Peptides antiparallel /?-sheet

Parallel /3-sheets tend to be more regular than antiparallel /3-sheets. The range of (f) and i/t angles for the peptide bonds in parallel sheets is much smaller than that for antiparallel sheets. Parallel sheets are typically large structures those composed of less than five strands are rare. Antiparallel sheets, however, may consist of as few as two strands. Parallel sheets characteristically distribute... 

To prevent insolubility resulting from uncontrolled aggregation of extended strands, two adjacent parallel or antiparallel yS-peptide strands can be connected with an appropriate turn segment to form a hairpin. The / -hairpin motif is a functionally important secondary structural element in proteins which has also been used extensively to form stable and soluble a-peptide y9-sheet arrangements in model systems (for reviews, see [1, 4, 5] and references therein). The need for stable turns that can bring the peptide strands into a defined orientation is thus a prerequisite for hairpin formation. For example, type F or II" turns formed by D-Pro-Gly and Asn-Gly dipeptide sequences have been found to promote tight a-pep-tide hairpin folding in aqueous solution. Similarly, various connectors have been... 

Tab. 2.7 Comparaison of selected backbone torsion angles for strand segments in antiparallel sheet-forming jff-peptides 117-119, 121, 122 [109, 154, 191-194]...

As a consequence of their different turn geometry a 10-membered turn closed by H-bonds between NH and C=0 +i and a 12-membered turn closed by Id-bonds between C=0 and NH +3, antiparallel hairpins formed by y9-peptides 121 and 122 display opposite sheet polarities (see Fig. 2.30A and B). Comparison of backbone torsion angles (X-ray and NMR) for selected y9-amino acids residues within extended strand segments of peptides 117-122 are shown in Tab. 2.7. The observed values are close to ideal values for y9-peptide pleated sheets =-120° (or 120°), 01 = 180°, (/ =120°(or-120°). 

Fig. 2.38 sheet forming y-peptides. (A) Crystal structure of the two stranded antiparallel sheet formed by a,j -unsaturated y-dipeptide 152 with a-methyl substituted residues [208], Both intermolecular H-bonds are characterized by a N---0 distance of 2.84 A and an angle (N-H- -O) ofl54.2°. (B) Crystal structure of the infinite parallel sheet arrangement formed by vinylogous dipeptide 153 [208], Intermolecular H-bonds are characterized by a N -O distance of 2.88 A and 3.24 A and an... 

Fig. 1.3.1. Examples of three different types of peptide structure a-helix, antiparallel / -sheet, and y-turn.

Figure 6. A 3u and a 12/10 helix, a parallel and an antiparallel sheet (with hair-pin turn) and a stack (clockwise from top left) formed by p-peptides consisting exclusively of simple open-chain homologated a-amino-acid residues (Ala, Val, Leu, Lys side chains) and of a-Methyl-P-homo-Ala and Leu residues [12,21,36].

The cooperative effects in secondary protein structures, helix and sheet have been reported [53]. The linear chain of formamide which resembles peptides has large cooperativity in H-bond, which is 2.5 times that of formamide dimer. For the parallel and antiparallel sheet in secondary protein structures, there was no cooperativity in the parallel direction, while significant cooperativity exists in perpendicular direction. In methanol solvent system, the cooperative effects were reduced, indicating that the cooperativity is due to the polarization effect. 

The molecule is amphipathic with one flexible loop between residues 4 and 12, a helix from residues 15 to 23, and an antiparallel sheet (residues 24-31 and 34-40). The insect defensins are structurally similar to peptides such as charybdotoxin that contain an a-helix stabilized by cysteine bridges... 

Peptide chains (or two parts of the same chain) connected with each other through multiple hydrogen bonds form fi-sheets. The participating chains can be parallel or antiparallel to each other. An antiparallel sheet is shown here including a view along the general plane of the sheet to indicate its pleated character ... 

Some rationally designed peptide nanotubes are based on the antiparallel -sheet model. The formation of either tubes or spheres gives an insight into the mechanism of their formation, suggesting that their formation from fundamentally similar peptides is consistent with the closure of a two-dimensional layer. High-level quantum chemical... 

The major stmctural feature of the HAz chain (blue in Figure 5.20) is a hairpin loop of two a helices packed together. The second a helix is 50 amino acids long and reaches back 76 A toward the membrane. At the bottom of the stem there is a i sheet of five antiparallel strands. The central i strand is from HAi, and this is flanked on both sides by hairpin loops from HAz. About 20 residues at the amino terminal end of HAz are associated with the activity by which the vims penetrates the host cell membrane to initiate infection. This region, which is quite hydrophobic, is called the fusion peptide. 

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