Regular helix

Proline Pro HN T 1 F1 H—C- ch2 COOH 6.1 Imino acid — distorts the regular -helix structure... 

Fig.25 Representations of parts of the solid state structure of the salt [TTF+]2[C6(COO)6 H42-]. (i) The hydrogen bonds between the mellitate ions, (ii) The helically arranged TTF cation radicals in the double helix of mellitate anions, (iii) Disorder in the stacks, (a) shows the regular helix, (b) and (c) two dimer faults and (d) the average column with disorder in the A site. Adapted and reprinted with the permission of the Royal Society of Chemistry [191]...

Starch, in foods like potatoes, contains the polysaccharides amylose and amylopectin. Like cellnlose, amylose, is a linear polymer of D-glucose. However, the linkages are now 1,4-a instead of 1,4-, as illustrated schematically in Figure 9-36. This seemingly minor change results in a significant difference in properties, however. Amylose is a much more flexible molecule than cellulose and can form a regular helix with six residues per turn. 

Theoretical calculations showed that in a-helices trans proline peptide bonds can best be accepted up to the fourth position within the helix because there is neither a disruption of the hydrogen bonding network in this region nor does the bulk of the pyrrolidine ring seriously interfere with the regular helix geometry. Analyses of the Brookhaven database in 1991 also showed that the highest fre-... 

Some linear homopolymers tend to form a regular helix. A case in point is amylose, which tends to form a helix in aqueous solutions. Nevertheless, also in this case Eq. (6.2) may remain... 

The condis crystal shows only equatorial X-ray diffraction spots, indicative of a regular (hexagonal) packing between the chains, but without matching of the CFj-motifs along the chain to any regular helix. Calculation of the energetics of the isolated polymer chain yields four rotational isomers jKr C—C bond Because of... 

An amylose chain twists around a lipid molecule to form a regular helix. The inside of the helix is hydrophobic, with most of hydroxy groups of glucose units remaining outside the helix. In this way, the crystallographic V-pattem of starch is... 

The above helices are all related to a linear helix axis. If the molecules contain monomeric units with geometries that differ in a random manner, the polymer will no longer be a regular helix. 

The number of observables described here is not sufficient to determine uniquely the several parameters of the theory. We can, however, assign p and u on the basis of independent information concerning the conformation of aqueous amylosic chains in the absence of iodine. A realistic model of aqueous amylose (31) discloses that perhaps 25% of an amylose chain in water might be classified as nearly regular helix at any instant, but the chain conformation is extremely labile, and there is no evidence for any conformational cooperativity in the absence of iodine. Hence, the cooperativity parameter u may be set equal to unity, and for convenience we also take p = 1, which implies equal proportions of helix and coil in the absence of iodine. Calculations not reported in detail here reveal that the results described below are quite insensitive to the exact numerical value of p, provided u = 1 and p is of order unity. 

Melt fracture can be minimized by increasing the die length, smoothly tapering the entrance to the die, and raising the die temperature. Melt fracture leads to surface irregularities on the finished parts such as a regular helix or irregularly spaced ripples. 

In Zubay s model, the polynucleotide sequence is bent in the middle. Under conditions in which all except five bases are paired to form hydrogen bonds of the same type as those found in Watson and Crick s model for DNA, the base pair sequence is twisted to form a regular helix. At the acceptor end of the molecule, the pCpA sequence is free and can therefore accept the amino acid, and the first cytosine of the pCpCpA sequence forms hydrogen bonds with a complementary guanine. The loop of the helix is formed by three free bases and plays an important role in connecting the tRNA molecule with messenger RNA. Zubay s model was soon superceded by new and more sophisticated models that were proposed when the base sequence of transfer RNA became known. 

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