Modular structure

In the previous sections composition variation has been attributed more or less to point defects and extensions of the point defect concept. In this section structures that can be considered to be built from slabs of one or more parent structures are described. They are frequently found in mineral specimens, and the piecemeal way in which early examples were discovered has led to a number of more or less synonymic terms for their description, including intergrowth phases, composite structures, polysynthetic twinned phases, polysomatic phases, and tropochemical cell-twinned phases. In general, they are all considered to be modular structures. 

Modular structures can be built from slabs of the same or different compositions, the slabs widths can be random or regular, and the slabs themselves can be ordered in a variety of ways (Fig. 4.19). Composition variation can then occur by variation of the amount of each slab type present, by variation in the degree of order present, or, when slabs have the same composition, by changes in atom ratios introduced at the slab boundaries. In addition, the planar boundaries that divide up a modular structure create new coordination polyhedra in the vicinity of the fault that are not present in the parent structure. These may provide sites for novel chemical reactions or introduce changes in the physical properties of significance compared to those of the parent structure. 

Apart from structures that are built of slabs, modular structures that can be constructed of columns in a jigsawlike assembly are well known. In the complex chemistry of the cuprate superconductors and related inorganic oxides, series of structures that are described as tubular, stairlike, and so on have been characterized. Alloy structures that are built of columns of intersecting structures are also well known. Structures built of linked columns, tunnels, and intersecting slabs are also found in minerals. Only one of these more complex structure types will be described, the niobium oxide block structures, chosen as they played a significant role in the history of nonstoichiometry. 

The difference in the two values is surprisingly large. The experimental value of the density, 5728 kg m-3, is in good accord with Model B, which assumes vacancies on the iron positions. This indicates that the formula should be written Fe0.945O, and that the point defects present are iron vacancies. 

The unit cell of the disordered material is the same as the unit cell of the parent phase, although the diffraction pattern will reveal evidence of this disorder in the form of diffuse or streaked diffraction spots, (see also Section 8.6). 

If the width of the component slabs in a modular structure is constant, no defects are present in the structure, because the unit cell includes the planar boundaries between the slabs as part of the stmcture. Take a case where two different slab widths 

As these two examples show, besides altered unit cell dimensions, a series of modular structures may or may not show a regular variation of composition. When only one slab type is present this will depend upon the nature of the planar boundaries involved. However, a change of composition must occur if the series of 

An examination of these structures shows that the overall composition is no longer SrTi03. The faulting arises because the crystal contains some Nb205. This gives each crystal a composition Sr(NbixTi1 x)03+ ( . Each Nb5+ ion that replaces a Ti4+ ion requires that extra oxygen 

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MMP genes show a highly conserved modular structure. They were first detected in vertebrates (1962), and human beings, but have since been found in invertebrates and plants as well. The MMP family... 

All NR proteins exhibit a characteristic modular structure that consists of five to six domains of homology (designated A-F, from the N-terminal to the C-terminal) based on regions of conserved sequence and function (Fig. 2). 

SHRs are built in a modular structure with similar structure elements. They contain a DNA-binding domain (DBD), a hinge region with a nuclear location signal (NLS), a ligand-binding domain (LBD) and several transcriptional activation functions (Fig. 1). 

The OPR developed by Alfa-Laval Vicarb is based on a modified HEX design [14] and consists of a modular structure built by stacking different plates (Figure 12.1) related to the circulation of reaction medium and utihty fluid (UE). ... 

The concept of the reactor is based on a modular structure built by the alternative stacking of reaction plates and of utility plates containing the thermal fluid (Figure 12.4). The reaction and utility plates are made of SiC the end plates are... 

Since the modular structure of (13a) is easily amenable to parallel synthesis, extensive structure-activity studies were carried out with this compound. This generated not only a large body of structure-activity data, but also led to a remarkable optimization of the metabolic and pharmacokinetic properties of (13a). 

Bacitracin biosynthesis requires a non-ribosomal peptide synthetase with three major protein components, BacABC. This synthetase has a modular structure. There are associated regulatory and transport systems. Biosynthesis of bacitracin has been engineered in the surrogate host, B. subtilis, by genetic techniques. A strain, B. subtilis KE 350, expresses the entire 49-kb bacitracin... 

A remarkable number of chiral phosphorus ligands (phosphoramidites, phosphites, and phosphines with modular structures) have been introduced into the copper-catalyzed conjugate addition of R2Zn reagents, and high enantio-selectivities (>90%) are now possible for all three different classes of substrates 2-cyclohexenones and larger ring enones, 2-cyclopentenones, and acyclic enones. 

Take care to note that a modulated structure is not the same as a modular structure. 

Modular structures are those that can be considered to be built from slabs of one or more parent structures. Slabs can be sections from just one parent phase, as in many perovskite-related structures and CS phases, or they can come from two or more parent structures, as in the mica-pyroxene intergrowths. Some of these crystals possess enormous unit cells, of some hundreds of nanometers in length. In many materials the slab thicknesses may vary widely, in which case the slab boundaries will not fall on a regular lattice and form planar defects. 

The PHOX ligands have a modular structure (Scheme 29.1). They are synthesized from chiral amino alcohols and benzonitrile or bromobenzonitrile the... 

Figure 15.12 The modular structure of methionine synthase. The four domains are connected by flexible hinges, which allow the CH3tetrahydrofolate-, AdoMet- or homocystein-binding domains to alternatively access the B12-binding domain. (From Bannerjee and Ragsdale, 2003. Reprinted with permission from Annual Reviews.)...

The core of the electrolyser is the cell block, which is made up of a large number of usually bipolar cells in a modular structure.8 Typical sizes of a cell block range from 1 to 800Nm3/h. (Most electrolysers sold today to laboratories, the semiconductor industry, etc., have a capacity less than 60 Nm3/h.) The biggest capacities realised are about 150 MW. An alkali solution, usually 20% to 40% potassium hydroxide (KOH), is used as the electrolyte that flows between the electrodes. In alkaline solutions, the electrodes must be resistant to corrosion, have good electronic... 

Emphasis to a similar approach has been given by Zvyagin (1993). He pointed out that many crystal structures can be represented as a composite of certain standard construction modules and various combinations, distributions and arrangements of these. The simplest example of a modular structure is the densest packing... 

The cytoplasmic NAD-reducing hydrogenase (SH) of the bacterium R. eutropha is a heterotetrameric enzyme, which contains several cofactors (Friedrich et al. 1996 Thiemermann et al. 1996). The Ni-containing subunit is called HoxH. This subunit plus the small subunit HoxY form the strictly conserved hydrogenase module with the Ni-Fe centre and a proximal [4Fe-4S] cluster. HoxF and HoxU represents the Fe-S/flavoprotein moiety which is closely related to a similar moiety in NADHrubiquinone oxidoreductase. The SH has been subject to molecular biological techniques in order to study its modular structure, mechanism and biosynthesis. 

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