Probing aggregation

Use of Near UV CD Spectra to Probe Aggregation in Polypeptides. Near UV CD spectroscopy can be employed to monitor the aggregation of a polypeptide species. Upon association, the aromatic groups will be in different environments than in the isolated monomers and the CD may change dramatically, especially in small peptides. This can be seen for insulin, where the near UV CD intensity of a monomeric form is only [e] - -150 deg cm2 dmoH. Yet, the hexameric form displays a signal nearly twice as intense [110-112],... 

We have shown PFG-NMR-derived diffusion coefficients can be used to calculate molar conductivity and compared with impedance measurements to understand ionicity. In a somewhat reversed experiment, diffusion coefficients measured electrochemicaUy have been compared to PFG-NMR self-diffusion coefficients to investigate proton transfer in protic ILs [19]. Aside from probing ionization dynamics in ILs, PFG-NMR has also been used for probing aggregation as shown in Fig. 5, especially for... 

Probing aggregation by simultaneous static and dynamic light scattering... 

Other chiral HT-PATs namely poly(3-(S -3, 7 -dimethyloctyl)thiophene) [109] confirms that CD spectra can be used, in chiral regioregular HT-PTs to probe aggregation states [110]. This particular polymer can also act as a sensor, by exhibiting large conformational changes induced by minute solvent variation. 

Jamil and Russo studied translational and rotational diffusion of poly-tetrafluoroethylene latex in aqueous sodium polystyrenesulfonate (NaPSS)(75). Spectra were single exponentials Dp and Dr were extracted from the dependence of the spectral linewidths. Addition of NaCl causes the probes to aggregate further addition of NaPSS reverses the salt-induced probe aggregation, even though it appears that NaPSS does not bind to the latex particles. The Dr tracks the changes in viscosity of the solution attendant on addition of polymer, in the sense that rjDr is independent of polymer concentration, other parameters being fixed. However, at fixed c, qDr does decrease as the salt concentration is increased. 

The primary motivation for early studies of probe diffusion was the relationship between Dp and rj. In simple fluids. Dp follows the Stokes-Einstein equation with Dp /r. If the Stokes-Einstein equation remained valid in polymer solutions. Dp would accurately track the solution viscosity, so that measuring Dp would be a replacement for classical rheological measurements. However, in polymer solutions, it is often found experimentally that Dpt depends strongly on c and M. Probe particles generally diffuse faster than expected from rj. Obvious experimental artifacts such as probe aggregation and polymer adsorption by the probes all lead to probes that diffuse too slowly, not too swiftly, and therefore cannot lead to the observed non-Stokes-Einsteinian behavior. 

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