Isoleucine

Ile · I · 131.17 Da

EssentialGlucogenic and ketogenic Essential

sec-Butyl

Isomeric with leucine: same formula, different branch point. Both glucogenic and ketogenic.

Mnemonic

I for Isoleucine, which branches at Cβ and so has a second chiral center.

Ionization ladder

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

pI = (2.36 + 9.68) / 2 = 6.02, 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.