Left-handed propellers

For a rnf(ethylenediamine) complex, in the A configurations (left-handed propeller), the A(555) / /-configuration 38 is most favored, and the A(XXX) ob-configuration 39, the least favored energetically. However, the intermediate X5 and SKK forms are statistically favored over the extreme forms. Recent... 

FIGURE 9-12 Right- and Left-handed Propellers, (a) Left-handed propeller and helix traced by the tips of the blades, (b) Right-handed propeller and helix traced by the tips of the blades. 

Twisted sheets. X-ray diffraction studies have shown that P pleated sheets are usually not flat but are twisted. In a twisted sheet the individual polypeptide chains make a shallow left-handed helix. However, when successive carbonyl groups are viewed along the direction of the chain, a rig t-handed twist is seen (Fig. 2-12). ° Such twisted P sheets are often found in the globular proteins. An example (Fig. 2-13) is the "nucleotide-binding" domain of a dehydrogenase enzyme. The twist of the sheet is seen clearly in this stereoscopic view. When such chains are associated into P sheets, whether parallel or antiparallel, and are viewed in a direction perpendicular to the chains and looking down the edge of the sheet, a left-handed "propeller" is seen. Such a propeller is visible in the... 

Fig. 6. Conformational model of chelated enterobactin (66). Bonds and atoms at the cycloester triseryl backbone are marked with heavier lines. Hydrogen atoms are not shown. A left-handed propeller configuration is shown for the ligands around the metal ion center, but this has not yet been established experimentally...

The PMR data can be interpreted as suggesting that the ligands have the left-handed propeller configuration shown in the model (Fig. 7), although a right-handed configuration cannot be ruled out. Molecular models do not show either configuration to be any more strained than the... 

Finally, there are for the fac isomer 37 (point group C3) (and indeed the mer isomer 38, C, or for any other geometry that has only axial symmetry, D3, C2, and so on) two forms of the molecule that differ, as do right-and left-handed propellers (39 and 40). Such an object (molecule) cannot be superimposed on its image in a mirror. The labels attached (by arbitrary convention) to propeller molecules, such as... 

As another example, consider triphenylborane (Eq. 6.1, where a, b, and c are just labels of hydrogens so that you can keep track of the rotations shown). Triphenylborane cannot be fully planar because of steric crowding, and so it adopts a conformation with all three rings twisted in the same direction, making a right- or left-handed propeller. The M or P descrip-... 

A particular case of helicity is that displayed by molecular propellers [47], aptly described by this picturesque name. An example of a 3-bladed propeller structure is provided by tri-o-thymotide, the (— )-enantiomer of which has now been shown to be the left-handed form (M configuration) (diagram XXXIX) [48]. 

We speak of a screw or a propeller as being right-handed or left-handed. ... 

P and M descriptors a helical, propeller or screw-shaped structure (e.g. S has a helical chain) can be right- or left-handed and is termed P ( plus ) or M ( minus ), respectively. This is illustrated with P)- and (M)-hexahelicene ... 

Such behavior raised many questions. What causes reversal of direction of the propeller Why do the bacteria tumble How does a bacterium "decide" when to tumble How is the flagellum changed from a left-handed to a right-handed superhelix How does this behavior help the bacterium to find food Most intriguing of all, what kind of motor powers the... 

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