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 | G | 2.34 | 9.60 | - | 5.97 | ||
| Alanine | A | 2,34 | 9.69 | - | 6.00 | ||
| Valine | V | 2.32 | 9.62 | - | 5.96 | ||
| Leucine | L | 2.36 | 9.60 | - | 5.98 | ||
| Isoleucine | I | 2.36 | 9.60 | - | 6.02 | ||
| Methionine | M | 2.28 | 9.21 | - | 5.74 | ||
| Proline | P | 1.99 | 10.60 | - | 6.30 | ||
| Phenylalanine | F | 1.83 | 9.13 | - | 5.48 | ||
| Tryptophan | W | 2.83 | 9.39 | - | 5.89 | ||
| Asparagine | N | 2.02 | 8.80 | - | 5.41 | ||
| Glutamine | Q | 2.17 | 9.13 | - | 5.65 | ||
| Serine | S | 2.21 | 9.15 | - | 5.68 | ||
| Threonine | T | 2.09 | 9.10 | - | 5.60 | ||
| Tyrosine | Y | 2.20 | 9.11 | - | 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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