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Amino acids melt at high temperatures and dissolve in water like salts, not like the amines or carboxylic acids they resemble on paper. The reason is the zwitterion. This post explains it, sets out NCERT's classification, and shows how to calculate an isoelectric point and use it to predict charge.
Where does this sit in the syllabus?
Section 10.2 of the rationalised NCERT Class 12 chapter on Biomolecules (Reprint 2026-27) covers amino acids, their classification and the zwitterion. The JEE (Main) 2026 syllabus asks for an "elementary idea of α-amino acids". The isoelectric point is not in the NCERT text; it is a standard next step in university chemistry and biochemistry, included here because it makes the zwitterion idea quantitative. Check the syllabus for your exam year.
What is a zwitterion?
NCERT explains that in aqueous solution the carboxyl group can lose a proton and the amino group can accept one, giving a dipolar ion called a zwitter ion. It is neutral overall but carries both a positive and a negative charge:
H₂N–CHR–COOH → H₃N⁺–CHR–COO⁻
OpenStax Organic Chemistry adds that zwitterions are internal salts, so amino acids have large dipole moments, dissolve in water but not in hydrocarbons, and are crystalline with relatively high melting points. NCERT adds that the zwitterion makes amino acids amphoteric: they react with both acids and bases.
How does NCERT classify amino acids?
Two ways:
Acidic, basic or neutral, by counting groups. Equal numbers of amino and carboxyl groups make it neutral (glycine, alanine). More carboxyl groups make it acidic (aspartic acid, glutamic acid). More amino groups make it basic (lysine, arginine).
Essential or non-essential. Non-essential amino acids can be made in the body; essential ones must come from the diet. NCERT marks the essential ones with an asterisk in Table 10.2.
NCERT also notes that every naturally occurring α-amino acid except glycine is optically active, and most have the L-configuration.
What is the isoelectric point?
OpenStax defines the isoelectric point, pI, as the pH at which the amino acid is exactly balanced between its cationic and anionic forms and exists mainly as the neutral zwitterion. The rules for calculating it:
Neutral amino acid: pI is the average of pKa₁ and pKa₂.
Acidic amino acid: the average of the two lowest pKa values.
Basic amino acid: the average of the two highest pKa values.
Worked example: three isoelectric points
pKa values are from OpenStax's Table 26.1.
Glycine (pKa 2.34 and 9.60). pI = (2.34 + 9.60)/2 = 11.94/2 = 5.97.
Aspartic acid (pKa 1.88, side chain 3.65, amino 9.60). Acidic, so average the two lowest: (1.88 + 3.65)/2 = 5.53/2 ≈ 2.77.
Lysine (pKa 2.18, amino 8.95, side chain 10.53). Basic, so average the two highest: (8.95 + 10.53)/2 = 19.48/2 = 9.74.
All three match the pI values OpenStax lists.
What charge does an amino acid carry at a given pH?
Compare the pH with the pI. Below the pI the molecule carries net positive charge; above it, net negative. In electrophoresis, OpenStax explains, negatively charged molecules move towards the positive electrode, positively charged ones towards the negative electrode, and a molecule at its pI does not move.
At pH 6.0, then:
Aspartic acid (pI 2.77) is negative and moves to the positive electrode.
Lysine (pI 9.74) is positive and moves to the negative electrode.
Glycine (pI 5.97) is almost neutral and barely moves.
The Henderson-Hasselbalch equation makes this exact. OpenStax works alanine at pH 9.00 with pKa₂ = 9.69: log([A⁻]/[HA]) = 9.00 − 9.69 = −0.69, so [A⁻]/[HA] ≈ 0.20, meaning about 17% is in the negatively charged form and 83% is zwitterion. Our post on buffers and the Henderson equation covers the equation itself.
Which mistakes cost marks?
Drawing an amino acid in water as H₂N–CHR–COOH. In solution, draw the zwitterion.
Averaging all three pKa values for an acidic or basic amino acid.
Reversing the direction of migration. Below pI means positive, which moves towards the negative electrode.
Calling glycine optically active. Its α-carbon carries two hydrogens.
A practice routine
Day 1: Draw glycine at pH 1, pH 6 and pH 12, with the correct charges.
Day 2: Calculate pI for glutamic acid, arginine and histidine from a pKa table.
Day 3: Predict migration for five amino acids at pH 4 and at pH 8.
Day 4: Link this to peptide bonds in our post on levels of protein structure.
If the acid-base reasoning feels shaky, Fermi's AI tutor works out what you already know and builds a plan for the session, so you start with the step you are missing.
Tonight, calculate the pI of alanine from pKa values of 2.34 and 9.69, and check that you get 6.01 or 6.02, depending on rounding.
