Isotropic susceptibility shift under MAS: the origin of the split water resonances in 1H MAS NMR spectra of cell suspensions.

Bulk susceptibility variations in a multiphase system such as cultured cells and tissue have two manifestations: a dipolar field component outside the regular heterogenous region which introduces linebroadening, and an isotropic field part which results in a frequency shift. Previous NMR studies have emphasized the utility of magic angle spinning for averaging the dipolar component, particularly if the spins of interest are limited to one phase of a multiphase system such as a sample of liquid with air pockets or glass beads. However, in analyzing spectra from complex multiphase systems, such as cell suspensions and tissues, etc., the isotropic part is often neglected, leading to questionable interpretation of experimental results. The present study demonstrates that under magic angle spinning, the water resonance in NMR experiments of cell suspensions is split into two resolved peaks due to the isotropic susceptibility shift. These two peaks are assigned to a central core of cell free water and an outer cylindrical ring of tightly packed cells in close association with water. A comprehensive theory for this splitting is provided based on a coaxis cylinder model with different susceptibilities. The frequency difference is shown to be dependent on the susceptibility difference and also on the angle of the rotor in the magnetic field. The splitting distance of the two water peaks can be used to measure the susceptibility difference of water in these two phases. The susceptibility difference was measured for three different cell types: 3T3 F442A preadipocyte cells, mouse embryonic stem cells, and human red blood cells.