Triple-stranded helicates

FIGURE 19.12 (See color insert.) VPL motor at (a) neutral and (b) acidic pH. (a) Front view of the partially a-helical triple stranded coiled coil. VPL motor is in the closed conformation, (b) VPL Motor in the open conformation. The random coil regions (white) are converted into well-defined helices and an extension occurs at lower pH. 

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. 

The T4 short tail fiber triple /l-helix is connected to a more globular head domain via residues 333-341, which form a very short a-helical triple coiled-coil. Residues 342-396, together with the C-terminal /1-strand composed of amino acids 518-527 (the collar ), are the only part of the structure in which the monomer has a recognizable fold. It may therefore be the first part of the protein to fold, followed by a zipping-up of the N-terminal domain and the top domain. The small, globular, domain contains six /1-strands and one a-helix and has some structural homology to gpl 1, also of bacteriophage T4. Three of the /1-strands and the a-helix formed by residues... 

The Watson and Crick model for DNA as a double helix is only a generalized model to describe much more complex structures. Along with the typical double helix there exist structural elements such as supercoils, kinks, cruciforms, bends, loops, and triple strands as well as major and minor grooves. Each of these structural elements can vary in length, shape, location, and frequency. Even the simple DNA double helix can vary in pitch (number of bases per helical turn), sugar pucker conformation, and helical sense (whether the helix is left-or right-handed). 

Triple-Stranded Left-Hand Helical Cellulose Microfibril in Acetobacter xylinum and in Tobacco Primary Cell Wall... 

Tobacco primary cell wall and normal bacterial Acetobacter xylinum cellulose formation produced a 36.8 3A triple-stranded left-hand helical microfibril in freeze-dried Pt-C replicas and in negatively stained preparations for transmission electron microscopy (TEM). A. xylinum growth in the presence of 0.25 mM Tinopal disrupted cellulose microfibril formation and produced a... 

Figure 1. Freeze-dried gel of A. xylinum cellulose ribbons deposited during normal growth. The arrows point to triple-stranded left-hand helical microfibrils averaging 36.8 3A in diameter (1). The sample was replicated with 17.3A Pt-C and backed with 90.2A of carbon. 

In tobacco primary cell wall the cellulose microfibrils observed individually or associated with bundles were also triple-stranded and left-hand helical. These observations are shown in Figure 10. Since cellulose is only 19% of the tobacco cell wall (17), the task of finding and identifying cellulose was complicated. For this reason A. xylinum which produces a pure ribbon of cellulose was used for studying cellulose structure. 

Submicrofibril and triple-stranded left-hand helical microfibrils are found in tobacco primary cell wall and bacterial A. xylinum cellulose. We suspect from our results and the literature survey outlined in reference (1) that the triple stranded structures are prominent in the primary plant cell wall. The highly crystalline cellulose of plant and algae secondary cell wall appears by X-ray fiber diffraction (18,19) and TEM lattice imaging (20-23) to be largely crystalline arrays of planar straight chains of (l-4)-/3-D-glucan chains. 

Andre, N. Scopelliti, R. Hopfgarter, G. Piguet, C. Biinzli, J.-C. G. Discriminating between lanthanide ions self-assembly of heterodimetallic triple-stranded helicates. Chem. Commun. 2002, 214-215. 

More recently, oligobidentate ligands were also used for the synthesis of various triple-stranded helical complexes around metals with a preference for octahedral coordination, e.g. Co(II) [85] or Ni(II) [86], or around Ag(I) [87] and Ga(III) ions [88]. 

Helix bundles. A third peptide chain can be added to a coiled coil to form a triple-stranded bundle.180-183 An example is the glycoprotein laminin found in basement membranes. It consists of three peptide chains which, for -600 residues at their C-terminal ends, form a three-stranded coil with heptad repeats.182184 Numerous proteins are folded into four helical segments that associate as four-helix bundles (Fig. 2-22).185-188 These include electron carriers, hormones, and structural proteins. The four-helix bundle not only is a simple packing arrangement, but also allows interactions between the + and - ends of the macro-dipoles of the helices. 

Lovejoy, B., Choe, S., Cascio, D., McRorie, D. K., DeGrado, W. F., and Eisenberg, D. (1993). Crystal structure of a synthetic triple-stranded a-helical bundle. Science 259, 1288-1293. 

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