The pKa Values for Ionizable Groups in Amino Acids
| Amino acid | α-COOH to α-COO– | α-NH3+ to α-NH2 | RH to R– or RH+ to R |
|---|---|---|---|
| Glycine (Gly, G) | 2.34 | 9.60 | NA |
| Alanine (Ala, A) | 2.34 | 9.69 | NA |
| Valine (Val, V) | 2.32 | 9.62 | NA |
| Leucine (Leu, L) | 2.36 | 9.60 | NA |
| Isoleucine (Ile, I) | 2.36 | 9.68 | NA |
| Methionine (Met, M) | 2.28 | 9.21 | NA |
| Proline (Pro, P) | 1.99 | 10.96 | NA |
| Phenylalanine (Phe, F) | 1.83 | 9.13 | NA |
| Tryptophan (Trp, W) | 2.38 | 9.39 | NA |
| Serine (Ser, S) | 2.21 | 9.15 | NA |
| Threonine (Thr, T) | 2.11 | 9.62 | NA |
| Asparagine (Asn, N) | 2.02 | 8.80 | NA |
| Glutamine (Gln, Q) | 2.17 | 9.13 | NA |
| Aspartate (Asp, D) | 1.92 | 9.87 | 3.87 |
| Glutamate (Glu, E) | 2.19 | 9.67 | 4.25 |
| Histidine (His, H) | 1.82 | 9.17 | 6.00 |
| Cysteine (Cys, C) | 1.96 | 10.28 | 8.18 |
| Tyrosine (Tyr, Y) | 2.20 | 9.11 | 10.07 |
| Lysine (Lys, K) | 2.18 | 8.95 | 10.53 |
| Arginine (Arg, R) | 2.17 | 9.04 | 12.48 |
| Average | 2.16 | 9.48 | NA |