Approximate Pka Values Of Ionizable Groups Of Amino Acids
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1
2-
-
-
-
-
COO
COO
COO
COO
COO
+
+
+
+
+
H
N
C
H
H
N
C
H
H
N
C
H
H
N
C
H
H
N
C
H
3
3
3
3
3
H
CH
CH
CH
CH
3
2
2
HO
CH
3
SH
OH
-
-
-
-
-
COO
COO
COO
COO
COO
+
+
+
+
+
H
N
C
H
H
N
C
H
H
N
C
H
H
N
C
H
H
N
C
H
3
3
3
3
3
CH
CH
CH
CH
CH
2
2
2
H
C
CH
H
C
C
CH
3
3
3
3
H
CH
C
C
2
-
H
C
CH
O
O
O
NH
3
3
2
-
-
-
-
-
COO
COO
COO
COO
COO
+
+
+
+
+
H
N
C
H
H
N
C
H
H
N
C
H
H
N
C
H
H
N
C
H
2
3
3
3
3
CH
CH
CH
CH
2
2
2
2
CH
CH
CH
2
2
2
NH
S
C
C
N
-
O
O
O
NH
2
CH
3
-
-
-
-
-
COO
COO
COO
COO
COO
+
+
+
+
+
H
N
C
H
H
N
C
H
H
N
C
H
H
N
C
H
H
N
C
H
3
3
3
3
3
CH
CH
CH
CH
CH
2
2
2
2
2
CH
CH
2
2
CH
CH
2
2
N
H
CH
NH
2
+
OH
NH
C
3
+
H
N
NH
2
2
values of ionizable groups of amino acids and peptides
Approximate pK
a
(side chains listed unless otherwise noted):
Aspartate (carboxyl):
4
Serine (hydroxyl):
13
Glutamate (carboxyl):
4
Threonine (hydroxyl): 13
Histidine (imidazole):
6
α-carboxyl of free amino acid:
2
Cysteine (sulfhydryl):
8.5
α-amino of free amino acid:
9.5
Tyrosine (hydroxyl):
10.5
C-terminal carboxyl of peptide: 3
Lysine (amino):
10.5
N-terminal amino of peptide:
8
Arginine (guanidino): 12.5
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