Polar, uncharged
Serine
Ser · S · 105.09 Da
Hydroxyl-containing
Non-essential
Hydroxyl (primary alcohol)
A phosphorylation target, and the nucleophile of the catalytic triad in serine proteases.
Ionization ladder
| pKa | Group | Change | Charge |
|---|---|---|---|
| 2.21 | α-carboxyl | -COOH → -COO⁻ | 0 → −1 |
| 9.15 | α-amino | -NH₃⁺ → -NH₂ | +1 → 0 |
pI = (2.21 + 9.15) / 2 = 5.68, 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.