Everyone knows that plants and animals consist mostly of water. Does this water behave the same as normal tap water? To find out, researchers from the University of Amsterdam and the Max-Planck-Institute for Polymer Research Mainz used pulsed lasers and high-frequency radio waves to observe the random thermal motion of water molecules in living cells. It turns out that most water molecules in cells move in the same superfast manner as they do in tap water, and that only a small fraction moves on a slower time scale. The results are published this week in Nature Communications.
Water in a living cell is brimful with biomolecules (proteins, sugar molecules, DNA, etc.) and it contains all kinds of dissolved salts. The molecules and salts influence the water, but to what extent is still unclear. Most experiments to investigate this issue were done on solutions of one particular type of biomolecule or salt. This situation is very different from the very crowded mix of molecules in the water of a real living cell.
A team of researchers of the University of Amsterdam (UvA), led by biologist Gertien Smits (Swammerdam Institute for Life Sciences), physicist Daniel Bonn (Institute of Physics) and physical chemist Sander Woutersen (Van ’t Hoff Institute for Molecular Sciences) has now experimented on water in living cells, using ultrafast lasers and high-frequency radio waves to directly observe the random motion of water in living cells.
The idea arose when biologist Smits found indirect evidence suggesting that water in yeast cells might behave differently from normal water; and there are scientists who believe that biomolecules change the structure of water over very long distances. Smits contacted Bonn and Woutersen, both experienced in water research. Together they decided to try ultrafast laser experiments on living cells: Escherichia coli (the bacteria that lives in our intestines, and a standard guinea pig in biochemistry), baker’s-yeast cells, and bacterial spores. Chemistry student Martijn Tros carried out the experiments, assisted in the parts by biology PhD student Linli Zheng.
The molecules of liquid water move in a random way, on a time scale of picoseconds (trillionths of a second). To observe this random motion in real time, the researchers used ultra-short infrared light pulses (about 0.1 picosecond long).
A first infrared pulse uniformly excites the O-H bonds of part of the water molecules. Due to the random motion of the water molecules, the direction of these O-H bond excitations rapidly becomes randomized. Using a second, slightly delayed infrared pulse, this change is measured. By repeating the experiment for different time delays between the pulses, the random (Brownian) motion of the water molecules can thus be tracked.
The laser measurements were later complemented with experiments at the Max-Planck Institute in Mainz, under the supervision of Johannes Hunger and Daniel’s brother Mischa Bonn. In these experiments the random motion of the water molecules was tracked using high-frequency (gigahertz) radio waves.
Both the laser and the microwave experiments show that most water molecules in a cell move randomly on a time scale of about 2 picoseconds. This is the same behavior as in pure water. A small fraction of the cell-water molecules moves on a slower time scale, mostly because they are stuck against protein molecules. The researchers conclude that the idea of a thick layer of “biological water” surrounding biomolecules can therefore be discarded: it appears most of the water in our cells behaves just like tap water.
Martijn Tros, Linli Zheng, Johannes Hunger, Mischa Bonn, Daniel Bonn, Gertien J. Smits & Sander Woutersen: “Picosecond orientational dynamics of water in living cells”, Nature Communications 8, Article number: 904 (2017) DOI:10.1038/s41467-017-00858-0