Pleat formation

Pleat formation can be avoided though the following changes on the mold set (Figure 1.169 (b)) ... 

Membrane cartridge filters are extremely flexible and high in tensile strength. The cartridge construction is based on a multi-layer combination of filter media in pleated format. Those polymers that have been used extensively as filtration media in coarser grades are now widely used as membrane filters. A typical format has a cartridge fabricated from a pleated filter pack, which contains a very fine polyolefin fibre prefilter layer, two nylon membranes of the same pore (0.2 pm) size, and a downstream polypropylene support. The layers of nylon microporous membrane and polypropylene prefilter are pleated together and snpported by an inner snpport core. The end-caps and core are melt sealed in polypropylene. 

FIGURE 5.8 Two structural motifs that arrange the primary structure of proteins into a higher level of organization predominate in proteins the a-helix and the /3-pleated strand. Atomic representations of these secondary structures are shown here, along with the symbols used by structural chemists to represent them the flat, helical ribbon for the a-helix and the flat, wide arrow for /3-structures. Both of these structures owe their stability to the formation of hydrogen bonds between N—H and 0=C functions along the polypeptide backbone (see Chapter 6). 

Comparisons between these toxins allow delineation of the variability of each position in the sequence. For instance, the residues which are extremely invariant (conservative) for both types of sea anemone toxin are the half-cystines, certain glycyl residues which are expected to be involved in )9-turns, and only a few other residues - Asp 5 or 6, Arg 13 or 14, and Tryp 30 or 31 (the numbering depends upon the toxin type) — expected to be important for folding or receptor binding. Rather surprising is the variation in the residues which NMR studies (22,23) have shown are involved in formation of the four stranded )9-pleated sheet. 

The thermal stability of these silicones is very high and even at 200° C over months no racemization of the chiral groups can be detected. We attribute this phenomenon to the formation of a specific pleated-sheet structure in an apolar environment, which stabilizes the L-configuration and prevents inversion at the as-symmetric carbon. 

The two major secondary structures found in nature— the helix and the sheet—are also major secondary structures found in synthetic polymers. The helix takes advantage of both the formation of intermolecular secondary bonding and relief of steric constraints. Some materials utilize a combination of helix and sheet structures such as wool, which consists of helical protein chains connected to give a pleated sheet. 

A third group is one in which the hydrogen bonding results in the formation of extended sheets, leading to structures with easy cleavage, as for instance, in succinimide (Fig. 8), or in glycine. Intermediate structures may occur in the form of ribbons or of slightly pleated sheets, such as those of methionine or of formamide. 

Ultrastructural patterns that arise when amino acids or small peptides interact with mineral surfaces have been studied in some detail99,100). A poly-L-alanine solution evaporated at 40 °C on a rhombohedral plane of R-quartz deposits the peptide principally in a-conformation. Chain-folded helices are aligned in the form of lamellae which exhibit a sharp phase boundary at the organic-mineral contact zone (Fig. 9). Frequently the lamellae are split along the direction of their fold axis ("zipper effect ). Insertion of 0-pleated sheets running perpendicular to the long axis of the lamellae act as dispersion forces and cause the formation of cross-0-... 

B is correct. The amino add bias and shapes of the secondary structures (a-hetix and -pleated sheets) is partially explained by the rigid structure of the peptide bond, whose double bond character prevents rotation. This rigidity provides steric constraints on hydrogen bond formation for some amino acids that precludes them from partidpating, or even breaking secondary structure. 

Assuming the pleated sheet-like motif and the ribbon-like motif as reference H-bonded core structures, a further increase in the steric demand of the residues, as for example the presence of aromatic rings as substituents on the diol or diamine moiety, or the absence of the required trans configuration, lead to a loss of efficiency in the recognition process with formation of incomplete ribbon-like or staircase-like structures. In the latter cases, competition experiments show that matched complexes with the pleated sheet-motif or with the ribbon-like motif of the core crystallize more easily than those characterized by incomplete or low organized core structures. Thus, when the complex 29 38 was melted with 1 equiv. of (i ,i )-tra x-l,2-cyclohexanediol (31) and the mixture was crystallized from benzene, again 29 31 was obtained as the only crystalline product (Scheme 22) [60]. 

This type of hydrogen bonds includes the N-H 0=C interactions which are the most predominant hydrogen bonds in fibrous and globular proteins. Because they are responsible for the formation of the commonly occurring secondary structure elements a-helix, -pleated sheet and / -turn, a large body of much less accurate data is available from protein crystal structures which will be analyzed in Part III, Chap. 19. The N-H 0=C type hydrogen bond is also the most common in the purine and pyrimidine crystal structures (Thble 7.14), and is one of the two important bonds in the base pairing of the nucleic acids. 

In filtration unit operation, especially in microfiltration, one usually differentiates between dead-end filtration (with cake formation) and cross-flow filtration [25] (Fig. 5). The cross-flow filter can have different geometries (Fig. 6) phase membranes, tubular membranes, or pleated membranes, of which the tubular and pleated ones are already accepted as cross-flow geometries in reactor technology, as mentioned above. In filtration engineering the cross-flow term means that the filtrate flows perpendicularly to the suspension stream. Cross-flow may not be considered a sufficiently illustrative term here [25]. A better term would be parallel filtration, but the term cross-flow filtration has been accepted generally and may be difficult to change at present. 

Direct flow filters are available typically in cartridge format, such as 10-40 in. (25-100 cm) lengths. Cartridges contain membranes that are pleated to provide the maximum amount of surface area within a single cartridge. Direct flow filters are also available as self-contained disposable capsules that do not require the use of housings. The ease of use associated with capsules has increased their use. 

Vims filtration membranes are made of various polymers such as hydrophilized PVDF, hydrophilized PES, and cellulose. They are available in pleated cartridge and hollow-fiber formats. At present almost all vims filtration applications are performed in the direct filtration mode. 

APPENDIX C. Pleat Capacities and Property Changes of Formation... 

Shifting from one interdisciplinary nomenclature to another we can view the bidentate molecule as an amino acid, the amide becomes a peptide and the polyamide a polypeptide or a protein. Hence, we have abjured organic chemistry in favour of biochemistry. Proteins are built up from approximately 20-25 different a-amino acids, the individual order of which decide the chemical and physical properties of a particular protein. Due to a combination of certain attributes of e peptide linkage, and the presence of functionalities enabling the formation of hydrogen bonds, protein strands fall into one of three geometrically different categories random coil, a-helix and pleated sheet. 

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