Helical wrapping

Helically wrapped fins are fabricated such that the fm height can be between about 3/8 to 3/4 of the tube diameter, but limited because of fabrication requirements to a maximum of about 2.54 cm (1.0 in.) in height. Fin spacings vary between about 275 and 450 fins per meter of tube length, while fin thicknesses range from 0.025 to 0.075 cm. For particular cases these parameters may be varied further. 

Electron diffraction studies [3] have revealed that hexagons within the sheets are helically wrapped along the axis of the nanotubes. The interlayer spacing between sheets is 0.34 nm which is slightly larger than that of graphite (0.3354 nm). It was dso reported [2] that the helicity aspect may vary from one nanotube to another. Ijima et al. [2] also reported that in addition to nanotubes, polyhedral particles consisting of concentric carbon sheets were also observed. 

Figure 5.27 Scanning electron micrograph of copper-coated DCg PC (38) tubules. Tubules are hollow with diameter of approximately 0.5 p.m (bar = 0.5 p,m). Helical wrappings evident in some tubules are all right handed. Reprinted with permission from Ref. 125. Copyright 1998 by the American Chemical Society. 

Coiling of a helices in a-keratins. Residues on the same side of an a helix form rows that are tilted relative to the helix axis. Packing helices together in fibers is optimized when the individual helices wrap around each other so that rows of residues pack together along the fiber axis. Helices in coiled coil (c) are oriented in parallel. 

A coiled coil is a protein bundle of 2-5 alpha helices wrapped around each other into a superhelix, also called a supercoil (Lupas, 1996a Mason and Arndt, 2004 Lupas and Gruber, 2005). In the simplest form of coiled coil, helical domains of two proteins wind around one another and bind via a distinctive knobs-into-holes pattern whereby an amino acid side chain of one helix (knob) inserts into a space surrounded by four side chains of the facing helix (hole) as first suggested by Francis Crick in 1952 (Lupas, 1996a Lupas and Gruber, 2005). 

Unlike the Ciy proteins, Cyt toxins have a much broader toxicity spectrum. They are able to lyse a wide range of cell types in vitro, corresponding to the fact that Cyt proteins are structurally and functionally distinct from Cry toxins. To date, 18 cyt genes have been identified, and most of them were isolated from dipteran-active B. thuringiensis. X-ray crystallography of Cyt2A revealed that this toxin consists of a single domain, in which two outer layers of a-helices wrap around mixed p-sheets [34]. 

The bis tridentate ligand L15 can assemble two lanthanide ions to give dinuclear 2 3 complexes, in which three helically wrapped bis tridentate ligands L15 are bonded to nine-coordinate tricapped trigonal prismatic lanthanide(III) ions at a metal-metal distance of 8.9 A.19,35... 

Another class of DNA-binding proteins are the restriction endonucleases which cleave DNA at specific sites. The recognition motif found in the complex between DNA and Eco R1 features a helices as well, but the interaction mode differs considerably from those found in the helix-turn-helix and zinc-finger proteins. In the Eco RI complex, a parallel bundle of four a helices penetrates the major groove, whereby only the ends of the helices interact with the DNA [99, 100]. In addition, an extension of the polypeptide chain of one of these helices wraps around the DNA and makes several contacts to bases. 

Individually Finned Tubes. In this fin geometry, helically wrapped (or extruded) circular fins on a circular tube as shown in Fig. 17.14a, is commonly used in process and waste heat recovery industries. The following correlation for j factors is recommended by Briggs and Young (see Webb [47]) for individually finned tubes on staggered tubebanks. 

A class of motile systems completely different from and unrelated to the actin-myosin contractile systems is used in cellular structures as diverse as the mitotic spindle, protozoan and sperm flagella, and nerve axons. These systems are constructed from microtubules, very long, tubular structures built from a helical wrapping of the protein tubulin (Figure 8.19). There are two kinds of tubulin subunits, oi and each of molecular weight 55,000. They are present in equimolar quantities in the microtubule, which can be considered a helical array of ot-/i dimers. Alternatively, we can view the microtubule as consisting of 13 rows, or protofilaments, of alternating ot and subunits. Because the oi and b units are asymmetrical proteins, with a defined and reproducible orientation in the fiber, the microtubule has a definite sense of direction. 

Microtubules are systems of very long tubular structures built from a helical wrapping of tubulin (Figure 8.19). Tubulin is involved in motility within cells, such as in the mitotic spindle, flagella, and nerve axons. Tubulin is composed of two subunits, ot and each of molecular weight 55,000. They are present in equimolar amounts in tubulin as dimers. 

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