Paramagnetic systems relaxation effects

The angle Qsin is that between the IN axis and the IS axis. The expression (3cos 0—1)/2 in Eq. (28) is characteristic for the cross-correlated relaxation effects. An analogous and somewhat more general expression for the case of anisotropic susceptibility was given by Bertini et al. (56). The crosscorrelation-driven coherence transfer phenomena between nuclear spins in paramagnetic systems with anisotropic susceptibility were even earlier considered by Desvaux and Cochin (65). 

R[m (Eq. (7.10)). This can be intuitively understood because cross relaxation in a non-selective experiment contributes less to signal recovery, especially at the beginning of the experiment, and the latter is dominated by paramagnetic effects (RjM). In any case, when pi in paramagnetic systems is very large (e.g. 100-1000 s ), it means that R[m dominates the relaxation processes and the results of selective and non-selective experiments are close to one another. 

For irradiation times of J short with respect to the relaxation time of / the NOE extent is independent of the relaxation time of the nucleus and provides a direct measurement of time required to saturate signal J is not negligible compared with t, the response of the system is not linear [18]. The truncated NOE is independent of paramagnetism as it does not depend on p/, which contains the electron spin vector S in the R[m term, and only depends on gkj), which does not contain S. If then the steady state NOE is reached, the value of p/ can also be obtained. This is the correct way to measure p/ of a nucleus, provided saturation of J can be considered instantaneous. In general, measurements at short t values minimize spin diffusion effects. In fact, in the presence of short saturation times, the transfer of saturation affects mainly the nuclei directly coupled to the one whose signal is saturated. Secondary NOEs have no time to build substantially. As already said, this is more true in paramagnetic systems, the larger the R[m contribution to p/. 

Cr(acac)3 in various perdeuterated solvents, Nilenls in en Paramagnetic relaxation effects on nonexchanging systems 2000 298 H. N 141... 

If the molecule has dynamic motions on the timescale of the EPR experiment, this motion will lead to relaxation effects on the EPR line. Depending on the timescale and size of these motions, these effects may be observable directly in the cw-EPR spectrum or indirectly by pulsed EPR measurements of the relaxation times. In many cases, different dynamics may simultaneously contribute to the relaxation behavior of the electron spin system, as, for example, vibrational and rotational motion, conformational dynamics, phonon coupling to the frozen solvent, and nuclear spin dynamics. In these cases, it will be difficult to obtain specific information from these relaxation measurements. On the other hand, it is possible to highlight a specific time-scale window by the selection of pulse sequences and microwave frequencies that can lead, in favourable cases, to a direct relation between measured relaxation times and interesting molecular dynamics at the paramagnetic site. In these cases, very interesting molecular dynamical aspects of electron-transfer, catalytic, or photo-reactions, unobservable by other structural methods, can be studied directly by pulse-EPR techniques. 

Next, we turn to the paramagnetic relaxation effects in protein systems. Koehler and Meiler reviewed the utility of measurements of this kind. Here again, I do not aim at providing a full literature review of the field, but rather to give the reader a limited selection of examples. Under certain conditions, the measurements of PRE can provide an alternative method to characterize invisible states in proteins. Anthis et applied this... 

A second clue as to the relaxation mechanism in paramagnetic systems may be obtained from measurements of chemical shift changes. Chemical shift changes observed upon addition of a metal ion for a rapidly exchanging halide nucleus in solution should be due to differences in chemical shifts in the "free" and complexed sites. Chemical shifts in diamagnetic halide complexes are not known. From Table 3.3 it can be estimated that they may at most be of the order of a few hundred ppm. Due to quadrupolar relaxation effects it can be envisaged that it should be very difficult to determine these shifts experimentally. No shift changes have so far been reported for Cl and Br in... 

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