Arginine

Arg · R · 174.20 Da

EssentialCharged Conditionally essential

Guanidinium

The guanidinium group spreads its positive charge over three nitrogens. Precursor of nitric oxide. Conditionally essential during growth, pregnancy and illness.

Mnemonic

aRginine → R. The most basic side chain of all, so it is essentially always positive.

Ionization ladder

pKa	Group	Change	Charge
2.17	α-carboxyl	-COOH → -COO⁻	0 → −1
9.04	α-amino	-NH₃⁺ → -NH₂	+1 → 0
12.48	side chain	protonated → neutral	+1 → 0

pI = (9.04 + 12.48) / 2 = 10.76, 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.