Nonpolar / hydrophobic
Methionine
Met · M · 149.21 Da
EssentialSulfur-containing
Essential
Thioether (S-CH₃)
Sulfur-containing, but a thioether cannot form disulfides. Precursor of SAM, the universal methyl donor.
Ionization ladder
| pKa | Group | Change | Charge |
|---|---|---|---|
| 2.28 | α-carboxyl | -COOH → -COO⁻ | 0 → −1 |
| 9.21 | α-amino | -NH₃⁺ → -NH₂ | +1 → 0 |
pI = (2.28 + 9.21) / 2 = 5.75, the midpoint of the two pKa values that bracket the neutral species.
Net charge across pH
pH 1+1
pH 40
pH 70
pH 10−1
pH 13−1
A group is deprotonated once the pH climbs past its pKa. Add the contributions and you have the net charge, with no memorization required.