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Chapter 3 Amino Acids, Peptides, and Proteins S-31
(c)
7. Comparing the pKa Values of Alanine and Polyalanine The titration curve of alanine shows the
ionization of two functional groups with pKa values of 2.34 and 9.69, corresponding to the ionization 
of the carboxyl and the protonated amino groups, respectively. The titration of di-, tri-, and larger
oligopeptides of alanine also shows the ionization of only two functional groups, although the experi-
mental pKa values are different. The trend in pKa values is summarized in the table.
(a) Draw the structure of Ala–Ala–Ala. Identify the functional groups associated with pK1 and pK2.
(b) Why does the value of pK1 increase with each additional Ala residue in the Ala 
oligopeptide?
(c) Why does the value of pK2 decrease with each additional Ala residue in the Ala 
oligopeptide?
Answer
(a) The structure at pH 7 is:
Note that only the amino- and carboxyl-terminal groups ionize.
(b) As the length of poly(Ala) increases, the two terminal groups move farther apart, sepa-
rated by an intervening sequence with an “insulating” nonpolar structure. The carboxyl
group becomes a weaker acid, as reflected in its higher pKa, because the electrostatic
repulsion between the carboxyl proton and the positive charge on the NH3
� group
diminishes as the groups become more distant.
(c) The negative charge on the terminal carboxyl group has a stabilizing effect on the
positively charged (protonated) terminal amino group. With increasing numbers of
intervening Ala residues, this stabilizing effect is diminished and the NH3
� group loses its
COO�
�
CH2
H3N H
CH3
C
CH CH3
COO�
�
CH2
NH3H
CH3
C
CH CH3
COO�
�
CH2
NH3H
CH3
C
C HH3CH3C
COO�
�
CH2
H3N H
CH3
C
C H
Amino acid or peptide pK1 pK2
Ala 2.34 9.69
Ala–Ala 3.12 8.30
Ala–Ala–Ala 3.39 8.03
Ala–(Ala)n–Ala, n 
 4 3.42 7.94
CH3
pK2 � 8.03 pK1 � 3.39
CH3H
�
H3N CH
CH3
C
O
N
H
CH C N CH
O
C O�
O
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