Mosaicity twist

Antiparallel tt-helix proteins are structures heavily dominated by a-helices. The simplest way to pack helices is in an antiparallel manner, and most of the proteins in this class consist of bundles of antiparallel helices. Many of these exhibit a slight (15°) left-handed twist of the helix bundle. Figure 6.29 shows a representative sample of antiparallel a-helix proteins. Many of these are regular, uniform structures, but in a few cases (uteroglobin, for example) one of the helices is tilted away from the bundle. Tobacco mosaic virus protein has small, highly... 

Tobacco mosaic virus protein has a small, highly twisted antiparallel j8 sheet at the base of the helix bundle, with two more helices underneath the sheet (see Fig. 72). Cytochrome bs looks remarkably similar (see Fig. 105), but the helices are much shorter. That structure could have been classified as an up-and-down helix bundle, but we have placed it in the small metal-rich proteins because its helix bundle is very small and distorted and the heme interactions appear more important than the direct helix contacts. 

A7.10 X-ray diffraction characterisation ofGaN-based materials rocking curve analysis D1 Tilt Mosaicity versus Twist Mosaicity... 

According to [18,19] and our experience, it is usual that the smaller FWHM for symmetrical reflections (measuring mostly tilt mosaicity) corresponds to a larger FWHM for asymmetrical reflections (measuring together twist and tilt mosaicity). For example, for the MOCVD GaN layer which possessed a 00.2 RC of only 40 arc sec, the 10.2 reflection exhibited an FWHM of 740 arc sec [18], Similarly, for a 0.76 pm MBE layer, Amano et al [19] reported 48 arc sec for the 00.2 reflection and 5226 arc sec for the 10.0 reflection (grazing incidence geometry). The other sample (1.7 pm) possessed 365 arc sec and 1581 arc sec, respectively, for those two reflections. 

For GaN layers on sapphire annealed at a high pressure of nitrogen (10 - 15 kbar) at temperatures up to 1500°C [20], the 00.2 FWHM decreased from about 700 arc sec down to about 400 arc sec upon annealing (tilt-mosaicity decrease), whereas the FWHM of the -21.4 reflection increased from 240 arc sec up to 440 arc sec (twist mosaicity increase). For samples with 00.2 FWHM of less than 400 arc sec, annealing induced no changes. 

Yip et al [29] reported the following 00.2 FWHMs 516, 275 and 478 arc sec for layers 0.2, 0.57 and 1.4 pm thick, respectively. For all three layers, twist mosaicity did not change much (10.1 reflection of 1692, 1728 and 1982 arc sec, respectively). The results of these authors seem to be typical the tilt mosaicity decreases up to a certain thickness (dependent on different growth conditions), and then it starts to increase. 

The four-helix bundle is a common motif in which (usually) antiparallel a helices are packed side by side. It is found in myohemerythrin, various cytochromes, and a number of other proteins. A viral example is the coat protein of tobacco mosaic virus (TMV) (Bloomer et al, 1978). TMV represents the most common type, in which the helix axes are nearly antiparallel, off by 18°, coiled with a left-handed superhelical twist (Fig. 5 see Color Insert). The slight misalignment of the individual helix axes allows the side chains to interdigitate efficiently, burying internal hydrophobic side chains. 

Leaves yellow and mottled stalks twisted plant dwarfed. Cause Celery mosaic. Destroy infected plants. Control aphids because they can spread viral diseases as they feed. Prevent problems by planting resistant cultivars such as Florida 683 and Utah-52-70R Improved . 

Twisted systems due to viruses. Certain viruses in the form of long cylinders can assemble into cholesteric phases (Narcissus mosaic virus, 6,39). The presence of a virus in a cell often leads to the differentiation of cholesteric networks of fibrils (6,16). 

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