Water is the most abundant chemical component present in living systems. It constitutes around 60% or more of the total mass of most of the organisms. The first organisms are believed to have originated in the aqueous environment. Most of the biological reactions are carried out in water; many of them directly involving water as one of the reactants or products. It will not be incorrect if we call water as the biological solvent. To understand the chemistry of the biological processes and the behavior of biological molecules in water, we need to understand the physical and chemical properties of water. Biochemical experiments are mostly carried out in aqueous solutions having a defined chemical composition and pH. In this lecture, we shall be discussing the physical and chemical properties of water, concepts related to solutions, and pH.

Hydrogen bonding and physical properties of water

Water (H₂O) is a molecule wherein the oxygen atom is sp³ hybridized; two of these hybridized orbitals make σ bonds with hydrogen atoms while the other two are occupied by the unshared pairs of electrons. Water, therefore, is a bent molecule with an H–O–H angle of 104.5º (Figure 2.1A). The large difference in the electronegativities of oxygen and hydrogen atoms make O–H a highly polar bond. Owing to the angular geometry of the molecule, the dipole moments of the two O–H bonds do not cancel each other out, imparting it a high dipole moment of 1.85 Debye. The oxygen atom is partially negatively charged while the hydrogen atoms are partially positively charged. This leads to an electrostatic attraction between the oxygen atom of one water molecule and the hydrogen atoms of the other. This interaction is referred to as the hydrogen bond (Figure 1.2B). It is due to these hydrogen bonds, that water molecules are held together making it a liquid at room temperature. Hydrogen bonds (H-bonds) between the water molecules impart it high melting point, boiling point, and heat of vaporization. Low molecular weight and the ability to form H-bonds also impart water a very high specific heat; it takes 1 calorie to increase the temperature of 1 gram of water by 1 ºC at standard temperature and pressure. Water, therefore, is as an excellent temperature buffer. Hydrogen bonds are not restricted to water; they are readily formed between an electronegative atom (called a hydrogen acceptor; usually fluorine, nitrogen, and oxygen) and a hydrogen attached to another electronegative atom (called a hydrogen donor). Hydrogen bonds are weak but play very crucial roles in the structure and function of the biomolecules. Common hydrogen bonds observed in biological systems are shown in figure 2.1C.

Figure 2.1: Structure of water and hydrogen bonding. Panel A shows the bent structure of water molecule; the big red sphere represents the oxygen atom while small spheres represent the hydrogen atoms. Panel B shows the hydrogen bonding between water molecules. Panel C shows the types of hydrogen bonds frequently found in biomolecules.