Tryptophan

Trp · W · 204.23 Da

EssentialGlucogenic and ketogenicAromatic Essential

Indole (bicyclic)

Largest amino acid at 204 Da. Dominates protein absorbance at 280 nm. Precursor of serotonin, melatonin and niacin.

Mnemonic

W = "double-u" = the double ring. The biggest letter for the biggest residue.

Ionization ladder

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

pI = (2.38 + 9.39) / 2 = 5.88, 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.