Three-fold helical conformation

Figure 10. Computer-generated projections of a left-handed three-fold helical conformation for Klebsiella K16...

The crystal polymorphism of the chiral but racemic P5MH1 is, to some extent, very reminiscent of that of isotactic polypropylene. It exists in two crystal modifications. One crystal modification is stable at high temperature, and was observed early on by Corradini et al [39]. Its structure has been redefined as a chiral, frustrated one based on a trigonal cell with three threefold helices per cell. We have also discovered a second crystal modification produced from solution. It has an orthorhombic unit cell that contains four chains in - again - three-fold helical conformation, for which one must assume coexistence of two right- and two left-handed helices. Contrary to the a and ft phases of iPP, the frustrated structure of poly( 5-methyl-hexene-1) is the more stable one [40]. 

Mesophase reveals intermediate order between amorphous and crystaUine phases. In the first studies it was labelled as smectic (Natta Corradini, 1960) or paracrystalline (Miller, 1960). Further studies revealed that mesophase is made up of bundles of parallel chains, which maintain typical for all polymorphic forms of polypropylene three-fold helical conformation. Bundles are terminated in the direction of the chain axis by helix reversals or other conformational defects (Androsch et al., 2010). In the bundles long range ordering maintains only along the chain axes, whereas in lateral packing a large amount of disorder is present (Natta Corradini, 1960). The mesophase is formed by quenching of the molten polypropylene (Miller, 1960 Wyckoff, 1962) or by deformation of the crystalline structure (Saraf Porter, 1988 Qiu, 2007). As for the fibres, the mesophase was observed in fibres taken at low take-up velocity (Spruiell White, 1975 Jinan et al., 1989, Bond Spruiell, 2001) in fibres intensively cooled in water with addition of ice or in the mixture of dry ice... 

Basically, in the extended conformation as well as in the three-fold helical conformation of the iPS chain, the phenyls do not form excimer states, because the distance between parallel phenyls in the three-fold helical conformation is 0.665 nm, as reported by Natta [62] and Sundararajan and co-workers [54]. This distance is too large for excimer formation of the sandwich type (which have a distance of 0.3-0.35 nm). In this situation, the excimer can be formed only outside the crystalline region, e.g., in the region of the lamellar borders, because in these areas some deformation of the helical conformation of the PS chain makes the excimer structure formation more probable. The excitation energy can effectively migrate along the helical structure [60, 63] to the lamellar border, where... 

Because of familiarity with x-ray crystallography and IR spectroscopy, Natta was able to show that solid polypropylene was a crystalline stereoregular polymer with a three fold helical conformation. As stated in Floiys book, which was read by Natta, C. Schildknecht had... 

The case of isotactic polypropylene (iPP) presents some differences with respect to those just discussed. While both sPP and PET adopt in their mesophases disordered, extended, essentially non-helical conformations, iPP is characterized by a unique, relatively well ordered, stable chain structure with three-fold helical symmetry [18,19,36]. More accurately we can state that an iPP chain segment can exist in the mesophase either as a left handed or as the enantiomeric right-handed three-fold helix. The two are isoener-getic and will be able to interconvert only through a rather complex, cooperative process. From a morphological point of view Geil has reported that thin films of mesomorphic iPP quenched from the melt to 0 °C consist of... 

The final Klebsiella polysaccharide that will be mentioned specifically is KZS, whose chemical structure was determined by Niemann et al (24) (Figure 30). This polysaccharide is of particular interesT because its backbone consists of a similar alternating 1, 3 diequatorial, 1, 4 diequatorial glycosidic linkage geometry to that found in the connective tissue polysaccharides, hyaluronic acid, chondroitin sulphate and dermatan sulphate (25). All these three polysaccharides have exhibited 3-fold helical conformations with axially projected chemical repeats in the range 0.95 - 0.97 nm which is comparable to 0.97 nm found in K25 which also forms a 3-fold helix (14). Further, left-handed helices were found to be more favourable in K25 as has previously been observed in the connective tissue polysaccharides. The similarities between these various different structures is apparent in Figure 31 which shows projections down the axis of K25 and several of the connective tissue polysaccharides. 

Sodiocellulose II crystallizes in a hexagonal unit-cell, with a = 1.0 nm and c = 1.51 nm. The meridional reflection on the third layer-line led to a three-fold helical structure. The unit cell contains two 3(—0.503) chains of sodiocellulose, six sodium ions, six hydroxyl ions, and at least six water molecules. Conformational calculations and packing analysis of the chains led to a structure which is similar in several features to that of poly[(l— 4)-j3-D-Xylp].16... 

In its hydrated, cationic form, ehitosan adopts helical conformations with eight residues per five left-hand or three right-hand turns,in four-fold helical conformations of dimeric units. The anhydrous, fully deacetylated ehitosan adopts a crystalline form much like that of cellulose II,with (jo = = 98°, P = -148° 92°), and the hydroxymethyl group adopting the gt confor-... 

Figure 13.30 Ribbon diagram of the structure of Src tyrosine kinase. The structure is divided in three units starting from the N-terminus an SH3 domain (green), an SH2 domain (blue), and a tyrosine kinase (orange) that is divided into two domains and has the same fold as the cyclin dependent kinase described in Chapter 6 (see Figure 6.16a). The linker region (red) between SH2 and the kinase is bound to SH3 in a polyproline helical conformation. A tyrosine residue in the carboxy tail of the kinase is phosphorylated and bound to SH2 in its phosphotyrosine-binding site. A disordered part of the activation segment in the kinase is dashed. (Adapted from W. Xu et al.. Nature 385 595-602, 1997.)...

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