Intrinsic relaxation rates

The intrinsic relaxation rate of a paramagnetic center can he enhanced hy spin—spin coupling with a nearhy fast-relaxing species. 

Ad. 1. Theoretical considerations including calculations based on the knowledge of anatomical features. While considering whether or not it is reasonable to believe that the non-mono-exponential transverse relaxation reflects an intra- and extra-cellular compartmentalisation, it should be questioned if the slow-exchange assumption is reasonable taking into account the macroscopic features of a muscle cell from a theoretical point of view. The NMR-timescale criterion for exchange limit is comparison of the quantity (rr1 + t"1), where r, and t0 are the average lifetime of a water molecule inside and outside the cell, respectively, with Rt — R , which is the absolute difference in the intrinsic relaxation rates. If R —Rfl > (h 1 + ), the... 

When the spin-diffusion rate is comparable to the values, the relaxation decay curves become non-single exponential, and when the spin-diffusion rate is less than 10% of the values, the relaxation decay curve can be described by a single exponential with an intrinsic relaxation rate. 

From these semiquantitative estimations, one can regard that, if the observed Ti values for the component polymers are the same, the size of the domains is small enough for spin diffusion to average polarization gradient among spins A and B created by different intrinsic relaxation rates. Then, the question arises how small the size of the domains should be to realize fast spin diffusion The maximum diffusive path length r by spin diffusion in three-dimensions for a time Ty may be given as [78-81]... 

The CPMG R2 experiment has long been applied to detect chemical exchange phenomena [95, 96, 102], A simple case is when the exchange rate is larger than the intrinsic relaxation rate, (determined by dipolar and CSA relaxation, details described below), and the observed R is the sum of R and the chemical exchange contribution, R, [98, 103] ... 

The constancy of P to changes in micelle concentration is also directly borne out in studies of counterion quadrupole relaxation rates for a large number of systems using Na, Cl, Rb, Cs, and Br NMR (reviewed in Ref 87). These studies also demonstrate that the intrinsic relaxation rates of micellar counterions are nearly independent of micelle concentration and the alkyl chain length of the surfactant. 

The fractional intensity change, F, for X on saturating K(Fj = (Xo — Xoo)X, where Xoo and Xo are the peak intensities with, and without, saturating Y, respectively), and the intrinsic relaxation rate for X is T J, yield d an observed rate... 

Fig. 5.13. Schematic NMR spectrum of a nucleus that may occupy two sites, A and B, with Larmor frequencies and Wg and with intrinsic relaxation rates I/T2A 1/T2b respectively, (a) illustrates the spectrum in the absence of chemical exchange between the two sites and (b) illustrates the spectrum after onset of chemical exchange...

---