HR-MAS of cells: A "cellular water shift" due to water-protein interactions?

Under HR-MAS conditions, cells are subjected to high centrifugal forces that may cause irreversible cell damage. First, conditions have been defined to monitor and keep to a minimum unwanted effects in HR-MAS spectra arising from the loss of cell integrity. Then, the HR-MAS spectra of reasonably intact cells have been analyzed. Cell suspensions subjected to MAS rates as low as 1 kHz split into a two-compartment system that is composed of a cell-rich phase (H₂O_i) and a cell-free phase (H₂O_o). Each of these phases is characterized by its own water ¹H-NMR signal. Transport of water molecules between the cell-rich and cell-free compartments is limited by the very low contact area between the two compartments, and water exchange dynamics consequently fall into the slow exchange limit on the NMR timescale. Since the exchange between the two water populations is "frozen," the separation between the H ₂O_o and H₂O_i water signals (Δν^water) detected in an HR-MAS experiment is not affected by chemical exchange but reflects only chemical differences in the two environments. Different cell lines show a different Δν^water, leading to the concept of "cellular water shift." This shift roughly correlates with the cellular protein content, supporting the view that the most important determinant of the cellular water shift is the interaction between water and proteins in the intracellular compartment.