Rotational dynamics heterogeneities

A H-NMR study of the dynamics of the phenyl ring of a side-chain liquid crystal polymer has been carried out. The rotation of the phenyl ring about its paru-axis has been attributed to the relaxation process and a Gaussian distribution of the activation energy points to a dynamic heterogeneity of the... 

Molecular dynamics simulations were employed to explore the dynamic heterogeneity in water motion around a protein-DNA complex. The complex chosen was the DNA-binding domain of human TRFl protein and a telomeric DNA (see Figure 11.1). It was observed that the slow water dynamics correlated with the relaxation time of HBs of the water molecules connected with the protein and the DNA. The restricted motion of such water molecules led to sluggish rotational and translational dynamics of the hydration layer near the protein and DNA molecules in their complexed forms as well as in their free forms (see Figure 11.2). 

Figure 22 Complex dynamics at the glass transition of soft matter from multidimensional NMR. (a) Correlation times of chain motion of atactic polypropylene from different NMR experiments, (b) Geometry of rotational motions, (c) Dynamic heterogeneities as visualized by computer simulation. Reproduced with permission from Spiess, H. W. J. Polym. Sci. 2004, A42,5031-5044. Copyright 2004, by John Wiley Sons, Inc.

When the orientation of a single dye molecule is plotted as a funaion of time, it could be seen that some dye molecules did not rotate during a long time period after which they suddenly started rotating much faster. The completely different rotational dynamics are a clear evidence of temporal heterogeneity. An example of such behavior is presented in Hgure 18. 

Pulsed deuteron NMR is described, which has recently been developed to become a powerftd tool for studying molectdar order and dynamics in solid polymers. In drawn fibres the complete orientational distribution function for the polymer chains can be determined from the analysis of deuteron NMR line shapes. By analyzing the line shapes of 2H absorption spectra and spectra obtained via solid echo and spin alignment, respectively, both type and timescale of rotational motions can be determined over an extraordinary wide range of characteristic frequencies, approximately 10 MHz to 1 Hz. In addition, motional heterogeneities can be detected and the resulting distribution of correlation times can directly be determined. 

The above experimental results largely relate to spectroscopic techniques, which do not give direct information about the spatial scale of the molecular motions. The size of the spatial heterogeneities is estimated by indirect methods such as sensitivity of the dynamics to the probe size or from the differences between translational and rotational diffusion coefficients (rotation-translation paradox). It might be expected that the additional spatial information provided by neutron scattering could help to discriminate between the two scenarios proposed. 

The involvement of lysine amino acid residues on cytochrome c in the heterogeneous reactions with functionalized electrodes seems to have been established. Importantly, it is now thought that the proposed protein-promoter complex is more likely to be dynamic, as revealed by the results of a recent investigation (28) of site-specific 4-chloro-3,5-dinitrophenyl (CDNP)-substituted cytochrome c. It was found that monosubstitution of either Lys 13 or Lys 72 did not result in any significant change in its electrochemical response, whereas two modifications greatly decreased the heterogeneous rate constant, and complete loss of electrochemical activity was observed upon modification of more lysines. It was proposed that the electrode reaction occurred in numerous rotational conformations. Therefore, for the mono-... 

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