Helix intertwining

The number of helical turns in these structures is larger than those found so far in two-sheet p helices. The pectate lyase p helix consists of seven complete turns and is 34 A long and 17-27 A in diameter (Figure 5.30) while the p-helix part of the bacteriophage P22 tailspike protein has 13 complete turns. Both these proteins have other stmctural elements in addition to the P-helix moiety. The complete tailspike protein contains three intertwined, identical subunits each with the three-sheet p helix and is about 200 A long and 60 A wide. Six of these trimers are attached to each phage at the base of the icosahedral capsid. 

The helices at the N-terminal regions of the two polypeptide chains are intertwined and make extensive contacts in the central part of the molecule to form a stable core. This core supports two "heads", each comprising the last three helices from one polypeptide chain. Alpha helix 3 in the middle of the subunit chain is quite long and forms the main link between the core and the head. 

Fibrous proteins can serve as structural materials for the same reason that other polymers do they are long-chain molecules. By cross-linking, interleaving and intertwining the proper combination of individual long-chain molecules, bulk properties are obtained that can serve many different functions. Fibrous proteins are usually divided in three different groups dependent on the secondary structure of the individual molecules coiled-coil a helices present in keratin and myosin, the triple helix in collagen, and P sheets in amyloid fibers and silks. 

H bonding also vitally influences the conformation and detailed structure of the polypeptide chains of protein molecules and the complementary intertwined polynucleotide chains which form the double helix in nucleic acids.Thus, proteins are built up from polypeptide chains of the type shown at the top of the next column. 

DNA is made up ot two intertwined strands. A sugar-phosphate chain makes up the backbone of each, and the two strands are joined by way of hydrogen bonds betwen parrs of nucleotide bases, adenine, thymine, guanine and cytosine. Adenine may only pair with thymine and guanine with cytosine. The molecule adopts a helical structure (actually, a double helical stnrcture or double helix ). 

Nucleotides are joined into a chain formation, as illustrated in Fig. A2.6a. In DNA, two nucleotide chains intertwine around each other in a double helix formation (Fig. A2.6b). The backbone of the two strands is the phosphate-sugar linkage. 

Base pairings of this type are only possible, however, when the polarity of the two strands differs—i. e., when they run in opposite directions (see p.80). In addition, the two strands have to be intertwined to form a double helix. Due to steric hindrance by the 2 -OH groups of the ribose residues, RNA is unable to form a double helix. The structure of RNA is therefore less regular than that of DNA (see p. 82). 

By far the most common form is B-DNA (2). As discussed on p. 84, this consists of two antiparallel polydeoxynucleotide strands intertwined with one another to form a right-handed double helix. The backbone of these strands is formed by deoxyribose and phosphate residues linked by phosphoric acid diester bonds. 

Figure 2.13 The dinucleating bis-bidentate ligand 14 forms with M1 metal ions of electronic configuration d10 (e.g., Cu1, Ag1) dimetallic complexes of formula [M2I(14)2]2 +, in which two molecules of 14 are intertwined to give a double helix. Ligands of the type 14 are named helicands and complexes such as 15 are called helicates. In this particular case, we have a double-strand helicate.

Nucleic acids are of great interest because they are the units of heredity, the genes, and because they control the manufacture of proteins and the functions of the cells of living organisms. Hydrogen bonds play an important part in the novel structure proposed for deoxyribonucleic acid by Watson and Crick.1,5 This structure involves a detailed eomplement riness of two intertwined polynucleotide chains, which form a double helix.117 The complementariness in structure of the two chains was attributed by Watson and Crick to the formation of hydrogen bonds between a pyrimidine residue in one chain and a purine residue in the other, for each pair of nucleotides in the chains. 

The dimensions of the xylan unit cell are slightly different a = b = 1.340 nm, (fibre axis) = 0.598 nm.) Atkins and Parker T6) were able to interpret such a diffraction pattern in terms of a triple-stranded structure. Three chains, of the same polarity, intertwine about a common axis to form a triple-strand molecular rope. The individual polysaccharide chains trace out a helix with six saccharide units per turn and are related to their neighbours by azimuthal rotations of 2ir/3 and 4ir/3 respectively, with zero relative translation. A similar model for curdlan is illustrated in Figure 6. Examinations of this model shows that each chain repeats at a distance 3 x 0.582 = 1.746 nm. Thus if for any reason the precise symmetrical arrangement between chains (or with their associated water of crystallization) is disrupted, we would expect reflections to occur on layer lines which are orders of 1.746 nm. Indeed such additional reflections have been observed via patterns obtained from specimens at different relative humidity (4) offering confirmation for the triple-stranded model. 

The achiral network of tetraphenylenes served as an inspiration for the design of a it-conjugated double helix. Double helical polymers, in which two polyphenylene helices are intertwined or tetraphenylenes are sequentially annelated, were recognized as the chiral building block of the network (Fig. 15.24) [107]. 

From a chemical perspective, the double-helix produced by two intertwining strands of oligomeric DNA is a fascinating and unique molecular structure. (See Fig. 1 for a structural model of a 12-base pair duplex of B-form DNA.) In it nucleic acid bases are stacked in pairs one on top of the other with a slight twist reminiscent of a spiral staircase [16]. The unique stacking and overlapping of the n- and Tr-electrons of DNA bases may provide a preferred path for electron transfer. Similarly, the exceptional closeness of the stacked bases may have important consequences for charge motion in DNA duplexes. Additionally, the... 

Figure 1 Molecular model of a 12-base pair duplex of canonical B-form DNA. The two 12-mer strands that intertwine to form the duplex are colored separately (black and gray). Nucleic acid base pairs are stacked perpendicular to the helical axis at 3.4-A intervals (center-to-center distance), and the duplex helix repeats its spiral structure every 10 base pairs. (Figure provided by Dr. Carolyn Kanagy using the Sybyl Version 6.3 molecular modeling program from Tripos, Inc. and standard B-form DNA substructures.)...

A third type of ordered structure giving x-ray patterns is the collagen helix. The basic collagen unit consists of three intertwined chains, in which one twist accounts for a distance of 9 A, with three amino acid residues present in each twist. 

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