Helix structures

In these p-helix structures the polypeptide chain is coiled into a wide helix, formed by p strands separated by loop regions. In the simplest form, the two-sheet p helix, each turn of the helix comprises two p strands and two loop regions (Figure 5.28). This structural unit is repeated three times in extracellular bacterial proteinases to form a right-handed coiled structure which comprises two adjacent three-stranded parallel p sheets with a hydrophobic core in between. 

The basic structural unit of these two-sheet p helix structures contains 18 amino acids, three in each p strand and six in each loop. A specific amino acid sequence pattern identifies this unit namely a double repeat of a nine-residue consensus sequence Gly-Gly-X-Gly-X-Asp-X-U-X where X is any amino acid and U is large, hydrophobic and frequently leucine. The first six residues form the loop and the last three form a p strand with the side chain of U involved in the hydrophobic packing of the two p sheets. The loops are stabilized by calcium ions which bind to the Asp residue (Figure S.28). This sequence pattern can be used to search for possible two-sheet p structures in databases of amino acid sequences of proteins of unknown structure. 

In addition to the antiparallel p-structures, there is a novel fold called the P helix. In the p-helix structures the polypeptide chain is folded into a wide helix with two or three p strands for each turn. The p strands align to form either two or three parallel p sheets with a core between the sheets completely filled with side chains. 

Blake, C., Serpell, L. Synchrotron x-ray studies suggest that the core of the transthyretin amyloid fibril is a continuous P sheet helix. Structure 4 989-998, 1996. 

The discovery of the ci-helix structure was only one of many achievements that led to Pauling s Nobel Prize in chemistry in 1954. The official citadon for the prize wa.s for his research into the nature of die chemical bond and its application to die elucidadon of the structure of complex substances. ... 

F. H. C. Crick. J. B. Watson and M. H. F. Wilkins (with Rosalind Franklin) establish the double helix structure of nucleic acids (Nobel Pnze 1962). 

In rarer cases the ODNs also prevent normal gene transcription by directly forming triplex-helix structures with target DNA. This does not destroy a gene but prevents its unwinding or its binding to a gene promoter. 

A helix-loop-helix motif is a DNA-binding motif, related to the leucine-zipper. A helix-loop-helix motif consists of a short a helix, connected by a loop to a second, longer a helix. The loop is flexible and allows one helix to fold back and pack against the other. The helix-loop-helix structure binds not only DNA but also the helix-loop-helix motif of a second helix-loop-helix protein forming either a homodimer or a heterodimer. 

Neutral salt-soluble collagen as well as add-soluble collagen show a CD spectrum (Fig. 8) having bands at 198 nm, 0 = -53 (MX) (deg cm2 dmol-1), and at 223 nm, 0 = 7500 (deg cm2 dmol-1), the ratio between both being 7 1. Pysh has calculated the CD spectra of a-helix, -structure, polyproline I and II127 ... 

Okamoto and his colleagues60) described the interesting polymerization of tri-phenylmethyl methacrylate. The bulkiness of this group affects the reactivity and the mode of placement of this monomer. The anionic polymerization yields a highly isotactic polymer, whether the reaction proceeds in toluene or in THF. In fact, even radical polymerization of this monomer yields polymers of relatively high isotacticity. Anionic polymerization of triphenylmethyl methacrylate initiated by optically active initiators e.g. PhN(CH2Ph)Li, or the sparteine-BuLi complex, produces an optically active polymer 60). Its optical activity is attributed to the chirality of the helix structure maintained in solution. 

COLLAGEN TYPE I IS COMPOSED OF A TRIPLE HELIX STRUCTURE FORMS FIBRILS... 

The central role played by DNA in cellular life guarantees a place of importance for the study of its chemical and physical properties. It did not take long after Watson and Crick described the now iconic double helix structure for a question to arise about the ability of DNA to transport electrical charge. It seemed apparent to the trained eye of the chemist or physicist that the array of neatly stacked aromatic bases might facilitate the movement of an electron (or hole) along the length of the polymer. It is now more than 40 years since the first experimental results were reported, and that question has been answered with certainty. 

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