section 2.4
Chemical Reactions of Amino Acids
31
TABLE 2-2
pK'and p i Values o f Selected Free Amino Acids at 25°C*
Amino Acid
pK j (a-COOH)
PK
' 2
pK
' 3
Alanine
2.34
9.69 (a-NH3+)
6 . 0 0
Aspartic acid
2.09
3.86 (ß-COOH)
9.82 (a-N H 3+)
2.98
Glutamic acid
2.19
4.25 (y-COOH)
9.67 (a-NH3+)
3.22
Arginine
2.17
9.04 (a-NH3+)
12.48 (Guanidinium)
10.76
Histidine
1.82
6 . 0 0
(Imidazolium)
9.17 (NH3+)
7.59
Lysine
2.18
8.95 (a-NH3+)
10.53 (e-NH3+)
9.74
Cysteine
1.71
8.33 (-SH)
10.78 (a-NH3+)
5.02
Tyrosine
2 . 2 0
9.11 (a-NH3+)
10.07 (Phenol OH)
5.66
Serine
2 . 2 1
9.15 (a-NH3+)
13.6 (Alcohol OH)
5.68
Pi
pK', + PK '2 j
pK'1+ pK'2j
PK '2 + pK'3 ^
p K ^ + p K ^ j
PK '2 + pK'3 j
Pr
1
+Pr 2j
Pr
1
+Pr 2j
pK'j + pK'2
\
*The pK' values for functional groups in proteins may vary significantly from the values for free amino acids.
carboxylic acids exhibit their greatest buffering capacities
in the two pH ranges near their two pK' values, namely,
pH 2.3 and pH 9.7 (Figure 2-7). Neither these amino acids
nor the a-amino or a-carboxyl groups of other amino acids
(which have similar pK' values) have significant buffering
capacity in the neutral (physiological) pH range. The only
amino acids with R-groups that have buffering capacity
in the physiological pH range are histidine (imidazole;
pK' 6.0) and cysteine (sulfhydryl; pK' 8.3). The pK' values
for R-groups vary with the ionic environment. The titration
profile of histidine is shown in Figure 2-8. The pi is the
mean of pK
2
and pK3.
Titration profiles of the basic and acidic amino acids
lysine and aspartic acid are shown in Figures 2-9 and
2-10. The R-groups are ionized at physiological pH
and have anionic and cationic groups, respectively. The
pi value for aspartic acid is the arithmetic mean of pKj
and pK2, whereas for lysine and histidine the pi values
are given by the arithmetic mean of pK
2
and pK3. The
pK' and pi values of selected amino acids are listed in
Table 2-2.
2.4 Chemical Reactions of Amino Acids
The reactions of amino acids with ninhydrin, carbon diox-
ide, metal ions, and glucose are described below. The last
three are of physiological importance.
Ninhydrin
(triketohydrindene hydrate) reacts with
a-
amino acids to produce CO
2
, NH
3
, and an aldehyde
with one less carbon than the parent amino acid. In
most cases, a blue or violet compound (proline and
hydroxyproline give a yellow color) is formed owing to
reaction of the liberated NH
3
with ninhydrin, as shown in
Figure 2-11. Color and CO
2
production provide a ba-
sis for the quantitative determination of amino acids.
