Amino acids — zwitterion, isoelectric point pI, classification (essential, non-essential): Question bank

Quick reminder of the ladder you will keep using: as pH rises, an amino acid moves cation +1 → zwitterion 0 → anion –1. The pI is the pH where net charge is zero. Tools you will lean on: Henderson–Hasselbalch equation, Acids and bases — pKa and pH.

True or false — justify

A zwitterion has a net charge of zero.

True — it carries a + on –NH₃⁺ and a – on –COO⁻ that cancel; "net zero" is not the same as "no charges present."

A zwitterion is a non-polar molecule because its charges cancel.

False — the + and – sit on different atoms, so it is strongly dipolar (an internal salt); that is exactly why it dissolves in water and not in oil.

At its pI, an amino acid stops moving in an electric field.

True — average net charge is zero, so there is no net force from the field; this is the whole basis of electrophoresis separation.

The pI is always the average of all the pKa values of the molecule.

False — only the two pKa's flanking the neutral (zwitterion) form are averaged; the others are spectators at that pH.

Glycine is chiral like the other amino acids.

False — in glycine R = H, so its α-carbon has two identical H's and is not a chiral centre; it is the one standard amino acid that is optically inactive. See Optical isomerism and chirality.

Naturally occurring amino acids are the D-form.

False — they are almost all the L-form; D-amino acids are rare and mostly non-biological.

Amino acids have high melting points because they have strong covalent bonds.

False — the high melting/charring point comes from the ionic lattice of zwitterions (electrostatic attraction between + and –), not from unusually strong covalent bonds.

"Essential" amino acids are the chemically most important ones.

False — "essential" means only that the body cannot synthesise them, so they must come from diet; it is a nutritional label, not a ranking of importance.

An amino acid is amphoteric.

True — it reacts with both acids (–COO⁻ grabs H⁺) and bases (–NH₃⁺ gives up H⁺), because it contains both an acidic and a basic group.

At a pH below its pI, an amino acid carries negative charge.

False — below pI there is excess H⁺, groups get protonated, so the molecule is a cation (+) and moves toward the cathode (– electrode).

Spot the error

"pI of aspartic acid = (2.0 + 3.9 + 9.9)/3."

Wrong — you never average all three. The neutral form is reached by losing the two COOH protons, so pI = (2.0 + 3.9)/2 = 2.95; the third pKa is above the neutral form.

"Lysine has a low pI because it has three ionisable groups."

Wrong — count of groups doesn't set pI; the basic side chain means the neutral form sits between the two highest pKa's, giving a high pI ≈ 9.75.

"Adding H⁺ to a zwitterion deprotonates the –COO⁻."

Backwards — adding H⁺ protonates –COO⁻ back to –COOH, producing the cation. Deprotonation needs base, not acid.

"At high pH the amino acid becomes a cation."

Wrong — high pH means excess OH⁻, which strips the proton from –NH₃⁺, giving the anion (–). Cation forms at low pH.

"The zwitterion is a high-energy, unstable form that quickly reacts away."

Wrong — the internal proton transfer (strong acid –COOH → good base –NH₂) is downhill in energy, so the zwitterion is the stable, dominant form in solid state and neutral water.

"Since Asp is acidic, at pH 7 it is neutral."

Wrong — pH 7 is well above pI(Asp) ≈ 2.95, so Asp is strongly negative at pH 7 and migrates to the anode.

"Histidine is essential for adults just like it is for children."

Careful — histidine (and arginine) are conditionally essential: required in the diet during childhood/growth, but the label is stated as essential-for-children specifically.

Why questions

Why does the proton "jump" from –COOH to –NH₂ inside the same molecule?

Because –COOH is a stronger acid (pKa ≈ 2) than –NH₃⁺ (pKa ≈ 9–10), so the acidic proton hops onto the better base; the internal-salt product is lower in energy.

Why is the pI the arithmetic mean of the two flanking pKa's, not some weighted value?

Adding the two Henderson–Hasselbalch equations for the flanking equilibria gives

2 pH = p K_{a 1} + p K_{a 2} + lo g \frac{[ anion ]}{[ cation ]}

; at pI those two concentrations are equal so the log term is zero, leaving the plain mean.

Why must we use only the two pKa's that "hug" the zwitterion?

Those are the two ionisations that create the +1 and –1 species flanking neutral; sitting at their midpoint balances cation and anion so net charge is zero — other pKa's don't change the charge near that pH.

Why does an acidic side chain lower the pI while a basic side chain raises it?

An acidic (extra –COOH) group means you must remove two low-pKa protons to reach neutral, so the balance point sits at low pH; a basic group means the flanking pKa's are both high, pushing the balance point to high pH.

Why is a zwitterion much more soluble in water than in benzene?

Water is polar and stabilises the separated + and – charges via ion–dipole attraction; non-polar benzene cannot, so the internal salt stays in the aqueous phase. This links to solubility being lowest at pI.

Why does solubility of an amino acid reach a minimum at the pI?

At pI the molecule is net-neutral (zwitterion), so molecules attract each other electrostatically and aggregate; away from pI a net charge makes molecules repel and stay dissolved.

Edge cases

What is the "net charge" of glycine at exactly pH = pI = 5.97?

Zero on average — the population is dominated by zwitterion, with equal tiny amounts of cation and anion cancelling out.

For glycine (R = H), is there any side-chain pKa to worry about?

No — R = H is not ionisable, so glycine has only two pKa's and pI = (pKa1 + pKa2)/2, the simplest case.

At pH far above pKa2 (say pH 13), what species dominates and what is its charge?

The anion H₂N–CHR–COO⁻ with net charge –1; both the amine is deprotonated and the carboxyl is already deprotonated.

At pH far below pKa1 (say pH 1), what species dominates?

The fully protonated cation H₃N⁺–CHR–COOH with net charge +1; both groups have grabbed protons.

If pH is nudged just above the pI, which way does the tiny net charge tip and where does it migrate?

Slightly negative (a hair more anion than cation), so it drifts slowly toward the anode (+) — the sign of migration flips exactly at the pI.

Can a molecule ever have a +2 charge, and if so when?

Yes — a basic amino acid like lysine at very low pH protonates both amino groups and the carboxyl-derived state, giving net +2 when all three ionisable groups are in their protonated form.

Does the peptide bond in a protein still contribute these terminal charges?

Only the free ends (N-terminus –NH₃⁺, C-terminus –COO⁻) and ionisable side chains carry charge; the internal α-groups are locked into the peptide bond and no longer ionise.

Recall One-line self-test before you close the page

Ladder direction as pH rises ::: cation +1 → zwitterion 0 → anion –1. pI formula (simple acid) ::: $pI = \frac{1}{2} (p K_{a 1} + p K_{a 2})$ , averaging only the two pKa's hugging the neutral form. Below pI the molecule is ::: positive (cation) → migrates to cathode.