Organic Chemistry

Section 23.4 The Isoelectric Point 967

Recall from the Henderson–Hasselbalch

equation that when , half the

group is in its acidic form and half is in

its basic form (Section 1.20).

pHpKa

An amino acid will be positively charged

if the pH of the solution is less than the

pI of the amino acid and will be nega-

tively charged if the pH of the solution

is greater than the pI of the amino acid.

The pI of an amino acid that does nothave an ionizable side chain—such as alanine—

is midway between its two values. This is because at half the molecules

have a negatively charged carboxyl group and half have an uncharged carboxyl group,

and at half the molecules have a positively charged amino group and half

have an uncharged amino group. As the pH increases from 2.34, the carboxyl group of

more molecules becomes negatively charged; as the pH decreases from 9.69, the amino

group of more molecules becomes positively charged. Therefore, at the average of the

two values, the number of negatively charged groups equals the number of

positively charged groups.

The pI of an amino acid that hasan ionizable side chain is the average of the

values of the similarly ionizing groups (a positively charged group ionizing to an un-

charged group or an uncharged group ionizing to a negatively charged group). For ex-

ample, the pI of lysine is the average of the values of the two groups that are

positively charged in their acidic form and uncharged in their basic form. The pI of

glutamate, on the other hand, is the average of the values of the two groups that

are uncharged in their acidic form and negatively charged in their basic form.

PROBLEM 8

Explain why the pI of lysine is the average of the values of its two protonated amino

groups.

PROBLEM 9

Calculate the pI of each of the following amino acids:

a. asparagine b. arginine c. serine

PROBLEM 10

a. Which amino acid has the lowest pI value?

b. Which amino acid has the highest pI value?

c. Which amino acid has the greatest amount of negative charge at

d. Which amino acid—glycine or methionine—has a greater negative charge at

PROBLEM 11

Explain why the pI values of tyrosine and cysteine cannot be determined by the method

just described.

pH=6.20?

pH=6.20?

pKa

pKa

pKa

pKa

+NH

3

O

pI===6.02

2.34 + 9.69

2

CH 3 CHCOH

12.03

2

pKa = 9.69

pKa = 2.34

alanine

pKa

pH=9.69,

pKa pH=2.34,

+NH

3

+NH

3

pKa = 8.95 pKa = 9.67

pKa = 2.18 pKa = 2.19

pKa = 10.79 pKa = 4.25

+

O

pI===9.87

8.95 + 10.79

2

H 3 NCH 2 CH 2 CH 2 CH 2 CHCOH

19.74

2

O O

pI===3.22

2.19 + 4.25

2

HOCCH 2 CH 2 CHCOH

6.44

2

lysine glutamic acid

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