Nonpolar / hydrophobic
Proline
Pro · P · 115.13 Da
Non-essential
Pyrrolidine ring
The only secondary amine. The ring fixes the backbone angle and removes the amide N-H, so proline breaks α-helices and β-sheets.
Ionization ladder
| pKa | Group | Change | Charge |
|---|---|---|---|
| 1.99 | α-carboxyl | -COOH → -COO⁻ | 0 → −1 |
| 10.96 | α-amino | -NH₃⁺ → -NH₂ | +1 → 0 |
pI = (1.99 + 10.96) / 2 = 6.48, the midpoint of the two pKa values that bracket the neutral species.
Net charge across pH
pH 1+1
pH 40
pH 70
pH 100
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.