Water Properties and Biomolecules

Hydrogen Bonding and Water's Properties

• Hydrogen bonding is an electrostatic attraction between water molecules.

• Water has high melting/boiling points due to hydrogen bonds.

• Energy for breaking OH hydrogen bond: $23 \, \text{kJ/mol}$, covalent bond: $470 \, \text{kJ/mol}$.

• Hydrogen bonds in liquid water break and reform constantly.

• Each H₂O molecule bonds with 3.4 others in liquid, 4 in ice.

Hydrogen Bonds and Melting Point

• Melting/evaporation absorbs heat, increasing entropy.

• $\text{H}2\text{O(solid)} \rightarrow \text{H}2\text{O(liquid)} \quad \Delta H = +5.9 \, \text{kJ/mol}$

• $\text{H}2\text{O(liquid)} \rightarrow \text{H}2\text{O(gas)} \quad \Delta H = +44.0 \, \text{kJ/mol}$

• $\Delta G = \Delta H - T\Delta S$

• Melting/evaporation occur spontaneously at room temperature because $\Delta G$ is negative due to increased entropy.

Biomolecules: Polar, Nonpolar, and Amphipathic

• Polar: Dissolve in water (e.g., Glucose, Glycine, Aspartate).

• Nonpolar: Don't dissolve in water (e.g., Typical wax, Phenylalanine).

• Amphipathic: Polar and nonpolar regions (e.g., Lactate, Glycerol, Phosphatidylcholine).

Water Interactions with Polar Solutes

• Hydrogen bonds form between hydrogen acceptors and donors.

Water as a Solvent

• Water dissolves salts/charged biomolecules by screening electrostatic interactions.

• Ions/molecules that dissolve easily are hydrophilic; entropy increases.

Water and Nucleophilic Reactions

• Nucleophilic reactions involve nucleophiles replacing groups in electrophiles.

• Water inhibits these reactions by crowding/shielding charges.

• Enzyme catalysis speeds up reactions.

Enzyme Catalysis: Glucose Phosphorylation Example

• Enzymes increase reactivity for metabolism.

• Uncatalyzed reactions take ~$1 \, \text{billion}$ seconds; catalyzed reactions take ~$1 \, \text{second}$.

• Example: Hexokinase phosphorylates glucose at position 6, regulated to act only when needed.

Solubility of Nonpolar Molecules in Water

• Nonpolar gases (CO₂, O₂, N₂) are hydrophobic, do not dissolve well.

• Dissolving decreases entropy. Polar molecules dissolve better.

Nonpolar Compounds and Water Structure

• Nonpolar compounds disrupt water's hydrogen bonding, increasing enthalpy ($\Delta H$), decreasing entropy ($\Delta S$).

• $\Delta G = \Delta H - T\Delta S$ is unfavorable, causing hydrophobic nature.

Ordering of Water Molecules Around Nonpolar Solutes

• Water forms cage-like structures around solutes to maximize hydrogen bonding.

Amphipathic Compounds in Aqueous Solutions

• Amphipathic compounds have polar/charged and nonpolar regions.

• Polar regions dissolve; nonpolar regions cluster.

Hydrophobic Effect

• Nonpolar regions cluster; polar regions interact with each other and solvent.

• Micelles are stable amphipathic structures.

Weak Interactions in Macromolecular Structure

• Noncovalent interactions are weak, constantly forming/breaking.

• Types: van der Waals, hydrogen bonds, ionic interactions, hydrophobic effect.

Enzyme-Substrate Complex Formation

• Release of ordered water favors complex formation.

• Stabilized by hydrogen bonding, ionic interactions, hydrophobic effect.

Cumulative Effect of Weak Interactions

• Stable structure maximizes weak interactions. Water binds to biomolecules.

Osmotic Pressure and Solute Concentration

• Solutes alter vapor pressure, boiling/melting points, osmotic pressure based on particle number.

• Osmosis: Water movement across membranes due to osmotic pressure.

Osmolarity and Water Movement

• Osmolarity measures solute concentration.

• Isotonic: No net movement. Hypertonic: Water moves out. Hypotonic: Water moves in, cell may burst.

Ionization of Water

• H₂O ionizes reversibly: $\text{H}_2\text{O} \rightleftharpoons \text{H}^+ + \text{OH}^-$

• Hydrogen ions form hydronium ions (H₃O⁺): $\text{H}2\text{O} + \text{H}^+ \rightarrow \text{H}3\text{O}^+$

• Proton hopping results in high ionic mobility.