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Biology LibreTexts

Structure and Property of the Naturally-Occurring Amino Acids

Amino Acid 3 Letter Code 1 Letter Code Structure pKa1
(
α-carboxyl)
pka2 (α-amino) pka3 (side chain)

Nonpolar Side Chains

Glycine Gly G 2.35 9.78 .
Alanine Ala A 2.35 9.87 .
Valine Val V 2.29 9.74 .
Leucine Leu L 2.33 9.74 .
Isoleucine Ile I 2.32 9.76 .
Methionine Met M 2.13 9.28 .
Proline Pro P 1.95 10.64 .
Phenylalanine Phe F 2.20 9.31 .
Tryptophan Trp W 2.46 9.41 .

Uncharged Polar Side Chains

Serine Ser S 2.19 9.21 .
Threonine Thr T 2.09 9.10 .
Asparagine Asn N 2.14 8.72 .
Glutamine Gln Q 2.17 9.13 .
Tyrosine Tyr Y 2.20 9.21 10.46
Cysteine Cys C 1.92 10.70 8.37

Charged Polar Side Chains10.54

Lysine Lys K 2.16 9.06 10.54
Arginine Arg R 1.82 8.99 12.48
Histidine His H 1.80 9.33 6.04
Aspartic Acid Asp D 1.99 9.90 3.90
Glutamic Acid Glu E 2.10 9.47 4.07