PJC H2 Chem 2013 Prelim P3 Soln

© PJC 2013 9647/03/Prelim/13 [Turn over PIONEER JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATION HIGHER 2 CHEMISTRY 9647/03 Paper 3 Free-Response 23 September 2013 2 hours Candidates answer on separate paper. Additional Materials: Writing Papers Data Booklet Cover Page READ THESE INSTRUCTIONS FIRST Write your Centre number, index number and name on all work you hand in. Write in dark blue or black pen on both sides of the paper. You may use a soft pencil for any diagrams, graphs or rough workings. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer any four questions. A Data Booklet is provided. You are reminded of the need for good English and clear presentation in your answers. The number of marks is given in brackets [ ] at the end of each question or part question. At the end of the examination, fasten all your work securely together. This document consists of 13 printed pages. Name: Index No.: CT Group: 12 1012

2 © PJC 2013 9647/03/Prelim/13 [Turn over Answer any four questions. 1 (a) With the aid of a diagram, describe how a polypeptide chain is held in the shape of an alpha helix. [3] The helix has a regular coiled spiral polypeptide chain held in place by hydrogen bonds between peptide C=O group of one amino acid residue and peptide N–H group of another amino acid residue in the covalently bonded sequence. There are 3.6 amino acids per helical turn. The –R groups (side chains) point outside of the helix or label R group in diagram. (b) Haemoglobin is the oxygen-carrying protein in red blood cells. (i) With reference to the haemoglobin molecule, describe and explain what is meant by the term quaternary structure of proteins. In your answer, you should state the type of bonding or interaction involved. Quaternary structure refe rs to the specific or ientation or spatial arrangement of polypeptide chains wit h respect to one another and the nature of interactions that stabilise s this orientation. Haemoglobin consists of four separate polypeptide chains of two types: two α chains and two β chains. The haemoglobin molecule is nearly spherical with the four polypeptide chains packed closely together. Each polypeptide carries a haem group to form one of the four subunits in a molecule of haemoglobin. Each subunit folds and coils into a compact globular shape and joined together by van der Waals’ forces, hydrogen bonds and ionic linkages. 1013

3 © PJC 2013 9647/03/Prelim/13 [Turn over (ii) A typical polypeptide chain of haemoglobin consists of 141 amino acids. The R groups of 3 of the amino acids are given below. amino acid formula of side chain (R in RCH(NH2)CO2H) glutamic acid CH2CH2CO2H valine CH(CH3)2 lysine CH2CH2CH2 CH2NH2 Use all three amino acids above to construct the disp