Positively charged (basic)
Lysine
Lys · K · 146.19 Da
EssentialPurely ketogenicCharged
Essential
Ammonium (-NH₃⁺)
Long flexible arm, positive at pH 7. Purely ketogenic, and the prime target for histone acetylation and methylation.
Ionization ladder
| pKa | Group | Change | Charge |
|---|---|---|---|
| 2.18 | α-carboxyl | -COOH → -COO⁻ | 0 → −1 |
| 8.95 | α-amino | -NH₃⁺ → -NH₂ | +1 → 0 |
| 10.53 | side chain | protonated → neutral | +1 → 0 |
pI = (8.95 + 10.53) / 2 = 9.74, the midpoint of the two pKa values that bracket the neutral species.
Net charge across pH
pH 1+2
pH 4+1
pH 7+1
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.