A Level Chemistry 9701: Amino Acid Zwitterions, Peptide Bonds and Electrophoresis

An amino acid, 2-aminobutanoic acid, has the structural formula $\text{CH}_3\text{CH}_2\text{CH(NH}_2\text{)COOH}$ .

(a) Give the structural formula of the zwitterion of 2-aminobutanoic acid, and state the type of intramolecular process that produces it from the structure shown above. [2]

(b) 2-Aminobutanoic acid reacts with glycine, $\text{H}_2\text{NCH}_2\text{COOH}$ , so that the carboxylic acid group of 2-aminobutanoic acid condenses with the amine group of glycine. Give the structural formula of the dipeptide formed, name the type of bond created, and give the formula of the small molecule lost. [3]

(c) A sample of this dipeptide is placed in a buffer solution at a pH well above its isoelectric point and subjected to electrophoresis. State, with a reason, the overall charge on the dipeptide under these conditions and the electrode towards which it will migrate. [2]

Part (a): The zwitterion of 2-aminobutanoic acid

2-Aminobutanoic acid, $\text{CH}_3\text{CH}_2\text{CH(NH}_2\text{)COOH}$ , contains both a basic $-\text{NH}_2$ group and an acidic $-\text{COOH}$ group in the same molecule. In aqueous solution, an intramolecular proton transfer (an internal acid-base reaction) occurs: the acidic $-\text{COOH}$ group donates its proton directly to the basic $-\text{NH}_2$ group of the same molecule. This converts the neutral structure into a zwitterion, a species that carries both a positive and a negative charge but has zero overall charge:

$\text{CH}_3\text{CH}_2\text{CH(NH}_3^+\text{)COO}^-$

Part (b): Forming a dipeptide with glycine

Glycine has the structural formula $\text{H}_2\text{NCH}_2\text{COOH}$ . In the condensation reaction described, the $-\text{COOH}$ group of 2-aminobutanoic acid reacts with the $-\text{NH}_2$ group of glycine: the $-\text{OH}$ of the acid and one $-\text{H}$ of the amine are lost together as a molecule of water, and a new $-\text{CONH}-$ linkage, a peptide (amide) bond, forms between the two amino acid residues.

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

The dipeptide formed is $\text{CH}_3\text{CH}_2\text{CH(NH}_2\text{)CONHCH}_2\text{COOH}$ : it still has a free $-\text{NH}_2$ group at one end (from the unreacted amine of 2-aminobutanoic acid) and a free $-\text{COOH}$ group at the other end (from the unreacted acid group of glycine), joined in the middle by the new peptide bond.

Part (c): Predicting electrophoresis behaviour above the isoelectric point

At its isoelectric point, a peptide exists predominantly as its zwitterion, with no overall charge, so it does not migrate in an electric field. Above the isoelectric point (i.e. in a more alkaline solution than the isoelectric pH), there is a higher concentration of $\text{OH}^-$ ions available to remove protons: the $-\text{NH}_3^+$ group(s) are increasingly deprotonated back to neutral $-\text{NH}_2$ , while the $-\text{COOH}$ group(s) remain deprotonated as $-\text{COO}^-$ . The dipeptide therefore carries an overall negative charge under these conditions.

In electrophoresis, negatively charged species are attracted to, and migrate towards, the positive electrode, the anode.

Final answers

(a) Zwitterion: $\text{CH}_3\text{CH}_2\text{CH(NH}_3^+\text{)COO}^-$ , formed by an intramolecular proton transfer between the amino acid’s own $-\text{COOH}$ and $-\text{NH}_2$ groups.
(b) Dipeptide: $\text{CH}_3\text{CH}_2\text{CH(NH}_2\text{)CONHCH}_2\text{COOH}$ ; a peptide (amide) bond forms, with a molecule of $\text{H}_2\text{O}$ lost.
(c) Overall negative charge; the dipeptide migrates towards the anode.