Field-Cycling Relaxometry in Biosystems

Abstract

Systems of biological origin tend to be heterogeneous in the sense that the investigated nuclei occur in different environments with different molecular dynamics and spin interactions. It is then a matter of exchange rates whether spin-lattice relaxation is determined by average intensity functions (“fast exchange” relative to relaxation times) so that monoexponential relaxation decays arise, or whether multiexponential distributions occur (see Chap. 23). In context with water relaxation in biological systems, the two-site/fast-exchange model turned out to be very successful in accounting for experimental findings. Note, however, that the term “exchange” in this context may refer to material transport as well as to (immaterial) spin diffusion [136, 225].