Motional flexibility

Finally, it is important to explain the strategy that we have used to construct model compounds on which to carry out ab initio calculations from which we can gain insight into the two classes of electron transfer discussed above. For the kind of polypeptides shown in Figures 1 and 4 and for most species used in ETD or ECD experiments, the positively charged sites reside primarily on side chains that possess great motional flexibility. This means that, as the peptide undergoes... 

Although rotary-motion flexible shafts are almost snakeUke in their ability to reach into inaccessible areas, there is a limit to how much they may be bent without damaging them. This is... 

Follow these simple, basic suggestions, and the design flexibihty of a rotary-motion flexible shaft will be applied to utmost advantage. 

A major advantage of radioscopy is the flexible inspection perspective that allows an optimum adjustment of beam direction and geometrical magnification to the inspection task at hand it also permits in-motion testing. 

Example I he distance between two ends of a large, flexible mole-cti le can provide in form ation about its structural properties or its interaction with solven t.. An alysis of an angle can reveal a h in ged motion in a rn acrorn olecu le. 

Rather than calculate the etithaltty conti ibutioti of the torsional states individually, an empirical sum that is an integral multiple of 0.42 kcal mol per torsional degree of freedom is assigned to flexible molecules in MM3, Torsional motion of a methyl gntup is iK)t added k) a calculated because it is included in the... 

Many simulations attempt to determine what motion of the polymer is possible. This can be done by modeling displacements of sections of the chain, Monte Carlo simulations, or reptation (a snakelike motion of the polymer chain as it threads past other chains). These motion studies ultimately attempt to determine a correlation between the molecular motion possible and the macroscopic flexibility, hardness, and so on. 

It is generally recognized that the flexibility of a bulk polymer is related to the flexibility of the chains. Chain flexibility is primarily due to torsional motion (changing conformers). Two aspects of chain flexibility are typically examined. One is the barrier involved in determining the lowest-energy conformer from other conformers. The second is the range of conformational motion around the lowest-energy conformation that can be accessed with little or no barrier. There is not yet a clear consensus as to which of these aspects of conformational flexibility is most closely related to bulk flexibility. Researchers are advised to first examine some representative compounds for which the bulk flexibility is known. 

The furanose rings of the deoxyribose units of DNA are conformationally labile. All flexible forms of cyclopentane and related rings are of nearly constant strain and pseudorotations take place by a fast wave-like motion around the ring The flexibility of the furanose rings (M, Levitt, 1978) is presumably responsible for the partial unraveling of the DNA double helix in biological processes. 

Increa sing the bulkiness of the alkyl group from the esterifying alcohol in the ester also restricts the motion of backbone polymer chains past each other, as evidenced by an increase in the T within a series of isomers. In Table 1, note the increase in T of poly(isopropyl methacrylate) over the / -propyl ester and similar trends within the butyl series. The member of the butyl series with the bulkiest alcohol chain, poly(/-butyl methacrylate), has a T (107°C) almost identical to that of poly(methyl methacrylate) (Tg = 105° C), whereas the butyl isomer with the most flexible alcohol chain, poly( -butyl methaciylate), has a T of 20°C. Further increase in the rigidity and bulk of the side chain increases the T. An example is poly(isobomyl methacrylate)... 

The labyrinth sleeve can be flexibly mounted to permit radial motion for self-ahguiug effects. In practice, a radial clearance of under 0.008 is difficiilt to achieve. 

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