pK_a values of  Common Amino Acids

Amino Acid

a-COOH pK_a

a-NH3⁺ pK_a

R to R^- pK_a

R⁺ to R pK_a

Alanine

2.4

9.7

Arginine

2.2

9.0

12.5

Asparagine

2.0

8.8

Aspartic Acid

2.1

9.8

3.9

Cysteine

1.7

10.8

8.3

Glutamic Acid

2.2

9.7

4.3

Glutamine

2.2

9.1

Glycine

2.3

9.6

Histidine

1.8

9.2

6.0

Isoleucine

2.4

9.7

Leucine

2.4

9.6

Lysine

2.2

9.0

10.5

Methionine

2.3

9.2

Phenylalanine

1.8

9.1

Proline

2.1

10.6

Serine

2.2

9.2

  ~13

Threonine

2.6

10.4

  ~13

Tryptophan

2.4

9.4

Tyrosine

2.2

9.1

10.1

Valine

2.3

9.6