(19) F NMR spectroscopy as a probe of cytoplasmic viscosity and weak protein interactions in living cells.

Protein mobility in living cells is vital for cell function. Both cytosolic viscosity and weak protein-protein interactions affect mobility, but examining viscosity and weak interaction effects is challenging. Herein, we demonstrate the use of (19) F NMR spectroscopy to measure cytoplasmic viscosity and to characterize nonspecific protein-protein interactions in living Escherichia coli cells. The origins of resonance broadening in Escherichia coli cells were also investigated. We found that sample inhomogeneity has a negligible effect on resonance broadening, the cytoplasmic viscosity is only about 2-3 times that of water, and ubiquitous transient weak protein-protein interactions in the cytosol play a significant role in governing the detection of proteins by using in-cell NMR spectroscopy.