Leucine

Leu · L · 131.17 Da

EssentialPurely ketogenic Essential

Isobutyl

One of the two purely ketogenic amino acids, and the core of the leucine zipper motif.

Mnemonic

Leu branches LATE (at Cγ); Ile branches EARLY (at Cβ).

Ionization ladder

pKa	Group	Change	Charge
2.36	α-carboxyl	-COOH → -COO⁻	0 → −1
9.60	α-amino	-NH₃⁺ → -NH₂	+1 → 0

pI = (2.36 + 9.60) / 2 = 5.98, 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.