Amino acid pKa and pKi values

The proteinogenic amino acids are amphoteric and have two or three pK values, depending on their side chains.  pKa1 is the α-carboxyl group, pKa2 is the α-ammonium ion, pKa3 is the side chain group if applicable and pI is the isoelectric point at which the amino acid has no net charge.  At a pH lower than pI the amino acid is positively charged, at higher pH than pI the amino acid is negatively charged.

Amino acids with neutral side chains

Amino acid                       Code                        pKa1                       pKa2                            pKa3                                 pI


Glycine G2.34 9.60 -5.97

Alanine A2,34 9.69  -6.00

Valine V2.32 9.62  -5.96

Leucine L2.36 9.60  -5.98

Isoleucine I2.36 9.60  -6.02

Methionine M2.28 9.21  -5.74

Proline P1.99 10.60  -6.30

Phenylalanine F1.83 9.13  -5.48

Tryptophan W2.83 9.39  -5.89

Asparagine N2.02 8.80  -5.41

Glutamine Q2.179.13 -5.65

Serine S2.219.15 -5.68

Threonine T2.099.10 -5.60

Tyrosine Y2.209.11 -5.66

Amino acids with ionisable side chains

Amino acid                    Code                    pKa1                 pKa2                     pKa3                         pI


Cysteine  C 1.96 10.28 8.18 5.07

Aspartic acid  D 1.88 9.60 3.65 2.77

Glutamic acid  E 2.19 9.67 4.25 3.22

Lysine  K 2.18 8.95 10.53 9.74

Arginine  R 2.17 9.04 12.48 10.76

Histidine  H 1.82 9.17 6.00 7.59


Data from: Carey and Giuliano (2011) Amino acids, peptides and proteins. Organic Chemistry 8 th Edition, 25, 1126 McGraw Hill, ISBN-13: 978-0077354770



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