Anomalous diffraction/scattering

Hence, by applying the Huygens principle to aplane just beyond the sphere ([200], Chap. 11 Sect. 3), the anomalous diffraction scattering amplitudes yield ... 

A teclmique that employs principles similar to those of isomorphous replacement is multiple-wavelength anomalous diffraction (MAD) [27]. The expression for the atomic scattering factor in equation (B1.8.2h) is strictly accurate only if the x-ray wavelength is well away from any characteristic absorption edge of the element, in which case the atomic scattering factor is real and Filiki) = Fthkl V- Since the diffracted... 

The multiple-wavelength anomalous diffraction method (MAD) relies upon sufficiently strong anomalously scattering atoms in the protein structure itself In this method, diffraction data must be collected at a number of different wavelengths, usually requiring data collection with... 

Alternative methods of solving the phase problem are also used now. When a transition metal such as Fe, Co, or Ni is present in the protein, anomolous scattering of X-rays at several wavelengths (from synchrotron radiation) can be used to obtain phases. Many protein structures have been obtained using this multiple wavelength anomalous diffraction (MAD phasing) method.404 407 408 Selenocysteine is often incorporated into a protein that may be produced in... 

While it is very easy, when one knows the structure of the crystal and the wavelength of the rays, to predict the diffraction pattern, it is quite another matter to deduce the crystal structure in all Its details from the observed pattern and the known wavelength. The first step is lo determine the spacing of the atomic planes from the Bragg equation, and hence the dimensions of the unit cell. Any special symmetry of the space group of the structure will be apparent from space group extinction. A Irial analysis may (hen solve the structure, or it may be necessary to measure the structure factors and try to find the phases or a Fourier synthesis. Various techniques can be used, such as the F2 series, the heavy atom, the isomorphous series, anomalous atomic scattering, expansion of the crystal and other methods. 

This appendix reports the weights for the moments of the particle size distribution obtained from an eight order Taylor Series approximation to the scattering efficiency for the anomalous diffraction case... 

The newer la.ser diffraction instrument allows measurement for particle sizes ranging from 0.1 pm to 8 mm (7). Most of the laser diffraction instruments in the pharmaceutical industry use the optical model based on several theories, either Fraunhofer, (near-) forward light scattering, low-angle laser light scattering, Mie, Fraunhofer approximation, or anomalous diffraction. These laser diffraction instruments assume that the particles measured are spherical. Hence, the instrument will convert the scattering pattern into an equivalent volume diameter. A typical laser diffraction instrument consists of a laser, a sample presentation system, and a series of detectors. 

The tunability of synchrotron radiation allows for data collection at or near the x-ray absorption edge of anomalous scatterers present in the protein or crystal to provide experimental phase information. Using techniques such as multi-wavelength anomalous dispersion (MAD) and single-wavelength anomalous diffraction (SAD) researchers are now able to solve macromolecular structures in a matter of days or weeks, a process that required months, or even years, a decade ago. 

To correlate the adsorption and diffusion properties to the cation distribution in the structure of CaNaA zeolites, temperature programmed desorption, anomalous X-ray powder diffraction and quasi elastic neutron scattering experiments were performed. It is shown that water adsorption and diffusion behaviour differs with the calcium content. Fourier maps obtained from anomalous diffraction experiments near the Ca K-edge allow to identify the calcium crystallographic sites unambiguously. 

The scattering pattern of anomalous diffraction may considerably deviate from those observed in the Fraunhofer limit. This is of particular importance when analysing laser diffraction measurements (cf. Sect. 2.3.4.3). 

Since anomalous diffraction is only relevant for forward scattering, it is not meaningful to discuss other cross sections than the extinction one. A first order approximation for real refractive indices was provided by van de Hulst (1981, p. 176) ... 

S. Zumer, Light scattering from nematic droplets Anomalous diffraction approach, Phys. Rev. A 37, 4006-4015 (1988). 

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