Hydrogen Bonds (DP IB Biology: SL)

• Hydrogen bonding plays an important role between many biological molecules
• Some key functions include:
  • Dissolving of solutes in water
  • The cohesion and adhesion of water molecules
    • These properties allow water to move up the trunks of really tall trees

  • Base-pairing between the two strands of DNA
  • Structure:
    • Hydrogen bonds help to form part of the secondary and tertiary levels of structure in proteins
    • The hydrogen bonds found between strands of cellulose and collagen give those molecules their tensile strength

  • Interactions between mRNA and tRNA during protein synthesis
  • Surface effects on membranes between polar phosphate groups and water

Hydrogen bonding in water

• Hydrogen bonding is a fundamental property of water
• Water is of the utmost biological importance
  • It is the medium in which all metabolic reactions take place in cells
  • Between 70% to 95% of the mass of a cell is water
  • Water is so fundamental to life that astronomers look for signs of water on other planets and moons, as indicators of possible extra-terrestrial life
  • As 71% of the Earth’s surface is covered in water it is a major habitat for organisms

• Water is composed of atoms of hydrogen and oxygen
  • One atom of oxygen combines with two atoms of hydrogen by sharing electrons (covalent bonding)

• Although water as a whole is electrically neutral, the sharing of the electrons is uneven between the oxygen and hydrogen atoms
  • The oxygen atom attracts the electrons more strongly than the hydrogen atoms, resulting in a weak negatively charged region on the oxygen atom (δ^-) and a weak positively charged region on the hydrogen atoms(δ⁺), this also results in the molecule's asymmetrical shape

• This separation of charge due to the electrons in the covalent bonds being unevenly shared is called a dipole
• When a molecule has one end that is negatively charged and one end that is positively charged it is also a polar molecule
• Water is therefore a polar molecule

The covalent bonds of water make it a polar molecule

• Hydrogen bonds form between water molecules
  • As a result of the polarity of water, hydrogen bonds form between the positive and negatively charged regions of adjacent water molecules

• Hydrogen bonds are weak, when there are few, so they are constantly breaking and reforming
• However, when there are large numbers present they form a strong structure
• Hydrogen bonds cause many of the properties of water molecules, that make them so important to living organisms:
  • Excellent solvent – many polar substances can dissolve in water
  • A relatively high specific heat capacity
  • A relatively high latent heat of vaporisation
  • Water is less dense when a solid (ice floats, allowing aquatic life to flourish beneath)
  • Water has high surface tension and cohesion
  • It acts as a reagent

The polarity of water molecules allows hydrogen bonds to form between adjacent water molecules

• Both methane (CH₄) and water (H₂O) are small, covalently-bonded molecules
• Methane is the simplest organic compound
• They have similar molecular weights, 16 and 18 respectively
• But water is polar and so forms hydrogen bonds, whereas methane is not polar and so does not form hydrogen bonds
• Water is liquid at room temperature, whereas methane is a gas
  • Other compounds, with a small molecular weight, such as ammonia (NH₃) and carbon dioxide (CO₂) are gases at room temperature
    • These compounds also do not form hydrogen bonds

  • This shows how water molecules are held together by hydrogen bonds, whereas the other gas molecules are free to move around in the gaseous state
  • Methane is a fuel with a high energy content in its bonds, whereas water is a final product of combustion and respiration

• Understanding the molecular properties of methane underlines the significance of hydrogen bonding in water

Methane’s structure determines its properties

Comparison of the Properties of Water and Methane Table