Double twist helical structures

The four histone groups that are composed of ho-mogeneous proteins, H2A, H2B, H3, and H4, make up the nucleosome core. Each core consists of two copies of the four histones. The double-stranded DNA is wrapped twice around each core in a left-handed superhelix. A superhelix is the name given to the additional helix made by the double-stranded, helical DNA as it is wrapped around the nucleosome core. A familiar superhelix in everyday life is a twisted spiral telephone cord. The nucleosome core of histones do not recognize specific DNA structures rather, they can bind to any stretch of DNA as long as it is not too close to a neighboring nucleosome. The order of contact of histones to the DNA is as follows ... 

The structures of ladder polysilanes were determined by X-ray crystallography.41,42 In Fig. 6, molecular structures of the ladder polysilanes with the anti structure are shown. A remarkable feature of the anti ladder polysilanes is a helical structure. These molecules can be regarded as silicon double helices, in which two polysilane main chains twist in one way and are bridged by Si-Si bonds. The twist angles between the terminal rungs are... 

FIGURE 24-12 Supercoiling induced by separating the strands of a helical structure. Twist two linear strands of rubber band into a right-handed double helix as shown. Fix one end by having a friend hold onto it, then pull apart the two strands at the other end. The resulting strain will produce supercoiling. 

Our template synthesis of knots implies that the target molecules are obtained as cationic dicopper(I) complexes. Therefore we considered the possibility of interconverting both enantiomers into a pair of diastereomeric salts [137, 138] by combining them with an optically active anion. Binaphthyl phosphate (BNP") [139] drew our attention because its chirality arises from the binaphthyl core, which is twisted. This helical structure is of the same type as that of die copper double helix, precursor of the knot. Besides, both compounds are aromatic and, thus, we could expect some potentially helpful stacking interactions [87],... 

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

We are now left with another question - what is this new structure that we have drawn with the three crossing points However we twist and turn the molecule (hint try this with pipe-cleaners) the crossing points remain. If we start at any point on the structure and trace around we eventually end up at the same point. In other words, there is only a single cyclic molecule, but the twisting means that it is not the same as the [2+2] macrocyclic product. This view serves to emphasise the relationship with the double-helical structures that... 

The increase in fluorescence intensity of TO upon binding to dsDNA is due to the restriction of rotation around the monomethine bridge upon intercalation of the dye into the double helical structure as the benzothiazole and quinolinium rings adapt to the propeller twist of the base pairs [49]. The monomethine bridge has a low energy barrier to rotation and hence is free to rotate in solution, allowing for the electronically excited dye to relax by non-radiative decay [49]. The quantum yield of free TO in solution has been reported to be 2 x 10-4 at 25 °C [43]. The binding constant for TO is 106 M 1 while that of ethidium bromide is 1.5 x 105 M 1 [59]. 

Figure 5 shows the molecular structures of rfs-2-nitro-7-(dimethylamino-)-9-(2,3,-dihydro-l J-f-naphtho[2,l-b]thiopyran-l -ylidene)-9J-f-thioxanthene ((P)-cis-17) and trans-dimethyl-[l-(2-nitro-thioxanthen-9-ylidene)-2,3-dihydro-lJ-f-ben-zo[f]thiochromen-8-yl]amine ((P)-trans-18). Anti-folded helical structures, in which the top and bottom parts are respectively tilted up and down relative to the plane of the central olefmic bond, are clearly observed. The extent of folding and twisting in these and related overcrowded alkenes can vary considerably. It should be emphasized that only minor deviation from planarity occurs at the central double bond (dihedral angles 5.4° (17) and 6.8° (18)), while normal bond lengths are found (1.353 A (17) and 1.338 A (18)).130,311... 

The two strands of the helix are held together by hydrogen bonding as well as stacking interactions between the aromatic rings of the nitrogenous bases. These strong interactions make the double helical structure very stable. It s also true, however, that the double helical structure has to be DNA looks like a ladder that has been twisted. The temporarily pulled apart when it s time to nucleotides,... 

There is a wide range of other chiral mesophases that are somewhat unconventional in terms of their structure and many of these are rather recent developments. Blue phases are cubic mesophases that have a double twist stracture. They are exhibited by highly chiral materials and appear at temperatures above the chiral nematic phase. These phases give characteristic platelet type textures that are often blue (hence the name) but they can also appear green, yellow and red. The twist grain boundary (TGBA ) phase also has a fmstrated complex helical stmcture. In this case the normal lamellar stmcture is... 

Imae et al. [91] have reported that gel-like solutions are obtained at medium pH for aqueous solutions of A-acyl-L-aspartic acid (C Asp, n = 12-18). The possible models of helical, fibrous assemblies are double stranded to form a twisted ribbon of a planar bilayer sheet. However, Al-dodecyl-P-alanine formed cylindrical fibers that have no helical structure. 

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