摘要:

AbstractThe problem of crystal structure analysis at macromolecular resolution is tackled using notions borrowed from small‐angle solution‐scattering studies. The mathematical procedure ‐ essentially a pattern recognition approach ‐ consists of generating all the phase combinations compatible with the data and screening them: the critical steps are the choice of the “pattern” and of the criteria which preside the “recognition”. Two strategies are adopted. One is based upon use of the histogram of the electron density maps, a function more dependent on chemical composition than on physical structure: the histograms of samples of different structure and similar chemical composition are compared in search for similarity. The other strategy operates on the electron density maps and involves the principle of maximum smoothness. From the practical viewpoint, one single parameter, (Δp)4, is found to play an outstanding role in the two cases. Algorithms are worked out, implemented on the computer and tested using a variety of lipid‐water phases. The comparison of samples of different structure and similar chemical composition is found to provide a powerful screening criterion. The principle of maximum smoothness is also of great help, at least to the extent that the electron density distribution is a fairly smooth function, devoid of sharp peaks (associated with heavy atoms). The practical aspects of the algorithms are discussed, as well as their possible application to crystallographic problems of mor

ISSN:0258-0322    

DOI:10.1002/masy.19880150103

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractContrast variation for macromolecular structure determination is usually achieved by isomorphous replacement of 1‐H by 2‐H (D) using small‐angle neutron scattering (SANS). This is particularly easy in aqueous solvents. By adding heavy water the contrast of dissolved proteins, nucleic acids and membranes changes drastically. It is the region inaccesible to solvent molecules, which acts as a label. Measurements of the scattering intensity at three different scattering densities of a solvent yields the three basic scattering functions. The contrast dependence of the radius of gyration receives particular interest.More recently smaller labels have been used. Their dimensions are smaller than those of the total particle by an order of magnitude. They are used forin situstructure determination of the labelled region. Contrast variation of the labelled region can be achieved by gradual deuteration of the label. Again, the measurements have to be done at three different contrasts in order to separate the scattering function of the label. ‐ By eliminating any intramolecular scattering density fluctuation (K. Nierhaus' principle of the ‘transparent’ ribosome) the structure of selectively labelled ribosomal proteins can be determinedin situby using only one derivative.Spin contrast variation of biomolecules relies on the nuclear spin dependence of neutron scattering by protons and, to a smaller extent, by deuterons. It is particularly well suited for labels rich in protons embedded in a deuterated matrix. Variation of the target polarization yields the three basic scattering functions from the same sample.In collaboration with CERN and ILL, a special set‐up for spin contrast variation has been installed at the reactor FRG1 of the GKSS Research Centre at Geesthacht. The dynamic nuclear polarization of proton spins in proteins, nucleic acids and ribosomes is achieved in the presence of Cr(V) at T = 0.4 K, H = 2.5 T and 4 mm microwave irradiation. Within two hours, the proton polarization will reach more than 70 % in favourable cases. Measurements have been carried out with various proteins, transfer ribonucleic acid (tRNA) and the large subunit of E.coli ribosomes, the latter also with its total protein deuterated (i.e. ribosomal ribonucleic acid (rRNA) acts as a label). There is an agreement between the nuclear spin polarization of protons and deuterons measured by NMR and the nuclear spin dependent change of polarized small‐angle neutron scattering.The results of spin contrast variation experiments are compared with those obtained from near‐edge anomalous X‐ray scattering using synchrotron radiation. Particular attention is given to the anomalous dispersion of light elements, like sulfur, which tend to show a strong dependence on the chemical bonding of the atom. In purple membrane, the anomalous scattering of sulfur in protein, bound to methionine and sulfates in the lipid matrix has been de

ISSN:0258-0322    

DOI:10.1002/masy.19880150104

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractDirect methods of small‐angle scattering data treatment and interpretation are reviewed. The possibilities of Tikhonov's regularization method for solving ill‐posed experimental data processing problems (desmearing, size distribution functions evaluation) are shown; comparison with other data treatment methods is made. Methods for direct reconstruction of the structure of particles in isotropic monodisperse objects are presented. The cases of particles described by one‐ and two‐dimensional density distributions and the possibilities of application of the multipole expansion theory are con

ISSN:0258-0322    

DOI:10.1002/masy.19880150105

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractSome aspects of small‐angle X‐ray scattering (SAXS) instrumentation for synchrotron radiation (SR) are considered below. The basic layout of instruments as well as some of the problems arising from the nature of the source and of the type of experiments are discussed. Further, a survey of the instruments available at major SR facilities is gi

ISSN:0258-0322    

DOI:10.1002/masy.19880150106

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractThe operation principles, constructions, advantages and shortcomings of the known Time‐of‐Flight Small‐Angle Neutron Scattering (TOF SANS) spectrometers built up on pulsed neutron sources are reviewed. The most important characteristics of TOF SANS apparatuses are rather a high luminosity and the possibility of measurement in an extremely wide range of the scattering vector at a single exposure. This is achieved by simultaneous employment of the white beam, TOF technique for Λ‐scan and the commonly known θ‐scan. However, the electronic equipment, data‐matching programs and the measurement procedure, necessary for accurate normalization of experimental data and their transformation into absolute cross‐section scale ‐ they all become more complex as compared with those for SANS apparatuses operating on steady‐state neutron sources, where o

ISSN:0258-0322    

DOI:10.1002/masy.19880150107

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractDeuterium labeling techniques have been widely used to study polymer chain configuration and motion, based on the assumption that the H‐ and D‐ chains are thermodynamically identical and that the interaction parameter between them, XHDis zero. By an appropriate choice of the degree of polymerization and temperature, both critical scattering and phase separation can be made to occur, due solely to the small but finite thermodynamic differences between protonated and deuterated molecules. These experiments helped delineate the range of validity of the assumptions which had been used hitherto in neutron scattering studies of polymers. The majority of previous experiments, carried out at modest molecular weights, have been shown to be in large part unaffected by such effe

ISSN:0258-0322    

DOI:10.1002/masy.19880150108

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractThe relative positions of the centers of mass of the 21 proteins of the 30S ribosomal subunit from Escherichia coli have been determined by triangulation using neutron scattering measurements in solution. This mapping is the first complete account of the quaternary structure of the small ribosomal subunit to be obtained by any method.

ISSN:0258-0322    

DOI:10.1002/masy.19880150109

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractThe use of small‐angle scattering techniques in the study of inhomogeneities and microstructural changes in inorganic materials is reviewed with emphasis on X‐ray scattering. Recent applications of anomalous scattering with synchrotron radiation and kinetic studies in amorphous and crystalline systems are stres

ISSN:0258-0322    

DOI:10.1002/masy.19880150110

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractRecent progress in two fields of small‐angle scattering is reviewed: (a) New procedures have been developed by Kotlarchyk and Chen and by Triolo, Griffith, and Compere for calculating the intensity of the small‐angle scattering from polydisperse systems of interacting particles of different sizes. These techniques have significantly increased the quantitative information that can be obtained from the scattering data. (b) The pore boundaries in many porous solids have been found to be fractal surfaces. In a porous solid in which the pores have an average diameter ϵ and the pore boundary surfaces have a fractal dimension D, the scattered intensity for qϵ,>>1 is proportional to q−(6‐D), where q = πλ−1sin(θ/2), θ is the scattering angle, and λ is the wavelength. Some small‐angle scattering studies of fractal por

ISSN:0258-0322    

DOI:10.1002/masy.19880150111

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractA new method is proposed for studying the structure and mutual arrangement of selected components in multicomponent particles. Here the difference between the scattering curve of a solution containing two types of structurally identical particles (differing only in the degree of deuteration) and the scattering curve of a solution containing particles of a third type (deuterated to an intermediate degree) is considered. This difference scattering curve differs only by a numerical multiplier from the “vacuum” scattering curve of the particle in which the scattering density is equal to the difference between the scattering densities of the particles of the first and second types. This means that any particle component which is deuterated (protonated) to the same degree in the particles of three different types does not contribute to the difference scattering curve and, consequently, is “invisible” for neutrons. The difference scattering curve depends neither on the isotopic content of the solvent nor on the inter‐particle interference and particle association. Possible applications of this method are

ISSN:0258-0322    

DOI:10.1002/masy.19880150112

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY