Twist diffusion

Figure 1. Schematic representation of remodelling mechanisms. (Adapted form Langst and Becker, 2004.) The schemes show nucleosomes from the top. (a) The twist diffusion model - Twisting of DNA moves it over the histone surface in one base pair increments. This changes the position of the DNA with respect to the histone, as shown by the open and closed circles, (b) The Loop recapture model - Extranucleosomal DNA is pulled into the nucleosomes to replace a DNA segment which consequently loops out. This loop is then propragated over the histone surface like ripples of a wave. The star,, indicates how this leads to a change in the position of DNA relative to the nucleosome. (See Colour Plate 4.)...

Akey CW, Luger K (2003) Histone chaperones and nucleosome assembly. Curr Opin Struct Biol 13 6-14 Aoyagi S, Hayes JJ (2002) hSWI/SNF-catalyzed nucleosome sliding does not occur solely via a twist-diffusion mechanism. Mol Cell Biol 22 7484—7490... 

The major alternative mechanism for nucleosome redistribution to the bulge propagation mechanism is twist diffusion. Supporting this, it is clear that the nucleosome core particle is capable of accommodating alterations to DNA twist of 1-2 bp per double helical turn without significant changes to the... 

Fig. 4. Proposed mechanisms of nucleosome mobility. From top to bottom the figure show octamer rotation, twist diffusion and bulge propagation.

An X-ray crystal structure determination of calciferol (vitamin D-2,71) showed that steric crowding in the s-cis diene system resulted in a twisted conformation with a dihedral angle of 53° [59], On irradiation with a mercury lamp, it was partially converted into ergosterol (72) and tachysterol (73) [60, 61]. When a solution of calciferol in light petroleum containing a trace of iodine was exposed to diffuse daylight, the vitamin was photoisomerized to (74) [62],... 

Along with CFPs, carbon cloths have also been widely used materials for diffusion layers in fuel cells. Figure 4.6 shows SEM pictures of typical carbon cloth materials used in fuel cells. The majority of these fabrics are made from PAN fibers that are twisted together in rolls. For details regarding how normal PAN fibers and carbon fibers are fabricated, please refer to Section 4.2.I.I. In this section, we will briefly discuss the fabrication process of carbon cloths. 

Interfacial transfer of chemicals provides an interesting twist to our chemical fate and transport investigations. Even though the flow is generally turbulent in both phases, there is no turbulence across the interface in the diffusive sublayer, and the problem becomes one of the rate of diffusion. In addition, temporal mean turbulence quantities, such as eddy diffusion coefficient, are less helpful to us now. The unsteady character of turbulence near the diffusive sublayer is crucial to understanding and characterizing interfacial transport processes. 

X-Ray analysis of crystals of 2-methyl-5-phenyltriazolo[4,5-, triazole 38, obtained by diffusion from methane/ethanol (PI space group), indicates there are two molecules per asymmetric unit. The first is essentially planar, with a mean deviation (non-H-atoms) of 0.03A and a maximum absolute torsion angle, C(2)-C(l)-N(5 )-N(4 ), of 3.4°. The second molecule has a more pronounced twist with C(2A)-C(1A)-N(5 A)-N(4 A) being 9.6° <1993ZK133>. 

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