Cell water dynamics on multiple time scales

Abstract

Water-biomolecule interactions have been extensively studied in dilute solutions, crystals, and rehydrated powders, but none of these model systems may capture the behavior of water in the highly organized intracellular milieu. Because of the experimental difficulty of selectively probing the structure and dynamics of water in intact cells, radically different views about the properties of cell water have proliferated. To resolve this long-standing controversy, we have measured the (2)H spin relaxation rate in living bacteria cultured in D(2)O. The relaxation data, acquired in a wide magnetic field range (0.2 mT-12 T) and analyzed in a model-independent way, reveal water dynamics on a wide range of time scales. Contradicting the view that a substantial fraction of cell water is strongly perturbed, we find that approximately 85% of cell water in Escherichia coli and in the extreme halophile Haloarcula marismortui has bulk-like dynamics. The remaining approximately 15% of cell water interacts directly with biomolecular surfaces and is motionally retarded by a factor 15 +/- 3 on average, corresponding to a rotational correlation time of 27 ps. This dynamic perturbation is three times larger than for small monomeric proteins in solution, a difference we attribute to secluded surface hydration sites in supramolecular assemblies. The relaxation data also show that a small fraction ( approximately 0.1%) of cell water exchanges from buried hydration sites on the microsecond time scale, consistent with the current understanding of protein hydration in solutions and crystals.

Fig. 1.

²H MRD profiles of cell water. (A) The water–²H relaxation rate R₁ was measured on E. coli cells (blue circles) and H. marismortui cells (red diamonds) in stationary growth phase at 27°C and pD 8.0. Control measurements were performed in parallel on a bulk D₂O reference sample (open circles). The curves are multi-Lorentzian numerical representations used for the model-free analysis. (B) The colored areas represent, for the E. coli sample, the contributions to R₁ from surface hydration water with rotational correlation time <2 ns (yellow) and from internal water molecules with residence time >160 ns (blue). In both images, Inset shows the high-frequency region on an expanded scale.

Fig. 2.

¹H MRD profiles and quadrupolar peaks. The water–¹H relaxation rate R₁ was measured on E. coli cells at 27°C (blue circles) and H. marismortui cells at 12°C (green triangles) or 27°C (red diamonds). The samples were prepared as in Fig. 1, but with H₂O at pH 7.6. The shoulder at 10–30 MHz is due to paramagnetic ions. Inset shows the quadrupolar peaks on an expanded scale for E. coli (blue circles) and H. marismortui (red diamonds) at 27°C. The plotted quantity R₁^Q was obtained by subtracting the baseline MRD profile. The curves serve only to guide the eye.