Water, the Solvent of Life

Water, the Solvent of Life

Overview of Water in Biological Systems

• Water constitutes approximately 70–90% of biological organisms.

• It serves as the medium in which chemical reactions occur, predominantly in an aqueous milieu (the environment or surrounding fluid).

• Water plays a critical role in determining the structure and function of essential biological macromolecules including proteins, nucleic acids, and biological membranes.

• The evolution of life in water may be partly attributed to the protective qualities it offers against UV radiation.

Physical Properties of Water and Other Solvents

### Table 2-1: Melting Point, Boiling Point, and Heat of Vaporization of Some Common Solvents

• Heat of vaporization refers to the energy required to convert 1.0 g of a liquid at its boiling point and at atmospheric pressure into its gaseous state at the same temperature, reflecting the energy needed to overcome intermolecular attractive forces.

Structure of the Water Molecule

• The structure of water molecules is dictated by the octet rule, which indicates that there are four electron pairs around a central oxygen atom:

  • Two of these pairs form covalent bonds with two hydrogen atoms.

  • The remaining two pairs are nonbonding (lone pairs).

• The geometry of a water molecule is described as a distorted tetrahedron.

• The oxygen atom's electronegativity causes a net dipole moment in the molecule, resulting in polar characteristics.

• Due to its dipole moment, water molecules can act as both hydrogen bond donors and acceptors.

Hydrogen Bonds

Definition and Characteristics of Hydrogen Bonds

• Hydrogen bonds are defined as strong dipole-dipole or charge-dipole interactions that occur between a hydrogen atom covalently bonded to an electronegative atom (donor) and a lone pair of electrons from another electronegative atom (acceptor).

• Frequently, hydrogen bonds involve pairs of electronegative atoms that are nitrogen and oxygen.

• The strength of hydrogen bonds is maximized in linear bonding arrangements where the bonded atoms and the hydrogen atom involved are in a line.

  • This linear alignment enhances the electrostatic interaction between the donor and the acceptor atoms.

Directionality of Hydrogen Bonds

• Hydrogen bonds exhibit directionality and are strongest when the acceptor atom is perfectly aligned with the hydrogen bond's covalent link between the donor atom and the hydrogen atom.

Hydrogen Bonding in Water

Summary of Properties

• Each water molecule can form up to four hydrogen bonds, contributing to water's unique properties:

  • Anomalously high boiling point

  • Anomalously high melting point

  • Unusually large surface tension

• Water displays significantly higher melting point, boiling point, and heat of vaporization compared to most other common solvents due to extensive hydrogen bonding.

• The hydrogen bond can be described as the electrostatic attraction between the oxygen atom of one water molecule and the hydrogen atom of another water molecule, typically measuring 0.2735 nm from oxygen to oxygen in hydrogen bonds.

Importance of Hydrogen Bonds in Biological Systems

• Hydrogen bonds contribute to the following biological functions:

  • Unique physical properties of water

  • The structure and function of proteins

  • The structure and function of DNA

  • The structure and function of polysaccharides

  • The binding of substrates to enzymes

  • The binding of hormones to their receptors

  • The matching of mRNA and tRNA

• Linus Pauling, in his work "The Nature of the Chemical Bond" (1939), expressed that the significance of the hydrogen bond for physiological processes could outweigh that of any other structural feature.

Biological Relevance of Hydrogen Bonds

• Hydrogen bonds readily form when a hydrogen atom covalently bonded to an electronegative atom (hydrogen donor) interacts with another electronegative atom (hydrogen acceptor).

Ice: Solid State of Water

• Water can exist in numerous crystalline forms, with hexagonal ice being the most prevalent.

• The structure of hexagonal ice exhibits an organized lattice, hence demonstrating low entropy.

• In this form, each water molecule maximizes hydrogen bonding, thereby forcing a consistent equidistance between water molecules, resulting in:

  • Lower density of ice compared to liquid water

  • The phenomenon of ice floating on water

Hydrogen Bonding in Ice vs. Liquid Water

• In liquid water, each water molecule forms hydrogen bonds with an average of 3.4 other molecules.

• In contrast, each water molecule in ice forms four hydrogen bonds.

Water as a Solvent

• Water is characterized as a poor solvent for nonpolar substances including:

  • Nonpolar gases

  • Aromatic moieties

  • Aliphatic chains

• The ionization of water at 37°C is minimal, calculated at 2.8 x 10⁻⁹, as the equilibrium reaction can be described as:
  $H_2O <br>ightleftharpoons H^+ + OH^-$

• However, water is an effective solvent for charged and polar substances such as:

  • Amino acids and peptides

  • Small alcohols

  • Carbohydrates

Dissolving Salts

• When salt (e.g., NaCl) is dissolved in water, ionic interactions are broken, increasing entropy. This dissolution process is thus thermodynamically favorable because it transitions the system from an ordered crystalline state to a disordered liquid state.

Solubility of Polar and Nonpolar Solutes

• Oxygen (O₂) and Nitrogen (N₂) do not form hydrogen bonds with water because they have equal electronegativity, which prevents any charge separation necessary for hydrogen bonding.

Amphipathic Molecules

• Molecules that contain both polar and nonpolar (hydrophobic) regions are referred to as amphipathic. They possess properties that allow them to interact with both aqueous and hydrophobic environments, influencing their biological functions and structures.

The END