Advanced Biochemistry 高等生化學 Water 陳威戎

1. Hydrogen bonding gives water its unusual properties. 2. Water forms hydrogen bonds with polar solutes. 3. Water interacts electrostatically with charged solutes. 4. Entropy increases as crystalline substances dissolve. 5. Nonpolar gases are poorly soluble in water. 6.Nonpolar compounds force energetically unfavorable changes in the structure of water. 7.van der Waals interactions are weak interatomic attractions. 8.Weak interactions are crucial to macromolecular structure and function. 9.Solutes affect the colligative properties of aqueous solutions. I. Weak Interactions in Aqueous Systems

1. Hydrogen bonding gives water its unusual properties.

Structure of the water molecule

Hydrogen bonding in ice

2. Water forms hydrogen bonds with polar solutes.

Some biologically important hydrogen bonds

Common hydrogen bonds with polar solutes

Directionality of the hydrogen bond

3. Water interacts electrostatically with charged solutes.

Water as solvent 4. Entropy increases as crystalline substances dissolve.

The force of ionic interactions in a solution F: the strength or force of ionic interactions in a solution Q: charges of the interacting molecules r: the distance between the charged groups  : the dielectric constant (water: 78.5 ; benzene: 4.6)

5. Nonpolar gases are poorly soluble in water.

6. Nonpolar compounds force energetically unfavorable changes in the structure of water. Weak Interactions in Aqueous Systems

Amphipathic compounds in aqueous solution – fatty acids 6. Nonpolar compounds force energetically unfavorable changes in the structure of water.

Amphipathic compounds in aqueous solution – micelles 6. Nonpolar compounds force energetically unfavorable changes in the structure of water.

Release of ordered water favors formation of an enzyme-substrate complex

7. van der Waals interactions are weak interatomic attractions.

8. Weak interactions are crucial to macromolecular structure and function.

Water binding in hemoglobin

Water chain in cytochrome

9. Solutes affect the colligative properties of aqueous solutions. Colligative properties: 1. Vapor pressure 2. Boiling point 3. Melting (Freezing) point 4. Osmotic pressure It depends only on the number of solute particles.

Osmosis and the measurement of osmotic pressure

Effect of extracellular osmolarity on water movement across a plasma membrane

Touch response in plants: an osmotic event (1)

Touch response in plants: an osmotic event (2)

1. Pure water is slightly ionized. 2. The ionization of water is expressed by an equilibrium constant. 3. The pH scale designates the H + and OH - concentrations. 4. Weak acids and bases have characteristic dissociation constants. 5. Titration curves reveal the pK a of weak acids. II. Ionization of Water, Weak Acids, and Weak Bases

Proton hopping 1. Pure water is slightly ionized.

2. The ionization of water is expressed by an equilibrium constant. For a generalized reaction The equilibrium constant For the reversible ionization of water The equilibrium constant

The ion product of water: K w

3. The pH scale designates the H + and OH - concentrations.

4. Weak acids and bases have characteristic dissociation constants. Strong acids: HCl, H 2 SO 4, HNO 3 ; Strong bases: NaOH, KOH Weak acids and bases: not completely ionized when dissolved Acids: proton donors ; Bases: proton acceptors Conjugate acid-base pair For a reversible reaction of acid ionization The equilibrium constant K a : dissociation constant

Conjugate acid-base pairs consist of a proton donor and a proton acceptor

5. Titration curves reveal the pK a of weak acids.

Comparison of the titration curves of three weak acids

1. Buffers are mixtures of weak acids and their conjugate bases. 2. A simple expression relates pH, pK a, and buffer concentration. 3. Weak acids or bases buffer cells and tissues against pH changes. III. Buffering against pH Changes in Biological Systems

1. Buffers are mixtures of weak acids and their conjugate bases.

Henderson-Hasselbalch equation can be derived as follows: 2. A simple expression relates pH, pK a, and buffer concentration.

Solving problems using the Henderson-Hasselbalch equation

Histidine residue, a component of proteins, is a weak acid 3. Weak acids or bases buffer cells and tissues against pH changes.

Blood, lungs, and buffer: the bicarbonate buffer system

The pH optima of some enzymes

1.Weak Interactions in Aqueous Systems 2.Ionization of Water, Weak Acids, and Weak Bases 3.Buffering against pH Changes in Biological Systems 4.Water as a Reactant 5.The Fitness of the Aqueous Environment for Living Organisms Water

Participation of water in biological reactions

Aqueous environments support countless species