Measurement of spin-5/2 relaxation in biological and macromolecular systems using multiple-quantum NMR techniques

Filtration of the NMR signal through a state of three- or five-quantum coherence is shown to facilitate quadrupolar relaxation studies of spin-5/2 nuclei in biological and macromolecular systems. The results of these multiple-quantum filtration experiments are analysed by directly fitting correlation times and quadrupolar coupling constants to the experimentally observed trilorentzian lineshapes or triexponential decays. Other experimental techniques are used to measure the exponential relaxation of four- and five-quantum coherences and provide an alternative and straightforward method of obtaining spin-5/2 motional parameters outside the extreme-narrowing limit. Irreducible spherical tensor operators are used to describe both the spin-5/2 quadrupolar relaxation and the multiple-quantum NMR techniques. Results are drawn from 25Mg NMR of magnesium ions binding to a large protein (bovine albumin) in aqueous solution, although the experimental techniques and fitting procedures are applicable to other spin-5/2 nuclei, such as 17O and 27Al