Phenylalanine

Phe · F · 165.19 Da

EssentialGlucogenic and ketogenicAromatic Essential

Phenyl (benzene ring)

Alanine with a benzene ring attached. Accumulates toxically in phenylketonuria.

Mnemonic

F = "Fenylalanine". Spell it with an F and the one-letter code follows.

Ionization ladder

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

pI = (1.83 + 9.13) / 2 = 5.48, the midpoint of the two pKa values that bracket the neutral species.

Net charge across pH

pH 1+1

pH 40

pH 70

pH 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.