Organic Chemistry

966 CHAPTER 23 Amino Acids, Peptides, and Proteins

on one atom and a positive charge on a nonadjacent atom. (The name comes from

zwitter, German for “hermaphrodite”or “hybrid.”)

A few amino acids have side chains with ionizable hydrogens (Table 23.2). The

protonated imidazole side chain of histidine, for example, has a of 6.04. Histidine,

therefore, can exist in four different forms, and the form that predominates depends on

the pH of the solution.

PROBLEM 4

Why are the carboxylic acid groups of the amino acids so much more acidic

than a carboxylic acid such as acetic acid

PROBLEM 5 SOLVED

Draw the form in which each of the following amino acids predominantly exists at physio-

logical pH (7.3):

a. aspartic acid c. glutamine e. arginine

b. histidine d. lysine f. tyrosine

SOLUTION TO 5a Both carboxyl groups are in their basic forms because the pH is

greater than their. The protonated amino group is in its acidic form because the pH

is less than its

PROBLEM 6

Draw the form in which glutamic acid predominantly exists in a solution with the follow-

ing pH:

a. b. c. d.

PROBLEM 7

a. Why is the of the glutamic acid side chain greater than the of the aspartic acid

side chain?

b. Why is the of the arginine side chain greater than the of the lysine side chain?

23.4 The Isoelectric Point

The isoelectric point(pI) of an amino acid is the pH at which it has no net charge. In

other words, it is the pH at which the amount of positive charge on an amino acid

exactly balances the amount of negative charge:

pI (isoelectric point)pH at which there is no net charge

pKa pKa

pKa pKa

pH= 0 pH= 3 pH= 6 pH= 11

+NH

3

OO

−OCCH

2 CHCO

−

pKa.

pKa’s

(pKa=4.76)?

(pKa'2)

pKa

+NH

3

+NH

3

+NH

HN NH^3

CH 2 CHCOH

+ HN NH

CH 2 CHCO−

+ NNH

CH 2 CHCO−

NNH

CH 2 CHCO−

NH 2

OOOO

pH = 0pH = 4pH = 8pH = 12

histidine

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