Post on 09-Apr-2022
Chapter 2. Aqueous solutions
• Properties of water• Acids and Bases• Buffers
Water• Biological solvent
– Metabolic reactions– Delivery of nutrients – Removal of wastes
• Buffer to temperature and pH change• Reactant for many biochemical reactions
Structure of water
Dipole moment of water
Water is polar
• forms H-bonds• interacts well with charged particles• It weakens polar and ionic interactions• interacts poorly with non-polar substances
(hydrophobic effect)
Hydrogen bond between two water molecules
Structure of ice
• Ice is a crystal of H-bonded H2O
• 4 H-bonds per H2O
• In liquid water this structure is broken down
— 15% less hydrogen bonds
Chemical bonds in biochemistry
• Covalent bonds - Strong• Noncovalent bonds – Weak
(Intra- or inter-molecular interactions) – Electrostatic interactions– Hydrogen bonds – van der Waals interactions – Hydrophobic interactions
Hydrogen bonds
Water as a solvent
• Good solvent for polar or ionic substances (hydrophilic)
• Poor solvent for non-charged/non-polar substances (hydrophobic)
Water as a solvent
Dipole-Dipole interactions
Ion -Dipole interactions
Hydrophobic interactionsNonpolar substance sticks together to minimize contact with water molecules
• Molecules with both polar and non-polar regions (amphiphilic or amphipathic)
• Form a micelle or a bilayer
Amphiphilic compounds
Head (polar)Tail (nonpolar)
Micelle and Bilayer
Hydrophobic interactions
• Membrane structure• Protein structure• Lipid-protein interactions• Protein-protein interactions
Proton mobility
Hydronium ion(H3O+)
Ionization of water
H2O H+ OH-+
[H2O][H+][OH-]
K=
K [H2O] = Kw = [H+][OH-]K [H2O] = Kw =
[H+][OH-]Kw = = 10-14M2
• In pure water [H+] = [OH-]= 10-7 M• [H+] >10 -7 M: acidic• pH = -log [H+]• [H+] < 10 -7 M : basic• pOH = -log [OH-]• Kw = [H+] x [OH-] = 10-14 M2
• pH + pOH = 14
Acid-Base reactions
• A Brønsted acid can donate protons to form a conjugate base
• A Brønsted base can accept protons to form a conjugate acid
HA + H2O H3O+ A-+
acid base conjugate acid
conjugate base
Strength of acids
HA + H2O H3O+ A-+
[H3O+] [A-][HA] [H2O]
K =
[H+] [A-][HA]
Ka = K [H2O] =
pKa = -logKa
Henderson-Hasselbalch equation
[H+] [A-][HA]
Ka =
[HA][H+] = Ka
[A-][HA]
-log [H+] = -log Ka
pH = pKa + log[A-][HA]
[A-]
Buffers
• Buffers are solutions that resist change in pH
• Structure and activity of biological molecules is affected by the pH
• Mixture of a weak acid and its conjugated base resist change in pH
• Buffer capacity is highest at the pKa• Buffers work within 1 pH unit of the pKa
Titration curve
Polyprotic acids
Buffers in blood