Picosecond orientational dynamics of water in living cells

Martijn Tros,Daniel Bonn,Sander Woutersen,Gertien J. Smits,Johannes Hunger,Linli Zheng,Mischa Bonn

doi:10.1038/s41467-017-00858-0

Martijn Tros, Daniel Bonn + Show 5 more

Open Access

https://doi.org/10.1038/s41467-017-00858-0

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Abstract

Cells are extremely crowded, and a central question in biology is how this affects the intracellular water. Here, we use ultrafast vibrational spectroscopy and dielectric-relaxation spectroscopy to observe the random orientational motion of water molecules inside living cells of three prototypical organisms: Escherichia coli, Saccharomyces cerevisiae (yeast), and spores of Bacillus subtilis. In all three organisms, most of the intracellular water exhibits the same random orientational motion as neat water (characteristic time constants ~9 and ~2 ps for the first-order and second-order orientational correlation functions), whereas a smaller fraction exhibits slower orientational dynamics. The fraction of slow intracellular water varies between organisms, ranging from ~20% in E. coli to ~45% in B. subtilis spores. Comparison with the water dynamics observed in solutions mimicking the chemical composition of (parts of) the cytosol shows that the slow water is bound mostly to proteins, and to a lesser extent to other biomolecules and ions.

Highlights

Cells are extremely crowded, and a central question in biology is how this affects the intracellular water
We combine these two spectroscopic methods to investigate the orientational dynamics of water in live cells of three prototypical species: a vegetatively growing bacterium (Escherichia coli) and a eukaryote (Saccharomyces cerevisiae, yeast) both living in aqueous environments, and Bacillus subtilis spores which can survive drought for many years and are resistant to heat, toxic chemicals and radiation
We find that in all three organisms most of the intracellular water exhibits the same random orientational motion as neat water, and that a smaller fraction of the intracellular water exhibits slower orientational dynamics

Summary

Introduction

A central question in biology is how this affects the intracellular water. The distribution of sub-ns reorientation times that underlie this average is difficult to access, and water molecules exhibiting picosecond orientational dynamics cannot be observed directly in NMR experiments The rotation of such rapidly reorienting water molecules can be tracked in real time using ultrafast time-resolved infrared spectroscopy, which directly probes the random orientational motion of the water–OH bonds (or OD bonds in the case of deuterated water). The collective orientational motion of the dipole moments of water molecules can be probed by measuring the electric-field induced polarization of a sample as function of field frequency using dielectric-relaxation spectroscopy (DRS)[53, 54] We combine these two spectroscopic methods to investigate the orientational dynamics of water in live cells of three prototypical species: a vegetatively growing bacterium (Escherichia coli) and a eukaryote (Saccharomyces cerevisiae, yeast) both living in aqueous environments, and Bacillus subtilis spores which can survive drought for many years and are resistant to heat, toxic chemicals and radiation. Additional experiments in which we study the orientational water dynamics in solutions that mimick the cytosol or parts of it indicate that most of the slow intracellular water is bound to proteins

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