Measurements of distances to half-integer quadrupolar nuclei in biological systems

Dipolar recoupling experiments performed under magic-angle spinning solid-state NMR allow the direct determination of internuclear distances, or the assessment of the proximity between NMR-active nuclei and their surrounding environment. This article describes the developments of methods to probe such proximities, and discusses the limitations and approaches toward accurate, efficient, and universal distance measurements between half-integer diamagnetic nuclei and their neighboring environment. Many metal ions, which have a spin larger than one-half, play a critical role in cellular processes as integral part of metaloproteins, as signal transducers, as small molecule cofactors, and so on. Therefore, the techniques described here lay the basis for characterizing the environmental structure of such nuclei.