Helical-shaped

Dou ble hel ix (Section 28 8) The form in which DNA normally occurs in living systems Two complementary strands of DNA are associated with each other by hydrogen bonds be tween their base pairs and each DNA strand adopts a helical shape... 

FIGURE 19.19 A representation of part of an a helix, one of the secondary structures adopted by polypeptide chains. The cylinder encloses the "backbone" of the polypeptide chain, and the side groups project outward from it. The thin lines represent the hydrogen bonds that maintain the helical shape. 

The work reported here used a tubular reactor of approx. 2.5 cm id and 150 meters in length. The reactor, lined with a fluorinated polymer, was coiled in a helical shape. The recipe employed standard concentrations of initiator and emulsifier. 

Ho et al. were able to verify the a-helical shape of the polymer by circular dichroism (CD) spectra. No structural elements were observed until the formation of the double helical DNA at which point they observed a right-handed a-helix in the polythiophene backbone. Their work demonstrates the power of fluorometric detection as they noted a seven order of magnitude increase in detection sensitivity (20 fM in 200 pi) simply through the use of fluorometric detection as opposed to UV-vis absorption. The polymer in solution has a high fluorescence yield with a maximum at 530 nm (Fig. 11a). Upon formation of the duplex the fluorescence is significantly quenched (Fig. lib), while with the addition of the complementary DNA and triplex formation, the fluorescence intensity is enhanced by a factor of 5 (Fig. 11c). The inherent sensitivity of the spectral shift even allowed distinction between DNA with only one and two mismatched bases (Fig. lOBd, e). 

Chemists commonly use a process called X-ray diffraction to determine the structure of a compound. Two pioneers of X-ray diffraction were Rosalind Franklin and Dorothy Crowfoot Hodgkin. Franklin s work on the DNA molecule was instrumental in the discovery of its helical shape. Nobel Prize winner Hodgkin discovered the structure of complex molecules, such as cholesterol, penicillin, insulin, and vitamin B-12. Find out about X-ray diffraction how it works and the types of scientists who employ this technology. 

When determining the range of likely helical shapes from intrinsic properties of amylose, this variability in monomer shape is almost as important as hindered rotation about the bonds linking the monomers. This conclusion is supported by conformational analyses of maltose such as shown in Figure 5 of the introductory chapter of this book. There are relatively small ranges (about 40 ) of allowed torsional rotation within one kcal/mol of the minimum (one must correct for the fact that there are two glucose residues in maltose when making such a coiqparison). ... 

The shape of macromolecules within a folded lamella is not the same for all polymers. In crystalline polyethylene, for example, the chains assume a planar zigzag conformation, but in some other polymers like polypropylene and polyoxymethylene the chains prefer a helical shape, as in proteins. The helix might have three, four, or five monomer units per turn, i.e., the helices are three-, four-, or five-fold (Fig. 1.12)... 

Propeller A propeller is a three- or four-bladed flow impeller, having helically shaped blades. The flow is primarily axial (discharge flow parallel to the agitator shaft) and is most effective in low-viscosity fluids. The marine-type propeller (Figure 3.15) is characteristically operated at relatively high speed, particularly in low-viscosity liquids (Treybal, 1980). 

One way to picture the DNA molecule is to consider a ladder twisted into a helical shape. The sides of the ladder represent the sugar- phosphate backbone, and the rungs are the amine base side chains. The amine bases forming the rungs of the ladder exist as complementary pairs. The amine base on one-strand hydrogen bonds to its complementary base on the opposite... 

Chirality due to a helical shape.52 Several compounds have been prepared that are chiral because they have a shape that is actually helical and can therefore be left- or right-handed in orientation. The entire molecule is usually less than one full turn of the helix, but this does not alter the possibility of left- and right-handedness. An example is hexahelicene,53 in which one side of the molecule must lie above the other because of... 

Figure 10.5. Power number, Np = PgJN3Dsp, against Reynolds number, NR = ND2p/p, for several kinds of impellers (a) helical shape (Oldshue, 1983) (b) anchor shape Oldshue, 1983) (c) several shapes (1) propeller, pitch equalling diameter, without baffles (2) propeller, s = d, four baffles (3) propeller, s = 2d, without baffles (4) propeller, s = 2d, four baffles (5) turbine impeller, six straight blades, without baffles (6) turbine impeller, six blades, four baffles (7) turbine impeller, six curved blades, four baffles (8) arrowhead turbine, four baffles (9) turbine impeller, inclined curved blades, four baffles (10) two-blade paddle, four baffles (11) turbine impeller, six blades, four baffles (12) turbine impeller with stator ring (13) paddle without baffles (data of Miller and Mann) (14) paddle without baffles (data of White and Summerford). All baffles are of width 0.1D [after Rushton, Costich, and Everett, Chem. Eng. Prog. 46(9), 467 (1950)].

Figure 2 shows the structures of four classes of so-called overcrowded alkenes (see Section 5.3.1), designed as molecular components for a chiroptical switch based on CPL irradiation.1191 Thanks to unfavorable steric interactions around the central ole-finic bond, the molecules are forced to adopt a helical shape. The chirality in these inherently dissymmetric alkenes - denoted M and P for left-handed and right-handed helices, respectively - therefore originates from distortion of the molecular... 

The sterically overcrowded alkenes shown in Scheme 6 have been exploited in our group since, from the perspective of molecular switches design, they combine a number of attractive structural features. Steric interactions between the groups attached to the central olefmic bond force these molecules to adapt a non-planar helical shape. The chirality of these so-called inherently dissymmetric alkenes 3, is therefore the result of distortion of the entire molecular structure. Beside the heli-cene-like geometry, both a cis- and a trans-stilbene chromophore are present in the same molecule. 

The SSE is an important and practical LCFR. We discussed the flow fields in SSEs in Section 6.3 and showed that the helical shape of the screw channel induces a cross-channel velocity profile that leads to a rather narrow residence time distribution (RTD) with crosschannel mixing such that a small axial increment that moves down-channel can be viewed as a reasonably mixed differential batch reactor. In addition, this configuration provides self-wiping between barrel and screw flight surfaces, which reduces material holdback to an acceptable minimum, thus rendering it an almost ideal TFR. 

Isotactic vinyl polymers -(CH2-CHX)- can, because of sterical hindrance, in most cases not lie in zig-zag shape the chain then takes, as the most regular conformation, a helical shape. An example is given in Figure 4.13, again as a projection on the plane of drawing, for PP, which as a helix crystallizes in a monoclinical lattice. 

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