Life Needs Water
All life is made up of the same fundamental biomolecular building blocks: lipids, that form cell membranes and isolate the inside of a cell from the outside world, proteins, which perform a host of biological functions from enzymes that catalyse biochemical reactions within your body, to structural proteins in the plant cell walls, to antibodies that fight infection, and DNA, which contains the genetic code required for life to grow, function, and develop. All these building blocks function due to the specific structure they adopt. Individual lipid molecules associate into bilayers, the folded structure of a protein determines what it binds to, and the stability and readability of DNA lies in its classic double helix structure. All these structures are determined by the way that the biomolecules interact with a single underlying species: water.
This water-induced structuring occurs because biomolecules are “amphiphilic”, meaning they contain both “hydrophilic”, or water-loving, sections, and “hydrophobic”, or water hating, sections. As the biomolecules are introduced in solution they will self-assemble or fold in such a way that they minimize contact of their hydrophobic regions with the surrounding water and maximize contact of their hydrophilic regions with the surrounding water. In a bizarre twist therefore, a free-flowing low viscosity liquid, free to adopt the shape of its container, results in the structuring of biomolecules and makes life possible.
The Structure of Water
The ability of life to thrive fundamentally depends on the interactions between water and biomolecules, and therefore on the properties of water itself. The properties of water arise from its molecular structure. A water molecule contains a single oxygen atom covalently bound to two hydrogen atoms, resulting in its molecular formula: H2O. The nature of a covalent OH bond means that the oxygen and the hydrogen each donate an electron, resulting in a shared electron pair between the two atoms which forms the bond. However, oxygen is more “electronegative” than hydrogen, which means the electrons are pulled closer to the oxygen atom and further away from the hydrogen atom. As electrons are negatively charged, this means that the oxygen atom in a water molecule is slightly negatively charged, and the hydrogen atoms are both rendered slightly positively charged. The electronic structure of oxygen also means that covalent bonding to two hydrogen atoms results in two lone pairs of negatively charged electrons on the central oxygen atom, located on the bottom of the molecule. A single water molecule can therefore be thought of as a tetrahedron, with an oxygen atom at its centre, a positively charged hydrogen at two apexes, and a negatively charged electron pair at the remaining two.
The tetrahedral nature of a single water molecule is translated to the structure of water as a complete liquid. Water molecules in solution will orient themselves such that the positively charged hydrogen atom on one water molecule will point towards the negatively charged electron pair on a neighbouring water molecule due to the attractive force between the opposite charges. This phenomenon is known as “hydrogen bonding” and is a crucial attractive force between water molecules. Without attractive hydrogen bonds between water molecules, water would be a gas at Earth’s surface temperature, and life couldn’t exist. The tetrahedral nature of the water molecule therefore results in a tetrahedrally ordered network of water molecules connected through hydrogen bonds.
How Pressure Changes Water Structure
This tetrahedral network of interconnected water molecules is sensitive to applied pressure. Under ambient conditions the tetrahedral network means that the average water molecule has four nearest neighbours. As pressure is increased the network begins to collapse and this number increases as the tetrahedral network is distorted. This is monitored through “radial distribution functions”, which show the local density of water molecules as a function of distance from a central water molecule. These show that water forms ordered hydration shells and as pressure is increased water molecules from the second hydration shell begin to be forced into the first hydration shell. The pressure induced network distortion also means that water molecules are less able to orient themselves appropriately to form hydrogen bonds with their neighbours, and the average water – water hydrogen bond becomes less stable. This in turn means that the hydrophilic and hydrophobic interactions between water molecules and biomolecules are changed, and biomolecules are less able to form the structures which life depends on. We would therefore expect increased pressure to hinder the ability of life to thrive, however organisms that survive under high pressure conditions found in the deep ocean are well observed. How is this possible?
How Life Survives Pressure
Life is able to compensate for the potentially damaging pressure induced changes to the hydrogen bonded network of water through a variety of mechanisms. One of the key methods of adapting to high pressure is an increased concentration of “compatible solutes” within the cells of the organisms. These are small molecules that have been shown to stabilise the biomolecular structure against a host of extremes, including pressure. A particularly prevalent example of this is trimethylamine N-oxide, or TMAO. A multitude of experimental and simulation studies have demonstrated that TMAO enhances the stability of hydrogen bonding between water molecules, and other experimental studies have shown that water containing TMAO is less compressible than pure water. We therefore suggest that the ability of TMAO to stabilise biomolecular structure against increased pressure is due to enhanced water hydrogen bonding which appropriately balances out the weakened water hydrogen bonding due to increased pressure. A solution of water and TMAO under pressure would therefore have the same hydrogen bonding characteristics of pure water under ambient pressure. How do we go on to test this hypothesis?
How We Study Water
To visualise the structure of water we need to turn to an experimental technique that is sensitive to the positions of the individual oxygen and hydrogen atoms within a liquid sample. An ideal technique to achieve this goal is neutron diffraction. Neutrons are subatomic particles which, along with protons, make up the nuclei of all atoms. In neutron diffraction a stream of neutrons is fired at a sample and data is collected on the how they emerge from a sample. This data is known as a “diffraction pattern”, and its form is depends on the structures present in the sample, and the elemental composition of the sample. By substituting hydrogen in the sample for its isotope deuterium, which is identical to hydrogen apart from an extra neutron in its nucleus, we can obtain different diffraction patterns for samples without changing the structures present in the sample. This is an important step in analysis of the diffraction patterns.
We can then use a simulation tool called “empirical potential structure refinement” (EPSR) to turn the diffraction patterns from several isotopic variants of a sample into a complete 3D picture which shows the positions of each atom in the sample. To achieve this a simulated box of atoms is built that matches the experimental sample in composition and density. This simulation is then slowly edited, until the diffraction patterns that the simulation would produce are a close match to the experimentally measured diffraction pattern. At this point we say that the simulation is an accurate representation of the experimental sample which we can go on to use to quantify hydrogen bonding, produce radial distribution functions, and a host of other structural parameters. By performing neutron diffraction experiments on pure water and water containing TMAO both at ambient and high pressure, and analysing the diffraction patterns with EPSR, we can test our hypothesis that TMAO restores the hydrogen bonding characteristics of water against external pressure.