Lecture 2 : Water and Biological Buffers

Question 1

What is the shape, hybrid orbital orientation. and degree of a water molecule?

Answer 1

Question 2

How many H-bonds can form per water?

Answer 2

4 bonds

2 on Hydrogen - 2 on electron pairs

Question 3

How much strength is required to break a hydrogen bond?

Answer 3

20 kJ/mol

Question 4

When water freezes what is the organized form?

Answer 4

Hexagonal, contains maximal hydrogen bonds/ water molecules, forcing the water molecules into equidistant arrangement

Question 5

Is waters dielectric constant high or low and why does it matter?

Answer 5

Waters high dielectric constant reduces attraction between oppositely charged ions in salt crystals; almost not attraction at large ( >40 nm) distances.

Question 6

What is the equation for strength of ionic interaction

Answer 6

F = (Q₁ * Q₂)/ (ε * r²)

Question 7

Does the mixture of an ionic compound and water lower or increase entropy?

Answer 7

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

Entropy increases

Question 8

What are the two components of van der Waals interaction

Answer 8

1. Attractive force (London dispersion, which depends on the polarizability
2. Repulsive force (steric repulsion) which depends on the size of atom

Question 9

When do attractive forces and repulsive forces dominate for van der Waals?

Answer 9

Attraction dominates at longer distances, typically .4 - .7 nm

Repulsion dominates at very short distances

Question 10

Why are van der Walls interactions important?

Answer 10

1. Determines steric complementarity
2. Stabilizes biological macromolecules (stacking in DNA)
3. Facilitates binding of polarizable ligands

Question 11

What is the hydrophobic effect?

Answer 11

refers to the association or interaction of nonpolar molecules or components of molecules in the aqueous solution

Question 12

Why is the hydrophobic effect important?

Answer 12

Protein folding

Protein-protein association

formation of lipid micelles

Binding of steroid hormones to their receptors

Question 13

Does the hydrophobic effect occur because of a direct attraction?

Answer 13

No, results from the system’s achieving the greatest thermodynamic stability by minimizing the number of ordered water molecules required to surround hydrophobic portions of the solute molecule.

Question 14

Why are nonpolar molecules poorly soluble in water?

Answer 14

The movement of molecules from the disordered gas phase into the aqueous phase decrease entropy

Question 15

Why do nonpolar portions of an amphipathic molecule aggregate?

Answer 15

So that fewer water molecules are ordered and entropy increases

Question 16

What is a micelle

Answer 16

With high enough concentration of amphipathic molecules, they completely aggregates into micelles

Question 17

How does the hydrophobic effect favor ligand binding?

Answer 17

• Binding site in enzymes and receptors are often hydrophobic
• Such sites can bind hydrophobic substrates and ligands such as steroid hormones

Question 18

What are colligative properties?

Answer 18

are properties that depend upon the concentration of solute molecules or ions, but not upon the identity of the solute. Colligative properties include vapor pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure.

Question 19

What is osmotic pressure?

Answer 19

Water moves from areas of high water concentration to areas of low water concentration. Osmotic pressure is the force necessary to resist the movement.

Question 20

Water ionizes, is it a good electrical conductor and where does the equilibrium lay?

Answer 20

1. very low electrical conductivity
2. Is strongly to the left

Question 21

What is solvation?

Answer 21

Solvation is the interaction of a solvent with the dissolved solute—in the case of water, solvation is often referred to as hydration

Question 22

What is the K_w of water?

Answer 22

K_w = K_eq [H₂O] = [H⁺][OH^-] = 1 x 10^-14