Transverse relaxation rates

Table I reports the observed NMR linewidths for the H/3 protons of the coordinating cysteines in a series of iron-sulfur proteins with increasing nuclearity of the cluster, and in different oxidation states. We have attempted to rationalize the linewidths on the basis of the equations describing the Solomon and Curie contributions to the nuclear transverse relaxation rate [Eqs. (1) and (2)]. When dealing with polymetallic systems, the S value of the ground state has been used in the equations. When the ground state had S = 0, reference was made to the S of the first excited state and the results were scaled for the partial population of the state. In addition, in polymetallic systems it is also important to account for the fact that the orbitals of each iron atom contribute differently to the populated levels. For each level, the enhancement of nuclear relaxation induced by each iron is proportional to the square of the contribution of its orbitals (54). In practice, one has to calculate the following coefficient for each iron atom ...

By now, water exchange has been studied on more than one hundred Gdm complexes with the help of 170 NMR, and the large body of data available has been reviewed recently (48). Variable temperature 170 transverse relaxation rate measurements provide the rate of the water exchange, whereas the mechanism can be assessed by determining the activation volume, AVt, from variable pressure 170 T2 measurements (49,50). The technique of 170 NMR has been described in detail (51). 

Bell et al. (2002) investigated the relationship between water mobility as measured by oxygen-17 NMR (transverse relaxation rate obtained from linewidth at half-height) and chemical stability in glassy and rubbery polyvinylpyrrolidone (PVP) systems. Reported results suggest that water mobility in PVP model systems was not related to Tg. The study did not find a link between water mobility and reaction kinetics data (half-lives) for degradation of aspartame, loss of thiamin and glycine, and stability of invertase. 

The linear relationship between H NMR transverse relaxation rate and (1 av) is shown in Figure 30 for pregelled potato starch (Hills et al., 1999). The change in slope at about 0.90 c/w corresponds to the bulk water break (i.e., the removal of bulk water) in a corresponding adsorption isotherm. Equation... 

Richardson, S.J. 1989. Contribution of proton exchange to the oxygen-17 nuclear magnetic resonance transverse relaxation rate in water and starch-water systems. Cereal Chem. 66, 244-246. Richardson, M.J. and Saville, N.G. 1975. Derivation of accurate glass transition temperatures by differential scanning calorimetry. Polymer 16, 753-757. 

Whereas 15N longitudinal and transverse relaxation rates can be determined with sufficient precision, the determined values of the 15N 1H -NOE differ significantly from the true values for residues involved in fast amide proton exchange. A comparison between the values for the 15N 1H -NOE for NPY in solution in the presence and in the absence of DPC is displayed in the Fig. 5.9. The comparison reveals two striking differences. 

Fig. 6.5 The dependence of transverse relaxation rates on static magnetic field B0 calculated for a C6 of Cytosine, b C8 of Guanine and c CV (average value for all nucleosides). Solid, dashed and...

This relaxation pathways cannot be influenced by TROSY. Only with the replacement of nonlabile protons with deuterons is the transverse relaxation significantly reduced further, as can be inferred from Tab. 10.1. In Tab. 10.1 the transverse relaxation rates of XH and 15N are predicted for a 23 kDa protein. In a conventional [15N,1H]-HSQC experi-... 

The measured spin relaxation parameters (longitudinal and transverse relaxation rates, Ri and P2> and heteronuclear steady-state NOE) are directly related to power spectral densities (SD). These spectral densities, J(w), are related via Fourier transformation with the corresponding correlation functions of reorientional motion. In the case of the backbone amide 15N nucleus, where the major sources of relaxation are dipolar interaction with directly bonded H and 15N CSA, the standard equations read [21] ... 

Here keg- describes the dynamic process (e.g. the transverse relaxation rate R2, or the diffusion coefficient D) and r describes a time constant typical for the experimental setup. By use of Eq.(l) the kegf can be written as follows ... 

The magnitude of the paramagnetic relaxation enhancement (PRE) caused by dipolar interactions depends on the square of the gyromagnetic ratios of both involved spins, the inverse sixth power of the inter-spin distance, and the correlation time rc of the vector connecting the two spins. For the transverse relaxation rate enhancement, R2para of a spin I,... 

Fig. 15.1 Principle of the Tip experiment (A) and of the SLAPSTIC experiment (B). The T-,p experiment makes use of the increased transverse relaxation rate of the ligand in the bound state, which leads to slightly reduced signal intensity (A). Signal intensity is drastically reduced or com...

The drastic paramagnetic effect on transverse relaxation rates can be used to make the distinction between binding and nonbinding compounds so clear that analysis of SLAPS-... 

Notice the presence of a spectral density at zero frequency in i 2 arising from bz t)bz Q)dt (which evidently does not require to switch to the rotating frame). This zero frequency spectral density will be systematically encountered in transverse relaxation rates and, in the case of slow motions, explains... 

The smaller contribution to solvent proton relaxation due to the slow exchanging regime also allows detection of second and outer sphere contributions (62). In fact outer-sphere and/or second sphere protons contribute less than 5% of proton relaxivity for the highest temperature profile, and to about 30% for the lowest temperature profile. The fact that they affect differently the profiles acquired at different temperature influences the best-fit values of all parameters with respect to the values obtained without including outer and second sphere contributions, and not only the value of the first sphere proton-metal ion distance (as it usually happens for the other metal aqua ions). A simultaneous fit of longitudinal and transverse relaxation rates provides the values of the distance of the 12 water protons from the metal ion (2.71 A), of the transient ZFS (0.11 cm ), of the correlation time for electron relaxation (about 2 x 10 s at room temperature), of the reorienta-tional time (about 70 x 10 s at room temperature), of the lifetime (about 7 x 10 s at room temperature), of the constant of contact interaction (2.1 MHz). A second coordination sphere was considered with 26 fast exchanging water protons at 4.5 A from the metal ion (99), and the distance of closest approach was fixed in the range between 5.5 and 6.5 A. 

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