Nitrogen Compounds: Question 10

Worked solution

Part (a): Acid hydrolysis of the peptide bond

Heating a peptide under reflux with excess concentrated hydrochloric acid is acid hydrolysis: a molecule of water adds across the $-\text{CONH}-$ peptide bond, breaking it and regenerating the two original amino acid residues:

$\text{H}_2\text{NCH}_2\text{CONHCH(CH}_3\text{)COOH} + \text{H}_2\text{O} \rightarrow \text{H}_2\text{NCH}_2\text{COOH} + \text{H}_2\text{NCH(CH}_3\text{)COOH}$

giving glycine and alanine. However, the solution also contains a large excess of concentrated hydrochloric acid, so both amino groups are protonated in the strongly acidic conditions, and the carboxylic acid groups remain undissociated as $-\text{COOH}$ (since the excess $\text{H}^+$ suppresses their own ionisation). The two products, as they actually exist in this solution, are the glycine cation, $\text{H}_3\text{N}^+\text{CH}_2\text{COOH}$, and the alanine cation, $\text{CH}_3\text{CH(NH}_3^+\text{)COOH}$ (each accompanied by a chloride counter-ion).

Part (b): The isoelectric point and behaviour during electrophoresis

The isoelectric point of an amino acid is the pH at which it exists predominantly as its zwitterion, the species in which the $-\text{NH}_2$ group is protonated to $-\text{NH}_3^+$ and the $-\text{COOH}$ group is deprotonated to $-\text{COO}^-$, so that the molecule carries no overall (net) charge, since the single positive charge exactly cancels the single negative charge.

If glycine is held in a buffer solution at exactly its isoelectric point, it exists almost entirely as this zwitterion, $\text{H}_3\text{N}^+\text{CH}_2\text{COO}^-$. Because the molecule has no net charge, it experiences no net electrostatic force from the applied electric field during electrophoresis, the pull on its positive end is exactly balanced by the pull on its negative end, so it does not migrate towards either the anode or the cathode, and stays at its starting position.

Final answers

• (a) Acid hydrolysis; products (in this strongly acidic solution) are the glycine cation, $\text{H}_3\text{N}^+\text{CH}_2\text{COOH}$, and the alanine cation, $\text{CH}_3\text{CH(NH}_3^+\text{)COOH}$.
• (b) The isoelectric point is the pH at which an amino acid exists predominantly as its (uncharged) zwitterion; at this pH glycine carries no net charge, so it feels no net force towards either electrode and does not migrate.