Triple strands

Because of the versatility of roller chain drives, certain standards have been established. With these standards, interchangeability of chains is possible between one manufacturer and another. As long as chains are identifiable, they can be cross-referenced easily without any serious operational problems arising. Table 58.7 shows the standard roller chain American Standards Association (ASA) number by pitch length for single, double, and triple strand chains. 

Single strand Double strand Triple strand... 

Another feature of this particular exopolysaccharide is that gel strength depends upon the temperature used. It is constant between 60-80°C, increasing in strength from 80-100°C and finally changing structure from a single to a triple stranded helix at temperatures over 120°C. This makes it particularly well suited for use as a molecular sieve, immobilised enzyme support and a binding agent. 

The leucine zipper motif was recently successfully employed for the construction of native-like triple-stranded coiled-coil in solution [29]. 

The following pairing rules apply to triple strands ... 

There are two levels of self-assembly in the formation of tetra-, penta-and hexa-nuclear products from the poly-bipyridyls (L) 20 and 21 and iron(II) salts FeCl2, FeBr2 or FeS04 - the products are anion-dependent. The coordination of three bpy units, from different ligand molecules, to the Fe2+ centers produces a helical structure interaction of these helical strands with anions results in further molecular organization to form the final toroidal product. The discussion draws parallels between the helical and toroidal structures here and secondary and tertiary structure in biological systems (482). Thermodynamic and kinetic intermediates have been characterized in the self-assembly of a di-iron triple stranded helicate with bis(2,2/-bipyridyl) ligands (483). 

A recently discovered subset of triple-stranded /l-helices from bacteriophage tail proteins (alternatively termed triple-stranded /1-solenoids ) represents another distinct group of /1-fibrous folds (Fig. 3B). In these structures, three identical chains related by threefold rotational symmetry wind around a common axis. These chains form unusual parallel /1-sheets with no intra- and only intermolecular -structural hydrogen bonding. Kajava and Steven (this volume) survey the distinguishing structural features of the known triple-stranded /1-solenoids, also documenting their notable diversity and differences in comparison to the single-stranded /1-solenoids. 

The abundance, location, stability, and folding of the triple-stranded /3-helices are also reviewed in a chapter by Mitraki, Papanikolopoulou, and van Raaij, which is dedicated to triple /3-stranded fibrous folds in the viral fibers. 

All the above-mentioned proteins have single-stranded folds based on solenoidal windings of one polypeptide chain. Recently, however, several triple-stranded /1-helices (alternatively, triple-stranded /l-solenoids ) have been described in bacteriophage tail proteins (Kanamaru et al., 2002 Smith et al., 2005 Stummeyer et al., 2005 van Raaij et al, 2001). In these structures, three identical chains wind around a common axis and their coils have an axial rise of 14.5 A, that is, 3 x 4.83 A (for details see Sections IV and V.D). In this chapter, triple-stranded /l-solenoids will be abbreviated as TS /l-solenoids, while the term /1-solenoid, if not otherwise qualified, will apply to the predominant group of single-stranded /l-solenoids. 

Most known //-solenoids are monomers. Recently, however, several triple-stranded (TS) //-solenoids have been described in which the three chains wind around a common axis. The three monomers are related by... 

Fig. 7. Triple-stranded (TS) /8-solenoids. (A) Ribbon diagram of a fragment of the T4 short tail-fiber gpl2 (van Raaij et al., 2001) and (B) cross-sectional shapes of the TS //-solenoids. Repetitive turn-strand elements of each individual chain of the TS //-solenoids are rendered in different colors.

D, Differences Between Sequences of Single-Stranded and Triple-Stranded (3-Solenoids... 

Smith, N. L., Taylor, E.J., Lindsay, A. M., Chamock, S.J., Turkenburg,J. P., Dodson, E.J., Davies, G. J., and Black, G. W. (2005). Structure of a group A streptococcal phage-encoded virulence factor reveals a catalytically active triple-stranded beta-helix. Proc. Natl. Acad. Sci. USA 102, 17652-17657. 

C. Role of Globular Domains in the Folding of Triple /.-Stranded... 

The three-dimensional structural architecture of plant defensins is exemplified by the structure of Rs-AFP, ° which comprises an N-terminal /3-strand followed by an ct-helix and two /3-strands (/3a/3/3 configuration). The /3-strands form a triple-stranded antiparallel /3-sheet. The three-dimensional structure is stabilized by three disulfide bonds. In general, in plant defensins two disulfide bonds form between the ct-helix and the central /3-strand. A third disulfide bond stabilizes the structure by linking the /3-strand after the helix to the coiled part after the ct-helix. This motif is called the cysteine-stabilized a/3-motif (CSa/3)" and also occurs in toxins isolated from insects, spiders, and scorpions.The fourth disulfide bond links the C-terminal end of the peptide with the N-terminal /3-strand. Two plant defensins, PhDl and PhD2, feature a fifth disulfide bond and have been proposed to be the prototypes of a new subclass within plant defensins." As a result of these structural features the global structure of plant defensins is notably different from o //3-thionins, which is one of the reasons for the different nomenclature. The structures of plant defensins Rs-AFP ° and NaDf are shown in Figure 6, where they are compared to the thionin /3-purothionin and the structurally more related drosomycin and charybdotoxin. ... 

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