Using Table 19 of the Data Booklet, deduce the structural formulas of the two dipeptides formed by the reaction of leucine (Leu) with valine (Val).

Dipeptide 1:

Dipeptide 2:

State the other substance formed during this reaction.

Explain how amino acids can be analysed using electrophoresis.

List two functions of proteins in the body.

Accept full or condensed structural formulas.

Penalize incorrect representation of peptide link (COHN or NHOC) once.

Award [1] for a correct peptide link if the rest of the structure is incorrect.

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

sample of amino acids/mixture placed/spotted on gel/polyacrylamide/PAGE/paper and buffer solution/solution of known pH;

potential difference/voltage applied;

Do not accept current.

Allow potential for potential difference.

Allow electric field applied.

if the (amino acid’s) isoelectric point is below the pH (of buffer) it is negatively charged / if the (amino acid’s) isoelectric point is above the pH (of buffer) it is positively charged;

different amino acids move different distances/rates according to their charge/isoelectric point / different amino acids move at different rates towards oppositely charged electrodes / OWTTE;

spray/develop with ninhydrin/organic dye / detected by staining/fluorescence under UV light;

measure distance travelled and compare with standards/isoelectric points;

Any two for [1]

structural / growth / repair

Allow more specific function eg, forms tendons/muscles/eye lens/ nails/hair, repair of tissue/cells etc.

enzyme / biological catalyst

hormone / chemical messenger

transport of molecules

Allow movement/carriage of molecules, and chemicals instead of molecules / OWTTE.

Do not award mark for transport alone.

storage of molecules

Do not award mark for storage alone.

Allow chemicals instead of molecules / OWTTE.

lubrication

(to make/produce) immunoproteins/antibodies

energy source

Do not accept energy storage.

Allow more specific examples of any of the above.

Part (a) was generally well answered. There were instances of careless mistakes where the side-chain of the amino acid was incorrectly copied or connected, and some instances where peptide links were incorrectly represented as COHN resulting in the loss of one mark. However, there were many cases where candidates had totally incorrect links between the amino acids in the dipeptide, and some scripts did not even attempt to connect the two amino acids.

Part (a) was generally well answered. There were instances of careless mistakes where the side-chain of the amino acid was incorrectly copied or connected, and some instances where peptide links were incorrectly represented as COHN resulting in the loss of one mark. However, there were many cases where candidates had totally incorrect links between the amino acids in the dipeptide, and some scripts did not even attempt to connect the two amino acids.

In part (b), all candidates were reasonably familiar with describing electrophoresis and some of them tried to explain how electrophoresis separated amino acids, but most candidates did not show a thorough understanding of the technique and only scored partial marks out of the 4 marks allocated. Most candidates did not clarify that amino acids moved at different rates through the gel depending on their charge (at the buffer’s pH), or the relation between isoelectric point, buffer pH and charge on amino acid.

The majority of candidates stated two correct functions of proteins in part (c). Energy storage was not accepted, and transport needed mention of molecules.