Direct phases

Hendrickson WA, Smith JL, Sheriff S. Direct phase determination based on anomalous scattering. Methods Enzymol 1985 115 44-55. 

Bricogne, G. (1993) Direct phase determination by entropy maximization and likelihood ranking status report and perspectives, Acta Cryst., D49, 37-60. 

Diazotization and formation of diazo dyes affords a general approach to pre-column derivatization, to be followed by direct phase or RP-HPLC with UVD or FLD. Thus,... 

Direct phase-transfer catalysed epoxidation of electron-deficient alkenes, such as chalcones, cycloalk-2-enones and benzoquinones with hydrogen peroxide or r-butyl peroxide under basic conditions (Section 10.7) has been extended by the use of quininium and quinidinium catalysts to produce optically active oxiranes [1 — 16] the alkaloid bases are less efficient than their salts as catalysts [e.g. 8]. In addition to N-benzylquininium chloride, the binaphthyl ephedrinium salt (16 in Scheme 12.5) and the bis-cinchonidinium system (Scheme 12.12) have been used [12, 17]. Generally, the more rigid quininium systems are more effective than the ephedrinium salts. 

It should be noted that F is a vector quantity with magnitude and direction (phase angle). 

Giacovazzo C (1998) Direct Phasing in Crystallography. Fundamentals and Applications, Oxford University Press Jansen, MSLS, This volume... 

Key words electron diffraction, structure determination, direct phasing, structure refinement... 

The problem of direct phasing can be simplified by the following heuristic rules ... 

Giacovazzo, C. (1998). Direct phasing in crystallography. Fundamentals and applications Oxford Oxford University Press and lUCr. 

Randrianantoandro, N. Mercier, A.M. Hervieu, M. Greneche, J.M. (2001) Direct phase transformation from hematite to maghemite during high energy ball milling. Mat. Letters 47 150-158... 

Direct phasing in crystallography fundamentals and applications C. Giacovazzo... 

T. Whittaker, Math. Ann. 57, 333-355 (1903) (an excellent paper which decomposes the scalar potential into a harmonic set of bi-directional phase conjugate longitudinal wavepairs). 

In words, the difference Patterson function is a Fourier series of simple sine and cosine terms. (Remember that the exponential term is shorthand for these trigonometric functions.) Each term in the series is derived from one reflection hkl in both the native and derivative data sets, and the amplitude of each term is (IFHpI — IFpl)2, which is the amplitude contribution of the heavy atom to structure factor FHp. Each term has three frequencies h in the u-direction, k in the v-direction, and l in the w-direction. Phases of the structure factors are not included at this point, they are unknown. 

G. Direct phasing Application of methods from small-molecule crystallography... 

The Structure of Atidine.—An X-ray crystallographic analysis of atidine [C22H33NO3 m.pt 182.3—183.5 °C] has been reported.21 This compound has been isolated from plants of Aconitum heterophyllum Wall. Its structure was solved by direct phasing methods and refined to a final R of 0.045. Since atidine... 

However, diffraction methods have severe drawbacks. Disordered crystals are often difficult to tackle. If the disorder is of dynamic nature, e.g. arising from small-or large-amplitude motions in the crystal, the use of devices for variable-temperature measurements is compulsory and can also yield very useful information (see below for some examples) on the existence of enantiotropic systems related by phase transitions. In some, not frequent, cases the crystals are sufficiently robust to be used for direct phase transition measurements on the diffractometer. Figure 3 shows an example of multiple diffraction data sets collected on the same specimen... 

In the area of surface science, a GA has been applied [105] for the direct phasing of surface diffraction data as part of a strategy to determine surface structure using direct methods. The method has been applied to both centrosymmetric and noncentrosymmetric two-dimensional structures. 

Fokine, A., Morales, R., Contreras-Martel, C., Carpentier, P., Renault, F., Rochu, D., ChahriCTe, E. (2003). Direct phasing at low resolution of a protein co-purified with human paraoxonase (PONl). Acta Cryst. D59 2083-7. 

Direct methods, direct phase determination A method of deriving relative phases of diffracted beams by consideration of relationships among the Miller indices and among the structure factor amplitudes of the stronger Bragg reflections. These relationships come from the conditions that the structure is composed of atoms and that the electron-density map must be positive or zero everywhere. Only certain values for the phases are consistent with these conditions. 

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