Molecular conformation changes

This is, for instance, the case of PTFE, which at atmospheric pressure presents two reversible first-order transitions at 19 °C and 30 °C [67], In the transition at 19 °C the molecular conformation changes slightly, from a 13/6 to a 15/7 helix and the molecular packing changes from an ordered structure with a triclinic unit cell (corresponding to a positioning of the chain axes nearly hexagonal) toward a partially disordered structure (partial intermolecular rotational disorder) with a... 

Quantum mechanical calculations are also useful in predicting minimum energy conformations of chromophores and for predicting properties such as dipole moments. Calculated dipole moments are shown for several chromophores in Table 4. Such calculations are particularly useful in understanding how chromophore molecular conformation changes with addition of substituents. 

Leahy [30], van de Waterbeemd [14] and Albery [16] discussed the meaning of the rate constants, koa, kao it is sufficient to state here that the primary process involved is the resolvation of the compound as it moves from the octanol to water environments, that is, the loss of any strongly associated octanol molecules and gain of water. Because of the mutual solubility of these solvents it seems reasonable to say that some solvent molecules may transfer along with the compound (which may not be true for other solvent systems where the mutual solubilities are lower). There may also be molecular conformation changes during this process of transfer, which would also be likely to affect solvation. 

Structural lability due to intermolecular association and molecular conformation changes in response to changes in pH, redox potential (Eh), electrolyte concentration, and functional-group binding... 

This shift was interpreted with the aid of DFT calculations in terms of molecular conformational changes of (NpS.. SNp)" radical anion due to the elongation of the S-S bond from 0.213 nm in a rigid matrix at 77 K up to 0.290 nm in fluid phase at 273 K. The (NpS.. SNp)" radical anion dissociates into napthylthiyl radical and thionaphtholate anion [reaction (38)] in DMF solution at 293 K with a first-order rate constant k= 10 s"h... 

Two of these directions correspond to up and down along the myosin duead, and the connections in the two lateral directions are also directed upwards and downwards. It is natural then to relate dtese four-coordinated channel regions of the Q surface to the myosin heads. Thus the possible connections via the 3i-centred actin threads of myosin threads have only two directions, which fulfil the crystallographic symmetry of the surface. It is proposed here that these two directions correspond to the initial direction of the myosin head before movement and the end direction after movement, respectively. In this model of the contraction, the time-phase of mobility represents a transient disorder condition of adjacent structure elements, whereas the structure as a whole fulfils the required crystallographic symmetry. If the individual molecular conformational changes must be accommodated within the periodicity of the surface, the necessary perfect long-range synchronisation of mobility over the entire muscle seems a natural consequence. 

Solid-state SS-NMR and XRPD take advantage of different phenomena to evaluate polymorphism. XRPD is sensitive to differences in unit cell dimensions, whereas SS-NMR is sensitive to conformational changes and differences in magnetic environments.94 One can think of two hypothetical situations. If two polymorphic forms differ exclusively in their unit cell dimensions but the conformation of the molecule is preserved, then the XRPD patterns are likely to be substantially different, whereas SS-NMR may not register any changes. On the other hand, if the unit cell dimensions are preserved while the molecular conformation changes substantially, SS-NMR is likely to pick up the differences and the XRPD patterns might be the same. In this sense, both techniques provide information that is complementary to the other.94... 

The relative accuracy of the M0DP0T/VRDD0 method along a potential energy surface (such as for molecular conformational changes or interactions between two species, 0.0001 - 0.0002 a.u.) is even greater than its absolute accuracy. 

The valinomycin study just described illustrates the value of close comparison of NMR and X-ray diffraction data. Correlation of NMR assignments with X-ray data for known structures may allow the subsequent elucidation of conformational features for related materials on the basis of NMR data alone. It is notoriously difficult to relate solid-state structures determined by X-ray diffraction to the solution conformation observed by conventional NMR spectroscopy, although data from the former technique are often the only type available for work on receptor recognition. However, solid-state NMR may provide a link between these two techniques by indicating whether molecular conformation changes do occur when the physical state is altered. In contrast to the valinomycin complexes discussed above, it has been found that significant differences in the solution and solid-state spectra of morphine... 

The molecular conformation change of the azo chromophore can be used to switch the conformation and hence properties of larger molecular systems to which it is attached. This is particularly interesting in the case of inclusion within molecular-scale biological systems. The bridging of biology and physical... 

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