Ch. 2 Water & Aqueous Solutions

Water

• Medium of life

• Life evolved in water

• Organisms typically contain 70-90% of water

• Chemical reactions occur in aqueous solution

• Water is a critical determinant of the structure and function of biological macromolecules

Water Molecules & Hydrogen Bonds

• Sp3 oxygen (two bonds and two lone pairs)

• 104.5* bond angle

• Net dipole moment

• Can serve as both a H-bond donor and acceptor

  • Typically 4-10 kJ/mol

  • Strongest when oriented to maximize the electrostatic interaction

  • Ideally the three atoms involved are in a line (180*)

Hydrogen Bonds

• Between the hydroxyl group of an alcohol and water

• Between the carbonyl group of a ketone and water

• Between peptide groups in polypeptides

• Between complementary bases of DNA

Hydrogen Bonding in Water

• Up to four H-bonds per water molecule gives water its

  • Anomalously high boiling point

  • Anomalously high melting point

  • Unusually large surface tension

• Hydrogen bonds between neighboring molecules are weak (20 kJ/mol) relative to the H-O covalent bonds (420 kJ/mol)

• 3.4 H-bonds per water molecule in liquid

Ice: Water in a Solid State

• Water has many different crystal forms, hexagonal ice is the most common

• Hexagonal ice forms a regular lattice, and thus has a low entropy

• Hexagonal ice contains more hydrogen bonds / water molecule

  • Ice has lower density than liquid water, therefore ice floats

Water as a Solvent

• Water is a good solvent for charged and polar substances

  • Amino acids and peptides

  • Small alcohols

  • Carbohydrates

• Water is a poor solvent for nonpolar substances

  • Nonpolar gases

  • Aromatic moieties

  • Aliphatic chains

The Hydrophobic Effect

• Refers to the association or folding of nonpolar molecules in the aqueous solution

• Is one of the main factors behind:

  • Protein folding

  • Protein-protein association

  • Formation of lipid bilayers and micelles

  • Binding of steroid hormones to their receptors

• Does NOT refer to attractive direct force between two nonpolar molecules

Entropy and Low Solubility

• Bulk water has little order and high entropy

• Water near a hydrophobic solute is highly ordered and low entropy

• Low entropy is thermodynamically unfavorable, thus hydrophobic solutes have low solubility

Lipids and Hydrophobic Effect

• If lipid molecules disperse in water, nonpolar tail of each lipid molecule is surrounded by ordered water molecules (entropy of the system decreases)

• If nonpolar portions of the molecule aggregate, fewer water molecules are ordered (released water molecules are more random, entropy increases)

• Polar “head” groups H-Bond to water

• Micelles maximize entropy by excluding the most water molecules

Ligand Binding & Hydrophobic Effect

• Binding sites in enzymes and receptors are often hydrophobic

• Such sites can bind hydrophobic substrates and ligands such as steroid hormones

• Many drugs are designed to take advantage of the hydrophobic effect

Water Dissolving Salt

• Polar water molecules surround ions to dissolve salts

• Strong electrostatic interactions between the solvated ions and water molecules lower the energy of the system

• Entropy increases as ordered crystal lattice is dissolved

Extracellular Osmolarity

• Water moves down its own concentration gradient from high conc. to low con.

• More solute = less water

• Cells in dilute salt solution are prone to bursting due to all the water moving from high (outside) to low (inside) concentration

• Cells are concentrated bags of solutes!

Ionization of Water

• O-H bonds are polar and can dissociate heterolytically

• Products are a proton (H+ ) and a hydroxide ion (OH– )

• Dissociation of water is a rapid reversible process

• Most water molecules remain un-ionized, thus pure water has very low electrical conductivity (resistance: 18 M•cm)

• The equilibrium is strongly to the left

• Extent of dissociation depends on the temperature

Proton Hydration

• H+ does not exist free in solution. – They are hydrated to form hydronium ions (H3O+ )

• A hydronium ion is a water molecule with a proton associated with one of the non-bonding electron pairs.

• Hydronium ions are solvated by nearby water molecules

• The covalent and hydrogen bonds are interchangeable. – This allows for an extremely fast mobility of protons in water via “proton hopping.”