ACJC_H2_CHEM_P3_Questions Prelim

2 © ACJC 2015 9647/03/Prelim/15 [Turn over Answer any four questions. 1 (a) Bradykinin is a nonapeptide ( polypeptide with nine residues) which is a pain - causing agent. Complete hydrolysis of this polypeptide yields the following amino acids. Amino acid Pro Ser Arg Gly Phe No. of residues 3 1 2 1 2 Partial acid hydrolysis of bradykinin gave the following di- and tripeptides. Phe-Ser; Pro-Gly-Phe; Pro-Pro; Ser-Pro-Phe; Phe-Arg; Arg-Pro Given that both the terminal residues in the nonapeptide are arginine (Arg), deduce the primary structure of bradykinin. [2] (b) Frederick Sanger used 2,4-dinitrofluorobenzene (2,4-DNFB) in a basic solution to bind to the free amino group of the N -terminal of a polypeptide thereby labelling the N-terminal residue leading to its identification after hydrolysis. The reaction is shown below: A nucleophilic aromatic substitution occurs between 2,4-DNFB and the N-terminal residue in the first step of the process. (i) Suggest why a basic solution is used in reaction step 1, involving 2,4-DNFB and the peptide. (ii) Generally, halobenzenes do not undergo nucleophilic aromatic substitution. However, in the above reaction, 2,4 -DNFB seemingly undergoes nucleophilic aro matic substitution. Explain why this is possible , in this instance. [3]

3 © ACJC 2015 9647/03/Prelim/15 [Turn over 1 (c) Emil Fischer used the following reaction scheme to convert serine into alanine. (i) Draw the structural formula for A and state the type (s) of reactions that have taken place. (ii) Draw the structure of B and use it to explain why the first step is necessary for the overall conversion of serine into alanine. . (iii) Identify C and hence state what type of reaction has taken place in the conversion of C to alanine. (iv) Would you expect the absolute configuration about the chiral centre to have changed in the conversion of serine to alanine in the above scheme? Explain. [8]

4 © ACJC 2015 9647/03/Prelim/15 [Turn over 1 (d) A small polypeptide, oxytocin, when treated with certain reducing agents ( eg. Na/NH3) brings about a single chemical change that can be reversed by air oxidation. Based on the structure of oxytocin shown below, describe and explain what chemical changes were involved. [2] (e) The guanidine group (in brackets) of arginine, shown below, is one of the most strongly basic of all organic groups, with a pK a of 12.5. Explain why this is so, using appropriate illustrations. [2] (f) There are three pKa values associated with arginine: 2.2, 9.0 and 12.5. Derive the pI value of t