ABSTRACT
This chapter investigates cross-relaxation phenomena. Multispin contributions to relaxation become important. In these cases, the nuclear Overhauser effect (NOE), which was earlier seen to be related to the cross-relaxation rate, provides a measure of a third relaxation parameter, often as a complement to T1 and T2 measurements for heteronuclei. Cross-relaxation studies offer possibilities to investigate the geometry of a molecule through the internuclear distance dependence of the dipole-dipole interaction. The heteronuclear NOE experiment is conceptually very simple, but is for most applications one of the most difficult relaxation experiments to perform in practice. The determination of heteronuclear NOE in proteins, in analogy with the determination of T1 and T2 relaxation time constants, demands the highest possible sensitivity and resolution. Proton relaxation is often difficult to quantify and interpret due to the fact that the spin systems easily become quite large in organic and biological molecules.
