Water, Weak Interactions and Buffers Summary

Water - General

  • Most abundant molecule in living organisms.
  • Passive role: Influences biomolecule structure and function (e.g., protein folding).
  • Active role: Participates in biochemical reactions (e.g., peptide bond formation).

Water - Structure/Function Relationship

  • Simple structure aids in understanding how it influences function.
  • Oxygen is more electronegative, giving water a permanent dipole.
  • Influences electrostatic interactions and hydrogen bond formation.

Hydrogen Bonds

  • Electrostatic interactions between hydrogen (donor) and electronegative atoms (acceptor).
  • Common in biomolecules (O, N).
  • Strength: ~5% of a covalent bond, geometry affects stability.

Unusual Properties of Water

  • Can form four hydrogen bonds per molecule, existing in liquid as dynamic clusters.
  • High heat of vaporization and specific heat due to hydrogen bonding.
  • Ice is less dense than liquid water due to ordered hydrogen bonds.

Water as a Solvent

  • Dissolves charged solutes via hydration layers.
  • Hydrophilic molecules are polar; hydrophobic molecules are non-polar.
  • Amphipathic molecules have both hydrophilic and hydrophobic regions.

Weak Interactions and Molecular Structure

  • Stability and dynamics of biomolecules due to non-covalent interactions:
    • Hydrogen bonds
    • Ionic interactions
    • Hydrophobic interactions
    • Van der Waals interactions

Buffers and pH

  • Biological systems must maintain constant pH.
  • Weak acids buffer systems (e.g. bicarbonate in blood).
  • Henderson-Hasselbalch equation relates pH, pKa, and relative concentrations of acid and base.

Ionization of Water

  • Water can ionize to H⁺ and OH⁻.
  • pH = -log [H⁺]; scale is logarithmic.

Weak Acids and Titration

  • Strong acids dissociate completely, weak acids partially.
  • Titration curves reveal pKa, indicative of buffering range.

Conclusion

  • Water's unique properties underpin biological processes and the functionality of biomolecules, with pH and buffer systems critical for maintaining life.