Plane Pleated sheet

Fig. 2. Protein secondary stmcture (a) the right-handed a-helix, stabilized by intrasegmental hydrogen-bonding between the backbone CO of residue i and the NH of residue t + 4 along the polypeptide chain. Each turn of the helix requires 3.6 residues. Translation along the hehcal axis is 0.15 nm per residue, or 0.54 nm per turn and (b) the -pleated sheet where the polypeptide is in an extended conformation and backbone hydrogen-bonding occurs between residues on adjacent strands. Here, the backbone CO and NH atoms are in the plane of the page and the amino acid side chains extend from C ...

Fig. 2.28 X-ray crystal structures of parallel sheet-forming and all-un//fce-/F -peptides 116 and 117 [10, 191]. Views along the parallel amide planes and crystal packing diagram show the parallel pleated sheet arrangement (view perpendicular to the amide planes).

The second structural element to be proposed by Pauling and Corey was the P-pleated sheet (Figure 4.7). These sheets are made up of P-strands, typically from 5 to 10 residues long, in an almost fully extended conformation, aligned alongside one another with hydrogen bonds formed between the C=0 bonds of one strand and the NH of the other, and vice versa. The P-sheets are pleated (i.e. they undulate) with the Ca atoms alternatively a little above, or a little below the plane of the P-sheet, which means that the side chains project alternatively above and below the plane. P-Strands can interact to form two types of pleated sheets. 

FIGURE 4-7 The /8 conformation of polypeptide chains. These top and side views reveal the R groups extending out from the /3 sheet and emphasize the pleated shape described by the planes of the peptide bonds. (An alternative name for this structure is /3-pleated sheet.) Hydrogen-bond cross-links between adjacent chains are also shown, (a) Antiparallel /3 sheet, in which the amino-terminal to carboxyl-terminal orientation of adjacent chains (arrows) is inverse, (b) Parallel f) sheet. 

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-... 

Fig. 19.15a-d. Distribution of out-of-plane (0) and in-plane (y) angles for N-H- -0=C hydrogen bonds in a a-helices, b / -turns (crosses) and 310-helices (dots), c parallel and d anti-parallel -pleated sheets [596]... 

The p-pleated sheet consists of extended strands of the peptide chains held together by hydrogen bonding. The C=0 and N-H bonds lie in the plane of the sheet, and the R groups (shown as orange balls) alternate above and below the plane. 

Two structures have been proposed for (Gly) I an antiparallel-chain pleated sheet (APPS) and a similar rippled sheet (APRS) (see Section III,B,1). These structures have different symmetries the APPS, with D2 symmetry, has twofold screw axes parallel to the a axis [C (a)] and the b axis [C (b)], and a twofold rotation axis parallel to the c axis [62(0)] the APRS, with C2h symmetry, has a twofold screw axis parallel to the b axis ( 2(6)], an inversion center, i, and a glide plane parallel to the ac plane, o-Sj. Once these symmetry elements are known, together with the number of atoms in the repeat, it is possible to determine a number of characteristics of the normal modes the symmetry classes, or species, to which they belong, depending on their behavior (character) with respect to the symmetry operations the numbers of normal modes in each symmetry species, both internal and lattice vibrations their IR and Raman activity and their dichroism in the IR. These are given in Table VII for both structures. 

Circular dichroism (CD) spectroscopy is widely used to determine the amount of a-helix, y3-pleated sheet, and random coil structures in a protein molecule. The principle of CD is based on the fact that asymmetrical structures absorb light in an asymmetrical manner. Natural light vibrates in all planes perpendicular to its direction of travel but its plane of polarization can be fixed to possess either left or right orientation. However, in circular polarization the direction of polarization rotates with the frequency of the light. If the rotation is clockwise, it is called right circularly polarized light and if counterclockwise it is called left circularly polarized light. 

Several parameters are involved in completely describing a polypeptide helix or pleated sheet. Helices can be either left-handed or right-handed. The number of amino acids per repeat of the structure can vary between two and five. Also, the planes of the peptide bond and angles in Figure 6.2) can be rotated about the oi carbon. If one considers theoretically that both and can rotate 180 in either direction (+180 to -180 ), then one can begin to construct a graphical representation of all and rotations about a peptide bond. 

Fig. 7.6. A (3-pleated sheet. In this case, the chains are oriented in opposite directions (antiparallel). The large arrows show the direction of the carboxy terminal. The amino acid side chains (R) in one strand are trans to each other, and alternate above and below the plane of the sheet, which can have a hydrophobic face and a polar face that engages in hydrogen bonding.

Fig. 10.5. (a) A schematic representation of the vibrational modes of the parallel-chain pleated sheet the arrows represent the components of the transition moments of peptide groups in the plane of the paper. The plus and minus signs represent the components of the transition moments perpendicular to the plane of the paper, the former pointing upward, and the latter pointing downward, (b) A schematic representation of the vibrational modes of the antiparallel-chain pleated sheet (Miyazawa, 1960c). (From Schellman and Schellman, 1964.)... 

Sheets of noncovalent polymers, which would be analogous to the p-pleated sheets of silk, are commonplace on water and solid surfaces. The molecular weight and extension of these surface mono- and bilayers is unlimited. Of current interest are well-defined rigid gaps within such monolayers. They provide small reactive domains which can be manipulated on a molecular scale. It was found possible, for example, to fixate porphyrin heterodimers with a plane-to-plane distance of 10 or 20 A within such gaps and to position tyrosine monomers as electron relays between them. These sheets can then be fixated on spherical colloids (see Fig. 3.4) and we are back to spherical polymers. ... 

Each peptide unit lies in a plane because it consists of a delocalized system of n electrons associated with n orbitals of the C and O atoms together with the lone electron-pair orbital of the N atom. Such an electron resonance structure is sufficient to produce significant diamagnetic anisotropy in the protein. " Some two dozen amino-acid residues make up polypeptides, but only glycine and proline have a first atom in the side chain R which is not a carbon atom. Thus, many features of regularity are present which may cooperate in forming energy bands, particularly in the extended arrays of a helices and ) -pleated sheets. In fact, spectra of DNA... 

Scleroproteins with pleated-sheet structures have little stretchability but high tensile strength. In the pleated sheets, the peptide chains lie in a plane. 

The crystal structure of VI viewed along the fe-axis is shown in Figure 4.7. There are two MTA cations and bromide anions and two o-iodophenol molecules in the asymmetric unit. The OH group of the o-iodophenol is hydrogen bonded with the bromide anions. The packing mode of the MTA cations and the bromide anions is nearly the same as that of the CTAB complex, crystal V. The pleated sheet is parallel to the flc-plane and the sheets are stacked along the fe-axis. 

TMs effective design, made out of coloured card (cardboard), is variation I of the Double sat Cut Away technique, repeated many times on a diagonally pleated sheet of card. The repeated use of even the simplest Cut Away form (here, a semicircle), can create patterns of gyeat beauty. Suspended from a thread and allowed to rotate, the ever-changng pattern of //gjhf and shade across the planes adds greatly to the effect. 

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