Barrel distortion

Fig. 105. Examples of small disulfide-rich or metal-rich proteins (shown on the right side) compared with their more regular counterparts in other structural categories (shown at the left), (a) Tobacco mosaic virus protein, an up-and-down helix bundle (b) cytochrome bs, a distorted up-and-down helix bundle (c) trypsin domain 1, a Greek key antiparallel /3 barrel (d) high-potential iron protein, a distorted Greek key /3 barrel (e) glutathione reductase domain 3, an open-face sandwich fi sheet (f) ferredoxin, a distorted open-face sandwich f) sheet.

A ribbon diagram of the trimer is shown in Figure 4B. Each monomer contains an eight-stranded (3 sheet and a five-stranded (3 sheet that join to form a distorted (3 barrel. Seven a helices surround this (3 sheet structure. 

Xiao et ah, 2007). The results indicate that the regular parallel lamella structure (the concentric cylinder barrel structure) appears if d and L0 are compatible in both flat and curved confinements, while a vertical or distorted vertical lamellae structure (sector column structure) is observed otherwise. Upon increasing the curvature of the exterior surface K = 1 /ReXr the compatibility span of d and L0 becomes smaller consequently, the formation of the parallel lamella structure (or the concentric cylinder barrel structure) is more difficult. Our MC results in this work are shown in Figure 28 and more details can be referred to (Xiao et ah, 2007). 

An excellent historical review and applications of this type of extruder were produced by White (53), and the flow was modeled by Janssen et al. (62) and Wyman (63), whose derivation we follow. The easiest way to visualize the conveying mechanism of a counterrotating intermeshing TSE is to place a robot viewer into the screw channel at point 0 in Fig. 6.37(d) and let it report its observations. We first stop the rotation of the screws and tell our viewer to explore the space around it. It will report that the space is entirely confined by steel walls the barrel surface from above, the flights of screw A at either side, and the intermeshing flight of screw B both up-channel and down-channel. The space is a helically distorted C-shaped channel that can be shown schematically, as in Fig. 6.46, or by molding silicone rubber into the space in Fig. 6.47. Clearly, by fully... 

Sometimes a plastic buffer material (usually white granulated polyethylene) is mixed with the shot to help prevent distortion during discharge. The buffer material may be found in the wound channel in close range shootings. Another innovation to prevent distortion is the use of a shot cup that encloses the shot as it travels through the barrel, with the shot cup falling away when it exits the muzzle. 

Many polymers exhibit neither a measurable stick-slip transition nor flow oscillation. For example, commercial polystyrene (PS), polypropylene (PP), and low density polyethylene (LDPE) usually do not undergo a flow discontinuity transition nor oscillating flow. This does not mean that their extrudate would remain smooth. The often observed spiral-like extrudate distortion of PS, LDPE and PP, among other polymer melts, normally arises from a secondary (vortex) flow in the barrel due to a sharp die entry and is unrelated to interfacial slip. Section 11 discusses this type of extrudate distortion in some detail. Here we focus on the question of why polymers such as PS often do not exhibit interfacial flow instabilities and flow discontinuity. The answer is contained in the celebrated formula Eqs. (3) or (5). For a polymer to show an observable wall slip on a length scale of 1 mm requires a viscosity ratio q/q equal to 105 or larger. In other words, there should be a sufficient level of bulk chain entanglement at the critical stress for an interfacial breakdown (i.e., disentanglement transition between adsorbed and unbound chains). The above-mentioned commercial polymers do not meet this criterion. 

Continuum Depiction Local oscillation of the melt-wall boundary condition in the exit region causes peturbations on the exit stress and die swell Oscillation of the overall stress due to unstable boundary condition produces cycles of melt compression and decompression in the barrel and fluctuations in the extrude swell The extrudate distortion arises from formation of secondary flow (vortices) in the barrel due to the strong converging flow near the die entry... 

The existence of -barrels was established for chymotrypsin at a very early stage in the now common protein crystal structure analyses. This enzyme contains two distorted six-stranded -barrels with identical topologies (Birktoft and Blow, 1972). A selection of -barrels in water-soluble proteins is given in Table I. The very abundant TIM-barrel consisting of eight parallel /1-strands was also detected rather early (Banner et al., 1975). Additional eight-stranded /1-barrels of this group are those of streptavidin (Hendrickson et al., 1989) and of the lipocalins (Newcomer et al., 1984). 

Crystal structures exist of two bacterial PI-PLC enzymes, the protein from B. cereus (Heinz et al., 1995), which can cleave GPI-anchors, and the PI-PLC from Listeria monocytogenes (Moser et al., 1997), which is not able to effectively release GPI-anchored proteins. While the sequence homology of these two proteins is limited, the structures are very similar. The bacterial PI-PLC proteins are folded into a distorted TIM-barrel, where the parallel (3-strands form an inner circular and closed barrel with a-helices located on the outside between neighboring (3-strands, that is structurally very similar to the catalytic domain of PLC8j, the only mammalian PI-PLC for which there is a structure (Essen et al., 1996 Heinz et al., 1998). The availability of structures and results of mutagenesis provide details on the catalytic mechanism for this type of enzyme (for review and more extensive references see Mihai et al. (2003)). 

The first structure was of the bluetongue virus VP7 protein (Grimes et al, 1995), which has two domains. The central part of the sequence (residues 121-249) forms a (3 sandwich (or distorted barrel) most like... 

Wrap a piece of tape around the 15/64" drill bit, 1-1/8 from the front. Carefully drill out the barrel to the depth of the tape mark, USe plenty of oil and don t let the hole become out of round. This drilling won t enlarge the barrel, it just removes the lands which cause the bullet to spin. It also removes the burrs caused by drilling the 1/8" holes. The undrilled portion of the barrel will still impart the needed spin, when the bullet reaches the drilled portion it is better to have the surface smooth to minimize bullet distortion as H passes the drilled holes. If a 15/64 drill is not a-vailable, a % may be used,... 

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