Draw the structure of a 2-amino acid.

(i)     Using Table 19 of the Data Booklet, draw the structure of the two dipeptides formed by the reaction of glycine with valine.

(ii)     State the other product of the reaction in (i).

Explain how a given protein can be broken down into its constituent amino acids and how these can be identified by electrophoresis.

 ;

Allow condensed structural formula (eg, R-CH(NH₂)COOH) or structure of a specific 2-amino acid (eg, Ala etc.).

(i)     

Accept condensed structural formulas.

Accept C₃H₇ for CH₃CHCH₃.

(ii)     \({{\text{H}}_{\text{2}}}{\text{O}}\)/water;

add hydrochloric acid/HCl to separate individual amino acids;

Accept strong acid/concentrated H⁺ or restriction enzymes but not sulfuric acid/H₂SO₄.

Award [4 max] for any four of:

mixture/amino acids spotted/placed on gel/PAGE/polyacrylamide/paper;

gel placed in buffer/solution of known pH;

voltage/potential difference applied;

Accept “applied electric field / positive and negative electrodes connected / anode and cathode connected” but not current.

amino acids move differently depending on size and charge/isoelectric point/pH of buffer / amino acids move to oppositely charged electrodes;

Accept any suitable diagram.

develop with ninhydrin/(organic) dye / detect by UV (light)/staining/fluorescence;

Accept any suitable development method.

measure distances moved / compare with known samples / measure isoelectric points;

In (a) most candidates drew either the general formula of a 2-amino acid or drew the structure of a specific 2-amino acid.

In (b) (i) the structures of the two dipeptides formed by the reaction of glycine with valine were usually correctly represented and water was identified by almost all.

In (c) electrophoresis was well understood as a biochemical analytical technique. However some candidates did not read the question carefully and dropped the first mark for not stating that hydrochloric acid needed to be added to separate the individual amino acids.