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