13012 13 α-amino acids are compounds which have a carboxylic acid group and an amino group bonded to the same carbon atom. (a) Draw the structure of the amino acids, glycine and alanine. glycine alanine [2] (b) The Strecker synthesis involves the reaction of aldehydes or ketones with ammonia and hydrogen cyanide to form an α-aminonitrile which is hydrolysed to form an amino acid. C O R1 R2 C NH2 CN R1 R2 C NH2 COOH R1 R2 NH3 HCN hydrolysis α-aminonitrile amino acid (i) State the IUPAC name of the amino acid which is formed via the Strecker synthesis from propanone. [2] 13012 [Turn over (ii) Draw the structure and state the IUPAC name of the aldehyde or ketone needed to form the amino acid valine, (CH3)2CHCH(NH2)COOH, via the Strecker synthesis. [2] (iii) Write an equation for the reaction of ethanal with ammonia and hydrogen cyanide to form the α-aminonitrile. [1] (iv) Write an equation for the hydrolysis of the α-aminonitrile formed in (b)(iii) above using hydrochloric acid. [2] 13012 (c) The structures of the amino acids lysine and valine are shown below. C COOH (CH )4 H NH2 H2N C COOH CH3 H H2N C H CH3 lysine valine (i) Draw the structure of lysine in solution at pH 1. [1] (ii) Draw the structures of two possible dipeptides formed between one molecule of lysine and one molecule of valine. Circle the peptide link in one of the structures you have drawn. [3] 13012 [Turn over (d) Chains of amino acids, bonded by peptide links, form the primary structure of proteins. Describe the secondary structure of proteins. [3] (e) Two-way paper chromatography can be used to separate and identify lysine and valine from a mixture of amino acids. (i) Describe how two-way paper chromatography is used to separate lysine and valine from the mixture of amino acids. [5] 13012 (ii) Describe how lysine and valine may be located and identified from the chromatogram. [4] (iii) Explain why two-way paper chromatography is a more effective method of separating amino acids than one-way paper chromatography. [1] 13012 BLANK PAGE DO NOT WRITE ON THIS PAGE (Questions continue overleaf) [Turn over