Water: Properties and Biomolecular Structure
Water is necessary for life. Many plant and animal adaptations conserve water — the thick skin of desert cacti and the intricate structure of the mammalian kidney are just two examples. Planetary scientists look for evidence of liquid water when speculating about the possibility of life on other planets such as Mars or Jupiter's moon, Titan.
Water has many remarkable properties, including:
- High surface tension: Despite being denser than water, small objects, such as aquatic insects, can stay on top of water surface.
- High boiling point: Relative to its molecular weight, water boils at a high temperature. For example, ammonia, with a molecular weight of almost 17, boils at −33° C, while water, with a molecular weight of 18, boils at 100° C.
- Density is dependent on temperature: Solid water (ice) is less dense than liquid water. This property means that lakes and ponds freeze from the top down, a benefit to the fish living there, who can overwinter without being frozen solid.
Water has a dipole, that is, a separation of partial electrical charge along the molecule. Two of oxygen's six outer‐shell electrons form covalent bonds with the hydrogen. The other four electrons are nonbonding and form two pairs. These pairs are a focus of the partial negative charge, and the hydrogen atoms correspondingly become partially positively charged. Positive and negative charges attract each other, so that the oxygen and hydrogen atoms form hydrogen bonds. Each oxygen in a single molecule can form H‐bonds with two hydrogens (because the oxygen atom has two pairs of nonbonding electrons). Figure shows such a hydrogen bond. The resulting clusters of molecules give water its cohesiveness. In its liquid phase, the network of molecules is irregular, with distorted H‐bonds. When water freezes, the H‐bonds form the water molecules into a regular lattice with more room between the molecules than in liquid water; hence, ice is less dense than liquid water.
In water, the nonbonding electrons are the H‐bond acceptors and the hydrogen atoms are the H‐bond donors. Biomolecules have H‐bond acceptors and donors within them. Consider the side chain of a simple amino acid, serine. The oxygen contains two pairs of nonbonding electrons, as water does, and the hydrogen is correspondingly a focus of partial positive charge. Serine thus can be both
an H‐bond acceptor and donor, sometimes at the same time. As you would expect, serine is soluble in water by virtue of its ability to form H‐bonds with the solvent around it. Serine on the inside of a protein, away from water, can form H‐bonds with other amino acids; for example, it can serve as an H‐bond donor to the nonbonding electrons on the ring nitrogen of histidine, as shown in Figure2
These H-bonds normally exist only when water is not present. If serine's side chain is found on the surface of a protein, it is very likely to form H-bonds, given the relatively high concentration of water available.