Poly pleated sheet conformations

Pleated sheet conformation, 30,31 PLEDs (polymeric light-emitting diodes), 218 Plexiglas, 62 Plunkett, Roy, 65-66 PMMA. See Poly(methyl methacrylate) Polartec (Polar Fleece), 194 Poly(6-aminohexanoic acid), 25 Poly(a methyl styrene), 20 Polyacetylene, 72, 73 Polyacrylamide, 20 Polyamides, 22, 28, 61, 146 biodegradable, 185 Polyaramids, 77, 86 Polybutadiene, 70,109,148,155 Poly butyl acrylate), 20 Poly(butylene isophthalate), 25 Polycaprolactam, 21 Polycarbonate (PC), 17, 48, 86, 140 biodegradable, 185 density of, 247 impact strength of, 143 permeability of, 163 Polychloroprene, 65 Polycondensation, 85, 90-91 interfacial, 91-92... 

Figure 23. Simultaneous resolution of circular dichroism (A) and absorption (B) curves of poly-L-serine in the antiparallel -pleated sheet conformation. The critical values for the resolved bands are included in Table 2B. Reproduced, with permission, from [73],...

Figure 24. A, Stereo pair plot of poIy-L-alanine in the antiparallel -pleated sheet conformation of Pauling and Corey [47]. Sheet is tilted 10°. Stereo pairs arranged for cross-eye viewing. B, Stereo pair plots of poly-L-alanine in the parallel -pleated sheet conformation of Pauling and Corey [47]. Stereo pairs are arranged for cross-eye viewing.

Figure 15.1 DOS plots corresponding to optimum GA compositions of aperiodic poly (gly-ala-ser) in (a) a-helical conformation and (b) p-pleated sheet conformation.

The fibers of the helical a-form of poly(L-leucine) are also wool-like but have not been commercialized. They have only been included in Table 38-3 to demonstrate the influence of macromolecular conformation on the fiber properties. For example, the P form or pleated-sheet conformation of poly(L-leucine) has, of course, quite different properties it is silklike. 

Poly(L-tyrosine), which adopts the antiparallel pleated sheet conformation in water at low ionization, has also been studied by far ultraviolet c.d. It was suggested that lower molecular weight polymers possessed only a few polypeptide chains folded into long open-chain / -structures, whereas a higher molecular weight polymer took up a more compact shape, in which the /S-sheet had folded over on to itself into a double-layer. This could be regarded as the first prototype of tertiary structure observed in a -forming homopolymer. 

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-... 

Figure 25. Circular dichroism spectra of the classical polypeptide conformations extended into the vacuum ultraviolet region. Solid curve, a-helical pattern averaged from poly-L-alanine and poly(y-methyl-L-glutamate) data. Dashed curve, antiparallel /5-pleated sheet CD pattern due to films of Boc-(l -Ala)7-OMe [78]. Dotted curve, parallel /S-pleated sheet patterns were calibrated by solution spectra. Dash-dot curve, disordered collagen to provide a measure of a random structure. Reproduced, with permission, from [79].

The secondary structure covers the spatially arranged conformations produced by hydrogen bonding between peptide bonds such as helix sequences and pleated sheet structures. Here, the tendency toward helix formation for the same amino acid residues in poly(a-amino acids) and proteins is mostly, but not always, of the same magnitude (Table 30-1). The peptide chains in the pleated sheet structure are mainly arranged antiparallel. Segments in the coil conformation are generally not included in the secondary structure. 

Poly(L-alanine) can stabilize the RH 3 18/5 hehx, the well-known a-helix, by intramolecular hydrogen bonds, indicated by the dotted lines in Fig. 5.16 A. Another stable conformation, called the pleated sheet, must make the H-bonds inter-... 

The poly-alanine stretches in spidroin 2 are somewhat longer than those of spidroin 1 and therefore would be more suitable for the formation of P-sheets. However, the proline residues in the glycine rich neighbourhood are rather detrimental to a formation of P-pleated sheets and would disrupt the integrity of the conformation. It was proposed that spidroin 2 should therefore be exclusively found in the amorphous matrix while solely motifs found in spidroin 1 are responsible for the formation of non-periodic lattices (NPL) in the crystalline fraction [57,58]. Yet, attempts to investigate the conformation of spidroin 2 with NMR failed as no labelled proline could be found [66]. Thus the role of spidroin 2 in the molecular structure remains speculative. 

Proteins and polypeptides have two major classes of chromophores, the amide groups of the peptide backbone, which absorb light in the far UV (below 250 nm) and the aromatic amino acid side chains and disulfide bonds, which absorb light in both the near (320-250 nm) and far-UV. Far-UV ORD and CD are useful for studying protein structure and folding because many conformations that are common in proteins, including a-helixes, j5-pleated sheets, poly-L-proline Il-like helices and turns, have characteristic spectra. Figure 1 illustrates representative CD spectra of model polypeptides with different secondary structures. In addition, the chromophores of the aromatic amino acids of proteins are often in... 

L-aspartate (7), y-[2-(9-carbazoIyl)ethyl]-L-glutamate(8), N -[(9-carbazolyl)-carbonyl]-L-lysine (9), and 2-[(9-carbazolyI)ethyl]-L-cysteine (10). No detailed information on the optical rotatory properties of the indicated polymers has been reported, whereas a great deal of attention has been focused on the dichroic absorption properties especially for different samples of poly(8). While the first three polymers are characterized by an a-helix structure in both solution and solid state, poly(lO) may assume antiparallel pleated-sheet and normal antiparallel P-forms. Apparently, the presence of the bulky side group does not substantially affect the conformational structure imposed by the polypeptide backbone. Additionally in concentrated solutions, the cholesterogenic propensity of polypeptides such as those based on L-glutamic acid, L-aspartic acid, and L-lysine is not appreciably influenced by the steric and electronic characteristics of the carbazole moiety anchored via different spacers to the peptide units. 

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