Helix helical twist

Collagen, the principal fibrous protein in mammalian tissue, has a tertiary structure made up of twisted a-helices. Three polypeptide chains, each of which is a left-handed helix, are twisted into a right-handed super helix to form an extremely strong tertiary structure. It has remarkable tensile strength, which makes it important in the structure of bones, tendons, teeth, and cartilage. 

H-DNA, proposed structure 226 helical twist 216 helix parameters 216 Holliday junction 228 - 230 hydration of 216 hydrolysis of 249, 251 hypersensitive sites 266 interaction with spermidine and... 

Double helix. A structure in which two helically twisted polynucleotide strands are held together by hydrogen bonding and base stacking. 

Scheme 12.19 Quinuclidines as a simple helix. Helicity of l-azabicyclo[2.2.2]octane and twist sense of quinidine-derived oxazatwistanes.

Su is the operator of the helical group that generates the set of coordinates for the wall sources located on the upper helical strand of the double helix that defines the channel the subscript u indicates upper. Here, is the displacement along the helical axis and t3 is the helical twist angle in a right-handed rotation down the axis. The second helical strand can be generated from the first simply by reflecting the first source in the x-z plane ... 

During the two decades after this important discovery, a tremendous amount of research has been directed toward the polymerization of sterically demanding achiral monomers with chiral initiators to create enantiomerically pure helical polymers (also known as helix-sense selective or screw-sense-selective polymerization ). These polymers, known as atropisomers, are stable conformational isomers that arise from restricted rotation about the single bonds of their main chains. Key aspects of these reactions are enantiopure initiators that begin the polymerization with a one-handed helical twist, and monomers with bulky side-chains that can maintain the helical conformation due to steric repulsion. Notable examples of this fascinating class of polymers that are configurationally achiral but conformationally chiral include [8, 38, 39] poly(trityl methacrylate), polychloral, polyisocyanates, and polyisocyanides. Important advances in anionic and metal-based enantioselective polymerization methods have been reported in recent years. 

Fig. 18.5. Correlation between observed helix-twist angles and sum function 1 (a) and correlation between observed roll angles and sum function (b) for the dodecamer [d(CGCGAATTCGCG)]2. A decrease in helical twist or an opening of the roll angle on the side of the base pair where a clash occurs will relieve the steric hindrance between adjacent purines of opposite strands. For example, the clash at the first G-C base pair step (R-Y, step 2) takes place in the major groove. To relieve the steric hindrance, the twist angle has to be decreased, and at the same time the twist at each of the two neighboring steps is increased by half that amount. Thus, the contributions to X) 1 for tbe three steps 1, 2 and 3 are +1, -2, -t-1. If the clash occurs in the minor groove (e.g. with the neighboring C-G step, step 3), each of the contributions has to be doubled. Thus, the contributions to 1 for the three steps 2, 3 and 4 are +2, -4, +2 etc. (for details, see [39, 40]). The correlation coefficients were 0.94 (a) and 0.92 (b). For the correlations, the contributions of terminal base-pair steps (1 and 11) were omitted to avoid the influence of end effects...

The ligands bind in a bis-monodentate coordination mode and form the strands of the helix that twist around the helical axis on... 

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