Reflections centric

Table 3. The phasing tree for permuting 4 centric reflections without an origin...

This might seem of scant use in protein crystallography, since we have no centric space groups. Crystals of biological macromolecules, as previously pointed out, cannot possess inversion symmetry. Sets of centric reflections frequently do occur in the diffraction patterns of macromolecular crystals, however, because certain projections of most unit cells contain a center of symmetry. The correlate of a centric projection, or centric plane in real space, is a plane of centric reflections in reciprocal space. A simple example is a monoclinic unit cell of space group P2. The two asymmetric units have the same hand, as they are related by pure rotation, and for every atom in one at xj, yj, Zj there is an equivalent atom in the other at —Xj, yj, —zj. If we project the contents of the unit cell on to a plane perpendicular to the y axis, namely the xz plane, by setting y = 0 for all atoms, however, then in that... 

The two exponential tenns are complex conjugates of one another, so that all structure amplitudes must be real and their phases can therefore be only zero or n. (Nearly 40% of all known structures belong to monoclinic space group Pl c. The systematic absences of (OlcO) reflections when A is odd and of (liOl) reflections when / is odd identify this space group and show tiiat it is centrosyimnetric.) Even in the absence of a definitive set of systematic absences it is still possible to infer the (probable) presence of a centre of synnnetry. A J C Wilson [21] first observed that the probability distribution of the magnitudes of the structure amplitudes would be different if the amplitudes were constrained to be real from that if they could be complex. Wilson and co-workers established a procedure by which the frequencies of suitably scaled values of F could be compared with the tlieoretical distributions for centrosymmetric and noncentrosymmetric structures. (Note that Wilson named the statistical distributions centric and acentric. These were not intended to be synonyms for centrosyimnetric and noncentrosynnnetric, but they have come to be used that way.)... 

A normalized centric index C was also introduced in order to reflect the topological shape of alkanes. By definition, C = 0 for n-alkanes (chain-graph) ... 

This index provides a measure of molecular branching the higher the value of B, the more branched the tree. It is called centric index because it reflects the topology of the tree as viewed from the centre. 

Table 11.2 shows 2003 production of both sawn timber and wood panels for various regions of the world. Structural panel production is dominated by North America, mainly because structural panel products have enjoyed a dominant position in residential construction. The United States is a substantial manufacturer of softwood plywood for domestic production but only 10% is exported whereas half that volume is imported as tropical hardwood plywood. Europe manufactures and uses comparatively little plywood and OSB. Instead the region relies on its own lower quality domestic wood resources for the manufacture of other wood-based panels, e.g. particleboard and fibre-based board. Amazingly, panel production in Asia equals that of lumber, reflecting the inter-regional China-centric supply chain. 

Patient-centric medicine is essentially personalized medicine, but with the accent only slightly taken off the pharmacogenomics and placed on the broader picture that essentially corresponds to holistic medicine to be discussed later. These distinctions are subtle and fragile, and it is perhaps clearer and truer to say that patient-centric medicine is a corporate strategy in the IT industry. But this is good it reflects the deep recognition by the IT industry of the kinds of principles discussed in this book. 

Miscellaneous techniques. The structure of Theta one(ll) was solved with neither ab initio calculations nor modelling, but by permuting all possible assignments of phases for a small number of reflections and then examining the Fourier maps. This may seem a tedious way to solve an unknown structure, but it was successful. The technique may indeed be a way of solving molecular sieve structures with centric space groups should all else fail. 

The end of the literature-centric system is well reflected in the evolution of literary journals. Symbolic of the change in the status of literature was the drop in circulation of thick literary monthhes today s printings constitute... 

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