Torsional flexibility

Besides the inherent lateral natural frequency characteristic, compressors are also influenced by torsional natural frequencies. All torsionally flexible drive trains are subject to non-steady or oscillatory excitation torques during normal operation of the system. These excitation torques can be an inherent function of either the driver or the driven equipment and, when superimposed on the normal operating torque, may appear to be of negligible concern. However, when combined with the high inertia loads of many turbomachinery trains and a torsional resonant frequency of the system, these diminutive ripples can result in a tidal wave of problems. 

Crystal interactions may lead to a number of readily observable consequences. The molecular geometries may be different in different polymorphic forms of a given compound or, as in the case above, in symmetry-independent sites in a given crystal a change of molecular substituents remote from the torsionally flexible bonds may change the molecular conformation, as a result of modification... 

Bending or torsional flexibility may be provided by bends, loops, or offsets by corrugated pipe or expansion joints of the bellows type or by other devices permitting rotational movement. These devices must be anchored or otherwise suitably connected to resist end forces from fluid pressure, frictional resistance to pipe movement, and other causes. 

Figure 16 Nicotinamide (pyridine-3-carboxamide) and isonicotinamide (pyridine-4-carbox-amide) ligands. Curved arrows indicated torsional flexibility at positions a-8. Reprinted from L. Brammer, J. C. Mareque Rivas, R. Atencio, S. Fang and F. C. Pigge, J. Chem. Soc., Dalton Trans., 3855-67 (2000). Reproduced by permission of the Royal Society of Chemistry.

Thouvenin, E., Laurent, S., Madelaine, M.-F., Rasschaert, D., Vautherot, J.-F., and Hewat, E. A. (1997). Bivalent binding of a neutralizing antibody to a calicivirus involves the torsional flexibility of the antibody hinge. / Mol. Biol. 270, 238-246. Roux, K. R. (1984). Direct demonstration of multiple VH allotopes on rabbit Ig molecules Allotope characteristics and Fab arm rotational flexibility revealed by immunoelectron microscopy. Eur. J. Immunol. 14, 459-464. 

The basic strategy in studying the conformation of polymers is to identify the principal factors that are responsible for their physical properties. Polysiloxanes constitute a suitable class of polymers, because they show two levels of flexibility in terms of molecular structure torsional flexibility, which... 

The methyl group at position 6 was incorporated for favorable interaction with Trp80. This also favorably restricted the torsional flexibility for the 5-substituent, and increased the inhibitory potency against human TS by 10-fold. The 2-methyl was replaced by an amino group, to create a hydrogen bond to a backbone carbonyl in the protein, and increased potency another sixfold. Compound... 

Katzenellenbogen et al. developed small molecules that inhibit the binding of coactivator peptides to ERa [9]. Their de novo design was based on a diverse set of heterocycle cores mimicking the helical scaffold of the coactivator peptides. Pyrimidines featuring side-chains mimicking Leu were found to be the most effective binders (8, Figure 2.3). Due to the intrinsic torsional flexibility of pyrimidine... 

Dlakic, M. Harrington, R. E. Bending and torsional flexibility of G/C-rich sequences as determined by cyclization assays. J. Biol. Chem. 1995 270 29945-29952. 

Even if the torsion of the chain of polythiophene could account for the chromic phenomena, we must still find good theoretical reasons why such a torsion should be possible. An ab initio theoretical study of the barriers to torsional flexibility in conjugated polymers in the gas phase [27], demonstrated that they are of the order of 0.7 ey clearly allowing conformational flexibility. This rationalizes the solubility of the polymers, but also warns that the polymer solid, where chain-chain interactions must be taken into account, is a rather different system. 

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