Two-sheet

This system has been applied also on a weld containing real defects. It consists of two sheets in ordinary steel, of equal thickness (35 mm), welded with a chamfer in X. 

In other areas, POD has been used to improve the wear resistance of a mbber latex binder by incorporation of 25% of Oksalon fibers. Heat-resistant laminate films, made by coating a polyester film with POD, have been used as electrical insulators and show good resistance to abrasion and are capable of 126% elongation. In some instances, thin sheets of PODs have been used as mold release agents. For this appHcation a resin is placed between the two sheets of POD, which is then pressed in a mold, and the sheets simply peel off from the object and mold after the resin has cured. POD-based membranes exhibit salt rejection properties and hence find potential as reverse osmosis membranes in the purification of seawater. PODs have also been used in the manufacturing of electrophotographic plates as binders between the toner and plate. These improved binders produce sharper images than were possible before. 

Fig. 9. A de novo designed P sheet protein, betabellin, formed by the dimerization of two identical four-stranded -sheets and a disulfide linking the two sheets. This model is for betabeUins 9 and later progenies the earher betabeUins contained a two-armed cross-linker connecting the sheets (51).

Fiae sihca particles are iacorporated ia the surface of plastic films to preveat the adhesioa of two sheets ia coatact with oae another. The apphcation is called antiblocking. The relatively low refractive iadex of the sihca particles makes them more difficult to detect. 

The jelly roll barrel is usually divided into two sheets... 

Figure S.28 Schematic diagrams of the two-sheet P helix. Three complete coils of the helix are shown in (a). The two parallel P sheets ate colored gieen and red, the loop regions that connect the P strands ate yellow, (b) Each stmctuial unit Is composed of 18 residues forming a P-loop-P-loop structure. Each loop region contains six residues of sequence Gly-Gly-X-Gly-X-Asp where X is any residue. Calcium Ions are bound to both loop regions. (Adapted from F. Jumak et al., Ciirr. Opin. Struct. Biol. 4 802-806, 1994.)...

In these p-helix structures the polypeptide chain is coiled into a wide helix, formed by p strands separated by loop regions. In the simplest form, the two-sheet p helix, each turn of the helix comprises two p strands and two loop regions (Figure 5.28). This structural unit is repeated three times in extracellular bacterial proteinases to form a right-handed coiled structure which comprises two adjacent three-stranded parallel p sheets with a hydrophobic core in between. 

The basic structural unit of these two-sheet p helix structures contains 18 amino acids, three in each p strand and six in each loop. A specific amino acid sequence pattern identifies this unit namely a double repeat of a nine-residue consensus sequence Gly-Gly-X-Gly-X-Asp-X-U-X where X is any amino acid and U is large, hydrophobic and frequently leucine. The first six residues form the loop and the last three form a p strand with the side chain of U involved in the hydrophobic packing of the two p sheets. The loops are stabilized by calcium ions which bind to the Asp residue (Figure S.28). This sequence pattern can be used to search for possible two-sheet p structures in databases of amino acid sequences of proteins of unknown structure. 

The number of helical turns in these structures is larger than those found so far in two-sheet p helices. The pectate lyase p helix consists of seven complete turns and is 34 A long and 17-27 A in diameter (Figure 5.30) while the p-helix part of the bacteriophage P22 tailspike protein has 13 complete turns. Both these proteins have other stmctural elements in addition to the P-helix moiety. The complete tailspike protein contains three intertwined, identical subunits each with the three-sheet p helix and is about 200 A long and 60 A wide. Six of these trimers are attached to each phage at the base of the icosahedral capsid. 

The canonical jelly roll barrel is schematically illustrated in Figure 16.13. Superposition of the structures of coat proteins from different viruses show that the eight p strands of the jelly roll barrel form a conserved core. This is illustrated in Figure 16.14, which shows structural diagrams of three different coat proteins. These diagrams also show that the p strands are clearly arranged in two sheets of four strands each P strands 1, 8, 3, and 6 form one sheet and strands 2, 7, 4, and 5 form the second sheet. Hydrophobic residues from these sheets pack inside the barrel. 

The relations (pA)a = (fijdp 5 (Fb) = (Pb)p would only by the merest chance form the solution of (2), hence there will not in general be a partition equilibrium between the ions when one is established between the neutral.molecules, but one solvent, say a, will contain more A ions than corresponds with ionic partition equilibrium. These will pass through the surface of contact into /3, and similarly B ions from /3 to a. The separation of the two kinds of ions will however set up an electrostatic field across the boundary, and the two kinds of ions collect there in two sheets very close together—in fact, we have an electrical... 

The charges on the two sheets composing the double layer are due to the accumulated ions, hence, from Faraday s law ... 

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