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Ideal torsion angle

Fig. 2.3 Model of the 2i- and 3i-helical structures proposed for PHB chains with ideal torsion angle values. The 2i-helix was determined by fiber X-ray diffraction of PHB [49-51] while the 3i-helical fold was constructed by using preferred dihedral angles found along the backbone in crystal structures of cyclic oligomers 9 ( oligolides ) [37, 43, 45]... Fig. 2.3 Model of the 2i- and 3i-helical structures proposed for PHB chains with ideal torsion angle values. The 2i-helix was determined by fiber X-ray diffraction of PHB [49-51] while the 3i-helical fold was constructed by using preferred dihedral angles found along the backbone in crystal structures of cyclic oligomers 9 ( oligolides ) [37, 43, 45]...
Fig. 2.23 Model of the 2g-helix formed by all-w/i///ce-jff -peptide 109 generated with ideal torsion angle values =-135°, =58°,... Fig. 2.23 Model of the 2g-helix formed by all-w/i///ce-jff -peptide 109 generated with ideal torsion angle values =-135°, =58°,...
Interestingly, the 28-hehcal fold identified by NMR analysis of /9-peptide 109 compares well with the model of a /9 -peptide consisting of 1-aminomethylcyclo-propanecarboxylic acid residues (Fig. 2.24). This model was generated using ideal torsion angle values ( = + 120°, 9i=-72°, ii/=0°, and < =180°) derived from crystal structures of dimer 110, trimer 111 and tetramer 112 [163] (Fig. 2.25). [Pg.74]

CO is the torsion angle, co i is the ideal torsion angle and is treated as a parameter. Bond torsion contributions to the overall energy may also need to include terms which are one-fold and two-fold periodic. [Pg.56]

Q, is the torsion angle, is the ideal torsion angle and is a parameter. Torsion contributions to the strain energy will also usually need to include contributions that are onefold and twofold periodic. These can be represented in the same manner as the threefold term. [Pg.214]

Isotactic polymers with two chain atoms per monomeric unit thus tend to occur in more or less ideal TG conformations. In addition, the low energy difference for slight deviations from the ideal torsion angle can lead to various helix types. Rapid crystallization of it-poly (butene-1) produces a 4i helix, for example, which, as a high-energy form, changes into a 3i helix on annealing (see also Chapter 10). [Pg.103]

Having established that our new size-dependent potential is quite effective for generating low-resolution structures of proteins below 100 residues using secondary structure derived from PDB coordinates, we next ask what the effect is of using ideal torsional angles for helices and strands as opposed to PDB-derived... [Pg.245]

At Its most basic level separating the total strain of a structure into its components is a qualita tive exercise For example a computer drawn model of the eclipsed conformation of butane using ideal bond angles and bond distances (Figure 3 8) reveals that two pairs of hydrogens are separated by a distance of only 175 pm a value considerably smaller than the sum of their van der Waals radii (2 X 120 pm = 240 pm) Thus this conformation is destabilized not only by the torsional strain associ ated with its eclipsed bonds but also by van der Waals strain... [Pg.111]

The most stable conformation of cyclohexane is the chair. Electron diffraction studies in the gas phase reveal a slight flattening of the chair compared with the geometry obtained when tetrahedral molecular models are used. The torsion angles are 55.9°, compared with 60° for the ideal chair conformation, and the axial C—H bonds are not perfectly parallel but are oriented outward by about 7°. The length of the C—C bonds is 1.528 A, the length of the C—H bonds is 1.119 A, and the C—C—C angles are 111.05°. ... [Pg.135]

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°). [Pg.81]


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See also in sourсe #XX -- [ Pg.130 , Pg.138 ]




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