Nonpolar / hydrophobic
Alanine
Ala · A · 89.09 Da
Non-essential
Methyl
The reference "average" residue and a strong α-helix former.
Ionization ladder
| pKa | Group | Change | Charge |
|---|---|---|---|
| 2.34 | α-carboxyl | -COOH → -COO⁻ | 0 → −1 |
| 9.69 | α-amino | -NH₃⁺ → -NH₂ | +1 → 0 |
pI = (2.34 + 9.69) / 2 = 6.01, 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.