Cross-relaxation water solution

Cyclo(Pro-Gly) (fig. 3) is a convenient model for demonstration of various aspects of 2D exchange spectroscopy. It is small rigid molecule with 10 protons, of which 8 are spectroscopically well resolved. It is well dissolved in dimethyl sulfoxide (DMSO)Zwater mixtures and stable at a broad range of temperatures. We used a 10 mM solution of cyclo(Pro-Gly) in 70/30 volume/volume mixture of DMSO/water. This solvent mixture is suitable for the cross-relaxation studies because it is rather viscous even at room temperature and does not freeze down to 223 K [29, 30]. Thus, molecules dissolved in this mixture can be studied at a broad range of temperatures (correlation times). 

Another interesting implication of the cross-relaxation results is relevant to the question of compartmentalization of water within tissue. Because relaxation rates in the presence of cross-relaxation must be derived by solution of coupled differential equations, the observed relaxation rates are not simply the sum of the intramolecular and cross relaxation contributions to the relaxation. For a typical situation, the solvent relaxation may be influenced substan-... 

Gerig has evaluated force field models for trifluoroethanol-water mixture in order to perform a reliable MD simulation of the system. In this study it was very important to obtain good description of solvent fluorine/solute hydrogen NMR cross-relaxation obtained from solute-solvent nuclear Overhauser effects (NOE). An additional requirement was good agreement between experimental and simulated diffusion coefficients as NMR cross relaxation depends on the diffusion of the components in the mfacture. It was found that OPLS-AA (TFE) and TIP5P-E (H2O) did do a reasonably good work. The mixture will be used as a solvent for small peptides in the future. 

Nevertheless, the T1 and NOE values are found to be unchanged from the solution to the gel of water content 75% and 53%. As discussed already in Section 2.1, it is very difficult to analyse such relaxation parameters in terms of a single correlation time. The calculated correlation times are found to give consistent values among those obtained from the Ti, T2, and NOE values with the width parameter of p = 18 (PVP solution) and p = 16 (PVP gel) and b = 1000. As the degree of cross-linking is increased, a discrepancy in the correlation times still occurs (a factor of 3 at 75%) but is much improved by this treatment. 

Fig. 17. The water proton spin-lattice relaxation rates as a function of magnetic field strength reported as the proton Larmor frequency in aqueous 1.8 mM samples of bovine serum albumin. The lower data set was taken on the solution, the open circles taken after the sample had been cross-linked with glutaraldehyde to stop rotational motion (89).

The Rhodonines, like other aromatic members of the carboxylic family are not very soluble in water. They would be expected to be soluble in less polar solvents like ether or alcohol. In dilute solution, their absorption cross section for both their isotropic and anisotropic spectra would be expected to be very small. To the extent they can be excited, their structure would continue to suggest, they will not relax via fluorescence or phosphorescence. 

Water-based permanent gels are not structurally different from other permanent gel systems. They acquire special character mainly when they are ionized and the ionic strength of the medium is sufficiently low. Suitable copolymers in aqueous solution can form a vast variety of reversible gel systems because of the moderately strong, primary interactions possible within the aqueous environment. Structural organization (mainly cooperative effects) are then used, to a limited extent, to create sufficiently long-lived bonds that form gels of adequately long relaxation and cross-link turnover times. 

Figure 7.10. The effect of light on the dynamic properties of physical cross-links between azobenzene-modified poly(acrylate) and poly(cyclodextrin). The relaxation modulus in stress relaxation experiments is plotted after application at time zero of a fixed strain to a 0.7% polymer solution in water (polymer structure, cf. Fig. 7.1, with n = 11 and x=3) with polycyclodextrin at 0.25% (lower curves) or 0.5% (higher moduli). The sample was either incubated for 24h in the dark (dark-adapted, closed symbols) or continuously exposed to UV before and after loading in the rheometer (open symbols). Details on the samples composition are given in Pouliquen et al. (2007).

Dielectric relaxation results are proven to be the most definitive to infer the distinctly different dynamic behavior of the hydration layer compared to bulk water. However, it is also important to understand the contributions that give rise to such an anomalous spectrum in the protein hydration layer, and in this context MD simulation has proven to be useful. The calculated frequency-dependent dielectric properties of an ubiquitin solution showed a significant dielectric increment for the static dielectric constant at low frequencies but a decrement at high frequencies [8]. When the overall dielectric response was decomposed into protein-protein, water-water, and water-protein cross-terms, the most important contribution was found to arise from the self-term of water. The simulations beautifully captured the bimodal shape of the dielectric response function, as often observed in experiments. 

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