Amino acid zwitterions and the isoelectric point template

Glycine’s three forms (net +1 below pH 2.34, zwitterion at pI 5.97, net −1 above pH 9.60) and its titration curve with pKa₁ = 2.34, pKa₂ = 9.60 and pI = 5.97 marked.

What the template shows

• Glycine is a cation in strong acid, a zwitterion near neutral pH and an anion in strong base; pI = (pKa₁ + pKa₂)/2 = 5.97
• Glycine at low pH, at its pI and at high pH
• Each arrow loses one proton; the form shown is the main species in that pH range
• low pH (< 2.34): net charge +1
• pH 5.97 (pI): zwitterion, net 0
• high pH (> 9.60): net charge −1
• Titration of glycine with NaOH
• Computed from the Henderson–Hasselbalch equation for both groups
• equivalents of OH⁻ added
• pKa₁ = 2.34 (–COOH)
• pI = (pKa₁ + pKa₂) / 2
• The flat regions are buffer zones (pH = pKa, half of that group titrated). The steep jump at 1 equivalent is the isoelectric point, where nearly every molecule is the zwitterion.
• At pH = pI the net charge is zero, so the amino acid does not move in an electric field (electrophoresis, isoelectric focusing) and is least soluble.
• Glycine’s –COOH (pKa 2.34) is far more acidic than acetic acid (4.76) because the nearby –NH₃⁺ stabilises the carboxylate.
• With an ionisable side chain, average the two pKa values on either side of the neutral form: the two lowest for Asp and Glu, the two highest for Lys, Arg and His.

Drawn with 15 atoms, 2 arrows, 392 shapes – every one editable in MolDraw.

Sources

• Nelson & Cox, Lehninger Principles of Biochemistry, 8th ed., 2021, ch. 3 Textbook
• Berg, Tymoczko, Gatto & Stryer, Biochemistry, 9th ed., 2019 Textbook
• Voet & Voet, Biochemistry, 4th ed., Wiley 2011 Textbook