CIE Nov 2013 v3 Paper 4 Q6

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Examiners For Use Naturally-occurring -amino acids, RCH(NH2)CO2H, can be classi ed as amphiprotic substances. An amphiprotic substance is one which can act as both a Brnsted-Lowry acid and base. 10 -amino acid R group alanine CH3 aspartic acid HO2CCH2 glycine lysine threonine serine H H2N(CH2)4 CH3CH(OH) HOCH2 (a) What is the Brnsted-Lowry de nition of an acid?[1] (b) (i) All -amino acids are soluble in water since they can form hydrogen bonds with water molecules and can also exist as zwitterions. Draw diagrams to show how the carboxylic acid and amino groups of alanine can form hydrogen bonds with water molecules. carboxylic acid amino group (ii) Draw the structure of the zwitterionic form of glycine. [5]UCLES 2013 9701/43/O/N/13<br /> Examiners For Use 11 (c) The amino acid alanine can be formed by the reaction of CH3CHCl CO2H with an excess of ammonia. Outline a mechanism for this reaction using curly arrows. (d) Amino acids can form different ions at different pH values. Suggest the structures of the ions formed from the -amino acids below at the respective pH value. lysine at pH 1 aspartic acid at pH 14 [3] [2] (e) (i) How many different dipeptides is it possible to synthesise, each containing two of the three amino acids alanine, serine and lysine?(ii) Write the structural formula of one of these dipeptides incorporating serine and alanine.UCLES 2013 9701/43/O/N/13 [3] [Turn over<br /> Examiners For Use 12 (f) Most naturally-occurring amino acids have a chiral centre and exhibit stereoisomerism. (i) De ne the term stereoisomerism.There are four optical isomers of threonine. Some of these optical isomers are drawn below. H2N HO2C HO2C H2N CH3 OH CH3 OH HO2C H2N H2N HO2C CH3 OH OH CH3 When answering this question, remember that completely free rotation about a CC single bond occurs in these compounds. (ii) Which of the structures G, H or J is identical to structure F?(iii) The other two of the structures G, H or J represent two of the three other possible optical isomers of threonine. Complete the following partial structure of the fourth optical isomer. CH3 [3] [Total: 17]UCLES 2013 9701/43/O/N/13<br /> 13 BLANK PAGE PLEASE TURN OVER FOR SECTION BUCLES 2013 9701/43/O/N/13 [Turn over<br />

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