Every amino acid carries an acidic COOH and a basic NH2 on the same carbon. That makes zwitterions, high melting points, charge that changes with pH, peptide bonds and the separation technique of electrophoresis.
An α-amino acid has the general formula RCH(NH2)COOH. In the solid and in neutral solution the COOH group donates H+ to the NH2 group, forming a zwitterion, H3N+CH(R)COO−: the strong ionic attractions between zwitterions give amino acids high melting points and solubility in water. The pH at which the amino acid has no overall charge is its isoelectric point. In acid the COO− is protonated, giving the positive ion H3N+CH(R)COOH; in alkali the NH3+ loses H+, giving the negative ion H2NCH(R)COO−. Amino acids join by condensation, losing water, to form peptide (amide) bonds, –CONH–; peptides are hydrolysed back to amino acids by heating with acid or alkali. In electrophoresis, ions move in an electric field at a set pH: the direction depends on the sign of the charge, and the speed on the size of the charge and of the ion.
Slide the pH in the simulator and watch the amino acid change form. It contains both a proton donor and a proton acceptor, so the form it takes depends on the pH around it.
| Conditions | Form of glycine | Overall charge |
|---|---|---|
| strong acid (low pH) | H3N+CH2COOH | +1 |
| isoelectric point (about pH 6.0) | H3N+CH2COO− | 0 (zwitterion) |
| strong alkali (high pH) | H2NCH2COO− | −1 |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What is a zwitterion?
Which form of glycine is present in strongly acidic solution?
What type of reaction joins two amino acids into a dipeptide?
Why do amino acids have high melting points?
Two amino acids join head to tail. Because either can supply the COOH group, two different amino acids give two different dipeptides.
| Process | Example | Key point |
|---|---|---|
| condensation | H2NCH2COOH + H2NCH(CH3)COOH → H2NCH2CONHCH(CH3)COOH + H2O | peptide bond –CONH– forms, water is lost |
| two dipeptides | gly-ala and ala-gly | different structures, both Mr 75.0 + 89.0 − 18.0 = 146.0 |
| hydrolysis | heat with 6 mol dm−3 HCl(aq) or NaOH(aq) | one H2O per peptide bond broken |
In acid, an amino acid is positive (NH3+, COOH) and moves to the cathode; in alkali it is negative (NH2, COO−) and moves to the anode. The peptide bond is an amide link made by condensation, not addition. Two different amino acids give two dipeptides, not one, because the order matters.
Unlocks once the four checks above are done. Worth more XP, written to A Level Paper 4 standard.
How many different dipeptides can be made from one molecule of glycine and one molecule of alanine?
A mixture of lysine, glycine and aspartic acid undergoes electrophoresis at pH 6.0. Which statement is correct?
What is the Mr of the dipeptide formed from glycine (Mr 75.0) and alanine (Mr 89.0)?
Two dipeptides have the same charge at the pH used for electrophoresis. Which one moves further?
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