry and difference thermal perturbation spectrophotometry it was established that both the Fc subunits within the intact immunoglobulins G of different subclasses, and the Fc fragments corresponding to them significantly differ in conformational properties. These differences are associated with a different energy of interactions between the CH2 and CH3 domains and also with a different rigidity of structure of the N- and C-terminal parts of the CH2 domains. The analysis of these data allowed to suggest that the "hinge region" interacts with the CH2 domains and the difference in the structure of this region affect the conformation of the Fc subunits. Possible models of the "hinge regions" in immunoglobulins G of the first, second and fourth subclasses, and also their arrangement relative to the CH2 domains were proposed. The analysis of curves of the small-angle scattering of X-rays shows an agreement between the theoretical models and experimental data.

Calorimetry, Differential Scanning↗
PubMed1999

Scanning small angle X-ray scattering analysis of human bone sections.

Scanning small-angle X-ray scattering (scanning SAXS) was applied for the first time on bone to compare results from SAXS directly with those from other position-sensitive methods, such as light and polarized light microscopy, back-scattered electron imaging, and radiographic imaging. Since scanning SAXS is a nondestructive method of investigation, images from all these techniques could be obtained from the same bone sections. Thus, it could be shown that both the collagen and the mineral crystals were predominantly aligned parallel to the trabeculae and, therefore, to principle stress directions. Moreover, the mean crystal thickness as determined by scanning SAXS was found to be different at various positions inside the trabecular and cortical structure. Finally, it could be shown that scanning SAXS is suitable for detecting local changes in bone material, e.g., due to fluoride treatment.

Adolescent↗
PubMed1998

Lysozyme crystal growth, as observed by small angle X-ray scattering, proceeds without crystallization intermediates.

A combination of small angle X-ray scattering and gel techniques was used to follow the kinetics of protein crystal growth as a function of time. Hen egg white lysozyme, at different protein concentrations, was used as a model system. A new sample holder was designed, in which supersaturation is induced in the presence of salt by decreasing the temperature. It had been shown previously that a decrease in temperature and/or an increase in crystallizing agent induces an increase in the attractive interactions present in the lysozyme solutions, the lysozyme remaining monomeric. In the present paper we show that similar behaviour is observed in NaCl when agarose gels are used. During crystal growth, special attention was paid to determine whether oligomers were formed as the protein in solution was incorporated in the newly formed crystals. From these first series of experiments, we did not find any indication of oligomer formation between monomer in solution and crystal. The results obtained are in agreement with the hypothesis that lysozyme crystals in NaCl grow by addition of monomeric particles.

Crystallization↗
PubMed2006

Probing fast kinetics in complex fluids by combined rapid mixing and small-angle X-ray scattering.

Stopped-flow mixing coupled to small-angle X-ray scattering (SAXS) is an established technique for investigating structural kinetics in solution down to the millisecond range. More recently, the emphasis has shifted to the sub-millisecond range using continuous flow microfluidic mixing devices. The aim of this article is to review the present status and limitations when applying mixing techniques to a wide range of soft matter and biological systems. In the case of SAXS, special consideration of the mixing quality is necessary for a quantitative description of the scattered intensity. This is demonstrated through two representative examples involving protein refolding and micellar self-assembly.

Journal Article↗
PubMed1998

Sample environments and techniques combined with small angle X-ray scattering.

The number of synchrotron radiation-based Small Angle X-ray Scattering beamlines has increased considerably over the last decade. With the high X-ray flux and collimation of these beamlines it not only has become possible to perform time-resolved experiments on time scales down to the millisecond/frame range, but also it allows experimenters to utilise new sample environments and use simultaneous several experimental techniques on one sample. An overview of recent developments in this field is given.

Animals↗
PubMed1999

Complementary information on bone ultrastructure from scanning small angle X-ray scattering and Fourier-transform infrared microspectroscopy.

Scanning small angle X-ray scattering (scanning SAXS) and Fourier-transform infrared microspectroscopy (FT-IRM) have previously been utilized independently to characterize the structural properties of bone in an anatomical position-resolved fashion. Whereas SAXS provides a direct measure of the physical characteristics of apatitic crystals, FT-IRM assesses structure of both mineral and organic matrix at the molecular level. In the present study both methods were applied to examine the same developing bone tissue from the L-4 vertebra of a 14-month-old (accidental death). A 200-microm-thick section was processed for examination by scanning electron microscopy and SAXS. Spectra were collected at 200 microm spatial resolution at specific locations in cortical and cancellous bone. Parameters determined included total SAXS intensity, crystal thickness (T), and degree and direction of predominant crystal orientation. For FT-IRM analysis, a section 4 microm thick was cut longitudinally from the top of the sample. Spectra of regions 100 x 100 microm2 were acquired from the same locations as the SAXS spectra. Integrated areas of the phosphate nu(1,3) collagen amide I, and carbonate nu2 absorbances, were calculated to obtain mineral: matrix and carbonate:mineral ratios. The relative quantities of types A, B, and labile carbonate (substituted for apatite hydroxyl, phosphate, and surface positions, respectively) were also evaluated. Polarized FT-IRM data were collected to determine molecular orientation of the apatite and collagen components. The results of this study show that the information obtained from the two techniques is complementary. Both SAXS and FT-IRM data revealed that the crystals were significantly larger in the cancellous region compared with the cortical region, that mineralization was greater in the cortex, and that the crystals were oriented to a larger degree in the cancellous compared with the cortical bone. The scanning SAXS measure of crystal thickness was significantly correlated to the FT-IRM measures of crystallinity, type A carbonate substitution, and crystal orientation. In conclusion, it was found that the combined use of SAXS and FT-IRM provides valuable, unique information on structural changes in bone at both the microstructural and ultrastructural level. Although each method can be used individually, the combination of techniques provides additional insights into the mechanism of bone crystal maturation.

Apatites↗
PubMed2002

Small-angle x-ray scattering studies of human breast tissue samples.

Small-angle x-ray scattering (SAXS) patterns are recorded from thin breast tissue samples containing healthy and cancerous regions. The SAXS patterns are compared with histo-pathological observations. The information available from SAXS is reviewed, and a model for scattering from collagen is presented. Scattering patterns of collagen at regions far from the tumours are essentially different from those at tumours. The axial period of collagen fibrils is 65.0 +/- 0.1 nm in healthy regions, and 0.3 nm larger in cancer-invaded regions. The average intensity of scattering from cancerous regions is an order of magnitude higher than the intensity from healthy regions. This is interpreted to arise from an increase of the specific surface area of the scatterers, which is due to a disruption of the molecular and supra-molecular structures in cancerous regions and invasion of new types of cells. The differences of the SAXS patterns are large and distinctive enough to suggest that these phenomena may be utilized in mammography.

Breast↗
PubMed1989

Small angle X-ray scattering study on Lingula unguis shell.

Lingula unguis shell yields a diffuse small angle X-ray scattering which is caused mainly by the scattering from particles of apatite. In this study, the distribution of particles smaller than 500A was analyzed using the small angle X-ray scattering technique. The non-linear Guinier plot indicates that the shell contains various sizes of apatite particles ranging about 20-460A. The small granules with size about 20-160A in diameter are located in the marginal part of the shell and progressively larger granules are observed internally towards the central part. The scattering is anisotropic in the lateral and central part, where the apatite crystals highly ordered. The combined analysis of wide and small angle diffraction indicates that the particles are elongated in the c-axis direction and, furthermore, their long axes are arranged almost parallel to the direction of growth in these parts. This suggests that small and isotropic particles are formed in the early stage of mineralization and larger ellipsoidal shaped particles are formed in the later stage of mineralization.

Animals↗
PubMed2006

Anomalous small angle x-ray scattering determination of ion distribution around a polyelectrolyte biopolymer in salt solution.

The distribution of counterions in solutions of high molecular mass hyaluronic acid, in near-physiological conditions where mono- and divalent ions are simultaneously present, is studied by small angle neutron scattering and anomalous small angle x-ray scattering. The solutions contain either sodium or rubidium chloride together with varying concentrations of calcium or strontium chloride. The effects of monovalent-divalent ion exchange dominate the amplitude and the form of the counterion cloud. In the absence of divalent ions, the shape of the anomalous scattering signal from the monovalent ions is consistent with the distribution calculated from the Poisson-Boltzmann equation, as found by other workers. In mixtures of monovalent and divalent ions, however, as the divalent ion concentration increases, both the diameter and the amplitude of the monovalent ion cloud decrease. The divalent counterions always occupy the immediate neighborhood of the charged polyanion. Above a given concentration their anomalous scattering signal saturates. Even in a large excess of divalent ions, ion exchange is incomplete.

Chemistry, Physical↗
PubMedDate not supplied

[Determination of the molecular weight and form of the molecule histidine decarboxylase by the method of small-angle x-ray scattering].

Shape and molecular weight of histidine-decarboxylase from Micrococus sp. n. were studied by the method of X-ray small-angle scattering. The inertion radius of the molecule: Rg-2,93 nm. The shape of the molecule is adequately approximated by rotation ellipsoids of two possible variants: the elongated and flattened ones. The eccentricity in both cases is 1.6. The volume of the enzyme molecule V=190 nm3. The molecular weight of histidine-dexarboxilase obtained from the X-ray experiment M=102 000 c.u.

Carboxy-Lyases↗
PubMedDate not supplied

[Comparative study of the crystallin supramolecular structure in the carp, frog, and rat lenses by small-angle roentgen ray scattering].

The supramolecular structure of crystallins in intact ocular lenses of carp, frog and rat as well as in the interior (nuclear) and outer (cortical) parts of these lenses was studied by the small-angle X-ray scattering method. The results show that the supramolecular structure of crystallins substantially varies both in lenses of different vertebrate species and in various parts of the same lens. In carp lens and in the cortical part of rat lens, crystallins have an ordered supramolecular structure, as indicated by a small-angle X-ray diffraction maximum in the region of Bragg distances 15-20 nm, whereas in frog lens and in the nuclear part of rat lens, the supramolecular structure of these proteins is disordered. The power-law X-ray scattering by rat lens nucleus may be evidence of fractal structures in the lens. A comparison of these results with literary data indicates that there is no obvious correlation between the type of supramolecular structure of crystallins and their polypeptide composition in lenses of different vertebrate species. The results suggest that the supramolecular ordering (short-range order) of crystallins is not a necessary condition for lens transparency.

Animals↗
PubMed2002

Preliminary study on extremely small angle x-ray scatter imaging with synchrotron radiation.

Among the medical physics community, there is nowadays a great interest in the possible implementation of scatter imaging techniques, especially in the field of breast imaging. It is well known that malignant lesions and normal tissue differ in their scattering signatures, and thus scattered radiation can provide a powerful tool to distinguish between the two cases. Up to now, most of the proposed techniques rely on the detection of radiation scattered at angles of the order of a few degrees, which in most cases results in very high contrast values. On the other hand, at those relatively large angles the scattered flux is relatively low with respect to the primary, which often implies the necessity of increasing the dose delivered to the sample in order to achieve sufficient statistics. Furthermore, most of these techniques are based on pencil beam set-ups, which results in an increase of the overall duration of the examination. We propose here an alternative approach based on the detection of radiation scattered at extremely small angles, of the order of approximately 100-200 microrad. This results in a relatively high scattered flux (5-10% of the primary) and in the possibility of utilizing a fan beam geometry, which reduces the acquisition times with respect to pencil beam set-ups. Images of several samples have been acquired, demonstrating that the proposed technique results in an increased contrast with respect to absorption imaging. Possible in vivo implementations of the technique at no dose expense are finally discussed.

Algorithms↗
PubMed2003

Nucleotide binding induces changes in the oligomeric state and conformation of Sec A in a lipid environment: a small-angle neutron-scattering study.

In Escherichia coli, SecA is a large, multifunctional protein that is a vital component of the general protein secretion pathway. In its membrane-bound form it functions as the motor component of the protein translocase, perhaps through successive rounds of membrane insertion and ATP hydrolysis. To understand both the energy conversion process and translocase assembly, we have used contrast-matched, small-angle neutron-scattering (SANS) experiments to examine SecA in small unilamellar vesicles of E.coli phospholipids. In the absence of nucleotide, we observe a dimeric form of SecA with a radius of gyration comparable to that previously observed for SecA in solution. In contrast, the presence of either ADP or a non-hydrolyzable ATP analog induces conversion to a monomeric form. The larger radius of gyration for the ATP-bound relative to the ADP-bound form suggests the former has a more expanded global conformation. This is the first direct structural determination of SecA in a lipid bilayer. The SANS data indicate that nucleotide turnover can function as a switch of conformation of SecA in the membrane in a manner consistent with its proposed role in successive cycles of deep membrane penetration and release with concommitant preprotein insertion.

Adenosine Triphosphatases↗