Amino Acids with Acidity Values

The structure to the right is a generic amino acid. All of the common amino acids except glycine are chiral (have a handedness). The form shown (the S-amino acid, or L-amino acid) is the most common form found in plants and animals. The mirror image of each amino acid can be found in nature, although they are less common.

The third through fifth columns give pKa values of groups: -NH3 refers to the protonated a-amino group, the CO2H refers to the carboxylic acid group on the a-carbon, and the side chain is just that. In all cases, these refer to the free amino acid, and not the amino acid incorporated into a polypeptide or protein chain. Usually, we assume that not much happens to the pKa of the side chain on incorporation, as a first guess. While that is not strictly true, there are so many other environmental effects on the values that this value works well as a starting point, until the value can be measured independently.

 

Amino Acid

R

-NH3+

-CO2H

Side chain

pI

Glycine, Gly

-H

9.6

2.34

5.97

Alanine, Ala

-CH3

9.69

2.35

6.02

Valine, Val

-CH(CH3)2

9.62

2.32

5.97

Leucine, Leu

CH2CH(CH3)2

9.60

2.36

5.98

Isoleucine, Ile

CH(CH3)CH2CH3

9.68

2.36

6.02

Phenylalanine, Phe

9.13

1.83

5.48

Tryptophan, Trp

9.39

2.38

5.88

Tyrosine, Tyr

9.11

2.20

10.07

5.65

Histidine, His

9.17

1.82

6.0*

7.58

Serine, Ser

CH2OH

9.15

2.21

5.68

Threonine, Thr

CH(CH3)-OH

10.43

2.63

6.53

Methionine, Met

CH2CH2SCH3

9.21

2.28

5.75

Cysteine, Cys

CH2SH

8.33

1.71

10.78

5.02

Aspartic Acid, Asp

CH2CO2H

9.82

2.09

3.86

2.87

Glutamic Acid, Glu

CH2CH2CO2H

9.67

2.19

4.25

3.22

Asparagine, Asn

CH2CONH2

8.08

2.02

5.41

Glutamine, Gln

CH2CH2CONH2

9.13

2.17

5.65

Lysine, Lys

(CH2)4NH2

8.95

2.18

10.53*

9.74

Arginine, Arg

9.04

2.17

12.48*

10.76

Proline, Pro

10.60

1.99

6.10

*Refers to the conjugate acid.