Water is a Polar Molecule
Hydrogen Bond Figure 2.1

Hydrogen bonds also form between other polar bonds
Figure 2.4

Ionic and polar substances dissolve in water
Example: Dissolution of sodium chloride in water
Figure 2.3
Water encapsulates dissolved
Ions.

An electrostatic interaction formed between charged residues in a protein.

Molecules can form hydrogen bonds with each other or water
Glucose contains five hydroxyl groups and a ring oxygen, each of which can form hydrogen bonds with water.

Hydrogen bonding between bases in DNA
Figure 2.8

Van der Waals forces are weak noncovalent interactions between any two atoms
Figure 2.6

The HYDROPHOBIC EFFECT
Why do water and canola oil (or any hydrophobic material) spontaneously separate when first mixed?
Figure 2.9-Entropy increases when nonpolar molecules aggregate.
WHY????

The HYDROPHOBIC EFFECT
The hydrophobic effect drives the formation of micelles from amphipathic molecules
Amphipathi c molecules, such
As detergents, have a polar and
A nonpolar end.
Water

Figures 1.4
and 1.9
The HYDROPHOBIC EFFECT
- The formation of plasma membranes
Phosphate group

The HYDROPHOBIC EFFECT
Figure 2.10 Protein folding into the 3D structure
Hydrophobic groups aggregate while “squeezing” H
2 out of interior of protein.
O

Water has a slight tendency to ionize

How do we determine the amount of ions present in water?
What is the concentration of H
2
O?
1 liter of water = 1000g and 1 mol of water = 18 g
Therefore,
[H
2
O] = 55.5 mol/L or 55.5 M
K eq
= 1.8 x 10 -16 M at 25 o C
1.8 x 10 -16 M (55.5 M) = 1.0 x 10 -14 M 2 = [H + ][OH ]
K w
= 1.0 x 10 -14 M 2
Since water is electrically neutral, [H + ] = [OH ]
K w
= 1.0 x 10 -14 M 2 = [H + ] 2 or [H + ] = 1.0 x 10 -7 M

Strong acids completely dissociate in water.
Example: Hydrochloric acid (HCl)

Weak acids dissociate in water with a characteristic acid dissociation constant (K a
).
Example: Acetic acid, present in vinegar

a

Titration of acetic acid with aqueous base (OH )
Figure 2.12
The buffering region is +/- 1 pH unit from the pK a

Figure 2.11 Variety of conjugate acid-base pair important in biochemistry
H
2
CO
3
– in blood chemistry the pK a
= 6.1

Titration of a polyprotic acid (phosphoric acid) w/ aqueous base.

Calculations involving the Henderson-Hasselbalch equation
1. Pick a conjugate acid-base pair in Figure 2.11. Calculate the pK a using the K a
.
2. What is the weak acid in the reaction and what is the conjugate base of that acid?
3. What is the pH of a solution containing equal amounts of the acid and base?
4. What is the pH of a solution containing 10 times more acid than base?
5. What is the ratio of base to acid at pH = 7?
6. Draw a titration curve for the addition of base (OH ) added to a solution of acid.
7. In what pH range is the conjugate acid-base pair an effective buffer?
8. What is the pH of a solution containing 10 times more weak acid than conjugate base?

Maintenance of Blood pH in Humans pK a
= 6.1

Regulation of blood pH in mammals

Role of the lungs and kidneys in regulation of physiological pH
Respiratory  Lungs
Metabolic  Kidneys
Problems occur when pH drops (acidosis) or if pH increases (alkalosis).
Acidosis (decrease pH of blood):
Lungs: control supply of H
2 of CO
2 exhaled.
CO
3 in the blood by the amount
When blood level HCO
3
decreases the pH drops, the breathing rate must increase which increases CO
2
Respiratory acidosis in the blood.
-if too much CO
2 is retained expelled.
Kidneys: Control [HCO if [HCO
3
-
3
] by excrete more acidic urine.
] is too low  Metabolic acidosis
Causes: hypoventilation, emphysema, congestive heart failure, kidney failure, too much acidic drugs (aspirin) are taken.

Alkalosis (increase pH of blood):
Lungs: control supply of H
2 of CO
2 exhaled.
CO
3 in the blood by the amount
When blood level HCO
3
increase, the breathing rate must decrease which increases CO
2
(aqueous).
Respiratory alkalosisto little CO
2
(H
2
CO
3
) in blood)
Kidneys: Control [HCO if [HCO
3
-
3
] by excrete less acidic urine.
] is too high  Metabolic alkalosis
Causes: hyperventilation, kidney disease, high fevers

Ratio of HCO
(pK a
3
: H
2
CO
3
= 20 : 1  This results in pH = 7.4
= 6.1 at metabolic temperatures)
HCO
3
-
H
2
CO
3
= 24 - 27 mM
= 1.20 - 1.35 mM
Clinicians often monitor blood pH, HCO
3
and CO
2 concentrations.

Topics not covered:
Making Buffers in the Lab – Page 30