Triple-stranded left-handed helical

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. 

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. 

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. 

At high concentrations, the strands aggregate into large polymeric entities, initially via filament formation, followed by lateral, tree like growth. Figure 10.82a -c shows electron microscope images of the various mixtures under these conditions. Note, especially, the opposite handedness of the L- and D-triple helices (right- and left-handed helices, respectively). 

Figure 45 (a) ORTEP view of the molecular adduct 39 35 (H-bonds are represented by thin lines), (b) ORTEP view of the inclusion complex between benzene and adduct 39 35. (c) Side view of the H-bonding network of adduct 39 35. (d) Simplified representation of the view in (c) showing the right-handed helical motif of the ribbon like H-bonded core of the assembly, (e) Single strand for H-bonded units extracted from the triple-stranded heli-cate structure in 39 35 showing left-handed helicity. (f) Stereoview of the inclusion complex between benzene and adduct 39 35 [60],... 

Structure of tropocollagen. (a) The coiling of three left-handed helices of collagen polypeptides. The dotted lines indicate hydrogen bonds, (b) The right-handed triple-stranded superhelix. The curved arrow indicates the covalent linkage between two chains. [Reproduced with permission from I. Geis and R. E. Dickerson.]... 

Interestingly, the chirality of the diol and diamine components controls the tertiary structure of the complex, in other words the sense of chirality with which the cyclohexane rings wrap around the central core in the triple-stranded helicate. Thus, whereas the (S,S)-diamine 22 gives a left-handed helicate 25 with the (S,S)-diol 23, the (J ,J )-diamine 24 gives a right-handed helicate 26 with the same diol. 

Su Z, Fan J, Sun W-Y. Novel two-fold interpenetrated Zn-based metal-organic framework with triple-stranded right- and left-handed helical chains. Inorg Chem Commun 2013 27 18-21. 

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). 

Helical intertwined stmctures should exhibit chirality. However, the helicates obtained from strands 451 are racemic mixtures of the right- and left-handed double helices. Jodry and Lacour resolved a dinuclear triple helicate 459... 

Manipulation of DNA in such a precise manner demonstrates what can be achieved when the nature of nanoscale interactions is well understood and can be controlled. As more and more becomes known about similar molecular recognition motifs in the natural world the potential to use other biomaterials as structural components will increase. Other structural biomolecules that could be targets for unnatural manipulation include collagen [8] and clathrin proteins [9], shown in Fig. 8.2. The former is a polypeptide that forms a triple helical coil in which the individual strands are left handed but the resulting supercoil is right handed. It provides structure to bone and cartilage and has many other rigidifying functions. 

Figure 10.82 (a) Electron microscope images of materials formed by PLP + ULU (right-handed helix), (b) PDP + UDU (left-handed) and (c) PMP -I- UMU (nonhelical), (d) Schematic representation of the model for the triple-helical organisation of three self-assembled homochiral polymer strands (PTP, UTU)n (T = L or d not m). PU hydrogen-bonded pairs on the same strand are represented by circles of the same shading. Alkyl chains stick ont of the side of the cylinder. (Reproduced with permission from J.-M. Lehn, Supramolecular Chemistry, VCH Weinheim, 1995.)... 

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