Shape pocket

The smaller p -tert-butyl-calix [4] arenes have a rich coordination chemistry as well (65). Of these, however, only the upper-rim modified calixarenes seem to support metal complexes with confined binding sites (66), except in those cases where the lower-rim substituents form an appended cavity. Thus, Matt and coworkers have reported (67) a pocket-shaped calix[4]arene ligand L2 bearing two lower-rim [([Pg.410]

Reaction of trans-peroxynitrite with superoxide dismutase. The placement of positively charged amino acids around the active site facilitates the attraction of the negatively charged peroxynitrite anion. Because the copper of superoxide dismutase is buried in a pocket shaped to accommodate superoxide, only peroxynitrite in the trans configuration should be able to fit in the active site. Because the predominant form of peroxynitrite is the cis form, the isomerization from the cis to trans geometry limits the reaction of peroxynitrite with superoxide dismutase. 

It would have been impossible to identify the ligand in Figure 7.1(b) with the pocket shape suggested by the pocket extraction software. This example illustrates some of the potential pitfalls of automated receptor-based techniques. 

The pocket shape can affect the compacting characteristics of a roll system in a manner that is contrary to the basic theories of press action predicting that overall compaction is higher in a press in which the ratio of roller radius to mean cup depth is higher. 

Figure 252. Schematic representation of two extreme pocket shapes, (a) Half circle, no briquette production possible due to release difficulties, (b) Rationally shaped pocket (tear drop)...

Generally, three factors must be considered in optimizing the pocket shape for easy release of briquettes ... 

The detailed release geometry. This is governed by the pocket shape. A pillow shape with the axis of its partial cylinder across the rollers and conical sides of wide angle will probably be the best. It is suggested that at no point on the cup surface should the normal to the surface differ in direction from the roll radius by more than 65° (Figure 224). 

Design of the roller surfaces In designing a roller press surface, some fundamental considerations do influence the possible shape, size, and distribution of the cups or indentations.Limitations include the relationships of circumferential pocket dimension/roller circumference and pocket depth/roller diameter as well as the pocket shape. Associated with these are problems of release, clam-shelling (splitting), extrusion, removal of air from the compaction zone, and uneven distribution of feed as well as compaction pressure. In spite of those limitations, a large variety of shapes can be produced. 

Figure 277 is a photograph showing different products from roller presses. Application of electrochemical milling (ECM see below and Figure 281) provides complete freedom to produce pocket shapes. 

Electrochemical milling (ECM) offers complete freedom of choice of material and pocket shape. This technique employs a liquid electrolyte to erode the roll or segment material thereby reproducing exactly the shape of the electrode (Figure 281). Using this method, not only finally machined and hardened materials can be treated economically without any distortion but also no limitation whatsoever exists regarding pocket shape. The ECM technique can reproduce such detail that even identifying marks can be incorporated. 

Number of pockets Pocket shape Size of pockets... 

A refr%erated pocket-shaped receiver Is used to sublime an excess of NgOg directly onto CI2O in h h vacuum. The receiver... 

Eyeglass case An eyeglass case holds sunglasses and reading glasses and protects them while carrying them in a bag or pocket. Shapes and constmction vary widely, but it always resembles and closely fits the eyewear. 

Fig. 2. Carbohydrase subsite nomenclature according to Davies and colleagues (49). (a) Subsite numbering in a polysaccharide enc o-hydrolase, exemplified by the extended binding cleft of Humicola insolens Cel6A (49). (b) Subsite numbering in a hypothetical exo-/S-glucosidase with a pocket-shaped active site that recognizes the nonreducing end of an oligosaccharide substrate.

A template approach has been invoked by Stack et al. to immobilize a metastable [Fe"(OTf)2(phen )2] (phen , substituted 9,10-phenanthroline) complex 12 in a preformed binding pocket shaped by a thermodynamically stable [Cu(phen )2] template. After replacing Cu+ by Fe +, the iron(II)-containing polymer 12 was obtained. A [Fe (OTf)2(phen )2] intermediate is unstable in a homogeneous solution and rapidly leads to the tris(chelate) [Fe(phen )3] + complex and Fe + ions [21]. Thus, the matrix prevents ligand scrambling and again kinetically stabilizes a metastable species by site isolation. 

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