Pleated-sheet model

Banding has been observed in both lyotropic and thermotropic polymers examined by optical and electron imaging techniques [430, 431, 439-446]. Incomplete extinction has been observed for some of the aramids where 1 ands, normal to the fiber axis, are observed in polarized light [440] (Fig. 5.88). It is now weU known that the aramids exhibit axial banding having periodicities of about 500 nm, which is observed by dark field TEM [447]. Sinunens and Hearle [440] have proposed that the optical observations and the pleated sheet model of Dobb et ah [447] are compatible and that the optical bands are the bends or folds between the pleats which might well exhibit the local density differences observed by DF TEM. 

TEM [363]. Simmens and Hearle [358] have proposed that the optical observations and the pleated sheet model of Dobb et al. [363] are compatible and that the optical bands are the bends or folds between the pleats which might well exhibit the local density differences observed. 

Michael reactions and, 895 Beta-keto ester, 851 alkylation of, 859-860 cyclic, 892-893 decarboxylation of, 857, 860 Michael reactions and. 895 pKd of, 852 synthesis of, 892-893 Beta-lactam antibiotics, 824-825 Beta oxidation pathway, 1133-1137 mechanism of, 1133-1136 Beta-pleated sheet (protein), 1038 molecular model of, 1039 secondary protein structure and, 1038-1039 Betaine, 720 Bextra. structure of, 544 BHA, synthesis of, 629 BHT, synthesis of. 629 Bicycloalkane. 129 Bijvoet. J. M., 299 Bimolecular, 363... 

Molar absorptivity. 502 Molecular ion (M+), 410 Molecular mechanics. 130 Molecular model, dopamine, 930 acetaminophen, 29 acetylene, 18 adenine, 67 adrenaline, 323 alanine, 28, 1016 alanylserine, 1028 rr helix, 1039 p-aminobenzoic acid, 25 anti periplanar geometry, 387 a recoline, 79 aspartame, 29 aspirin. 17 ball-and-stick, 61 /3-pleated sheet, 1039 p-bromoacetophenone, 449 bromocyclohexane, 121 butane, 80... 

In an early inaccurate model for the structure of biological membranes, a phospholipid bilayer was coated on both sides by protein in an unfolded or jS-pleated sheet conformation. This model reflected the prevailing view of membrane structure from about 1940 until the early 1970s. 

Fig. 13. Schematic model of a PPT fibre showing radially oriented axially pleated sheets 91)...

In Fig. 30, a three-dimensional model is presented in which only the organic phases are shown. Hexagonal plates of MM alternate with pleated sheets of CP. The hydrophobic sides of MM are facing each other and encase the mineral phase. The relationship between hydrophobic bonding and accessible surface area in proteins, and the effect of polar and non-polar side groups on free energy values has recently been discussed246. For informations on hydrophobicity in protein systems see Refs.247-252. ... 

In pleated sheets, we know that successive carbonyl oxygens point in opposite directions. One or two carbonyls whose orientations are clearly revealed by the map can allow sensible guesses as to the positions of others within the same sheet. As mentioned previously with respect to map fitting, we use knowledge of protein structure to infer more than the map shows us. If our inferences are correct, subsequent maps, computed with phases calculated from the model, will show enhanced evidence for the inferred features and will show additional features as well, leading to further improvement of the model. Poor inferences degrade the map, so where electron density conflicts with intuition, we follow the density as closely as possible. 

Plate 15 A selection of common types of computer graphics models, all showing the same three strands of pleated-sheet structure from cytochrome b5 (PDB 3b5c). (a) Wireframe (b) ball and stick (c) space filling (d) ribbon backbone with ball-and-stick side chains. (For discussion, see Chapter 11.) Image SPV/POV-Ray. (Continues)... 

Thus, we feel that the a-helical and the extended" helical structure are well established in VCD, and that there exists a simple method for the interpretation of the data. The VCD features of the B-pleated sheet structure appear reasonably well established, too, although its interpretation is much more difficult. Since the data are mono-signate, the DECO model is not appropriate (it always predicts conservative couplets). Nafie and coworkers explained such monosignate VCD in terms of a model similar to one described earlier by Schellman [24], with nearly co-linear (and antiparallel) electric and magnetic dipole transition moments [30]. 

The result was one of the most extraordinary sets of papers in 20th-century science. Seven appeared together, dominating the May 1951 issue of the Proceedings of the National Academy of Sciences. There was a detailed description of the pleated sheet for silk. There was a new model for the protein in feathers, and new ideas about the structure of artificial proteins, globular proteins, and muscle. 

The jS form of keratin requires still additional models. And, continuing the order of decreasing certainty, these models are again less well corroborated by experimental observations than are the a-helix or a-keratin structures. Pauling and Corey (1598) presented the pleated sheets to explain /3-keratins. These sheets are made up of extended peptide chains H bonded essentially side by side. Two are shown in Fig. 10-7. 

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