Negatively charged (acidic)
Aspartate
Asp · D · 133.10 Da
Polar, chargedQuirkily coded
Non-essential
Carboxylate (-COO⁻)
Negative at pH 7. Donates the second nitrogen of urea, and sits in the catalytic triad of serine proteases.
Ionization ladder
| pKa | Group | Change | Charge |
|---|---|---|---|
| 1.88 | α-carboxyl | -COOH → -COO⁻ | 0 → −1 |
| 3.65 | side chain | neutral → deprotonated | 0 → −1 |
| 9.60 | α-amino | -NH₃⁺ → -NH₂ | +1 → 0 |
pI = (1.88 + 3.65) / 2 = 2.76, the midpoint of the two pKa values that bracket the neutral species.
Net charge across pH
pH 1+1
pH 4−1
pH 7−1
pH 10−2
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.