Polar, uncharged
Cysteine
Cys · C · 121.16 Da
Sulfur-containing
Non-essential
Thiol (-SH)
Serine with sulfur in place of oxygen. Two cysteines oxidize to a disulfide bond, the main covalent cross-link in secreted proteins.
Ionization ladder
| pKa | Group | Change | Charge |
|---|---|---|---|
| 1.96 | α-carboxyl | -COOH → -COO⁻ | 0 → −1 |
| 8.18 | side chain | neutral → deprotonated | 0 → −1 |
| 10.28 | α-amino | -NH₃⁺ → -NH₂ | +1 → 0 |
pI = (1.96 + 8.18) / 2 = 5.07, 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 13−2
A group is deprotonated once the pH climbs past its pKa. Add the contributions and you have the net charge, with no memorization required.