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Date November 2019 Marks available 2 Reference code 19N.3.sl.TZ0.7
Level SL Paper 3 Time zone TZ0
Command term Draw Question number 7 Adapted from N/A

Question

Aspartame is formed from the two amino acids aspartic acid (Asp) and phenylalanine (Phe).

Draw the structure of the dipeptide Asp–Phe using section 33 of the data booklet.

[2]
a.

The isoelectric point of amino acids is the intermediate pH at which an amino acid is electrically neutral.

Suggest why Asp and Phe have different isoelectric points.

[2]
b.

Markscheme

amide link (eg, CONH) ✔

correct order and structures of amino acids ✔

NOTE: Accept a skeletal formula or a full or condensed structural formula.
Accept zwitterion form of dipeptide.
Accept CO–NH but not CO–HN for amide link.

a.

«Asp isoelectric point lower than Phe and » Phe has a neutral/hydrocarbon side chain ✔
Asp side chain contains −COOH/carboxyl ✔

NOTE: Award [1 max] for “Asp has two carboxyl/−COOH groups and Phe has one carboxyl/−COOH group”.
Accept “Asp has an acidic side chain” for M2

b.

Examiners report

[N/A]
a.
[N/A]
b.

Syllabus sections

Core » Topic 10: Organic chemistry » 10.1 Fundamentals of organic chemistry
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Options » B: Biochemistry » B.2 Proteins and enzymes
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Core » Topic 10: Organic chemistry
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