Glycine

Gly · G · 75.07 Da

Non-essential

Hydrogen, no side chain

Smallest residue. The tiny side chain gives the backbone unusual freedom, so glycine appears at tight turns and at every third position in collagen.

Mnemonic

G for Glycine, the only achiral amino acid. Its side chain is a single H.

Ionization ladder

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

pI = (2.34 + 9.60) / 2 = 5.97, 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.