Amino Acid

Symbol

Structure*

pK1
(COOH)

pK2
(NH2)

pK R Group

Amino Acids with Aliphatic R-Groups

Glycine

Gly - G

2.4

9.8

Alanine

Ala - A

2.4

9.9

Valine

Val - V

2.2

9.7

Leucine

Leu - L

2.3

9.7

Isoleucine

Ile - I

2.3

9.8

Non-Aromatic Amino Acids with Hydroxyl R-Groups

Serine

Ser - S

2.2

9.2

~13

Threonine

Thr - T

2.1

9.1

~13

Amino Acids with Sulfur-Containing R-Groups

Cysteine

Cys - C

1.9

10.8

8.3

Methionine

Met-M

2.1

9.3

Acidic Amino Acids and their Amides

Aspartic Acid

Asp - D

2.0

9.9

3.9

Asparagine

Asn - N

2.1

8.8

Glutamic Acid

Glu - E

2.1

9.5

4.1

Glutamine

Gln - Q

2.2

9.1

Basic Amino Acids

Arginine

Arg - R

1.8

9.0

12.5

Lysine

Lys - K

2.2

9.2

10.8

Histidine

His - H

1.8

9.2

6.0

Amino Acids with Aromatic Rings

Phenylalanine

Phe - F

2.2

9.2

Tyrosine

Tyr - Y

2.2

9.1

10.1

Tryptophan

Trp-W

2.4

9.4

Imino Acids

Proline

Pro - P

2.0

10.6