Proline (Pro) is a naturally occurring amino acid. [Figure 7.1]

1. (a)(i)Proline is often found bonded to glycine (Gly) in a protein. Draw the dipeptide Pro-Gly. The peptide bond must be shown fully displayed.[2]
2. (a)(ii)Name the type of reaction that forms a dipeptide from two amino acids.[1]
3. (a)(iii)Proline is able to form a poly(proline) peptide chain. A section of a poly(proline) chain is shown. [Figure 7.2] Suggest why the secondary structure of poly(proline) cannot be stabilised by hydrogen bonding.[1]
4. (b)(i)The reaction scheme shows several reactions of proline. [Figure 7.3] Write an equation for the reaction of proline with NaOH(aq) in reaction 1. C₄H₄NHCO₂H + ..........................[1]
5. (b)(ii)Proline has a secondary amine functional group. Secondary amines react with acyl chlorides. For example, dimethylamine reacts with RCOCl according to the following equation. [Figure 7.4] Suggest the skeletal structure of R, C₇H₁₁NO₃, the product of reaction 2.[1]
6. (b)(iii)Suggest the reagent required for reaction 3.[1]
7. (c)(i)Proline was first synthesised in the laboratory using a multi-stage synthetic route. In stage 1, CH₂(CO₂C₂H₅)₂ and CH₂=CHCN react to form a single product U. [Figure 7.5] Name all the functional groups present in the reactants of stage 1. CH₂(CO₂C₂H₅)₂ CH₂=CHCN[2]
8. (c)(ii)Suggest the type of reaction that occurs in stage 1.[1]
9. (c)(iii)In stage 2, U reacts with reagent V to form W. [Figure 7.6] Suggest a suitable reagent V.[1]
10. (c)(iv)Stage 3 takes place in the presence of an acid catalyst. X and Y are the only products of the reaction. [Figure 7.7] Suggest the type of reaction that occurs in stage 3.[1]
11. (c)(v)Deduce the identity of Y.[1]
12. (c)(vi)After several further stages, Z is produced. [Figure 7.8] In the final stage of the synthesis, Z reacts via a nucleophilic substitution mechanism to form proline. Complete the diagram to describe the reaction mechanism of the final stage. Draw curly arrows, ions and charges, partial charges and lone pairs of electrons, as appropriate. Draw the structure of any organic intermediate ion. [Figure 7.9][3]
13. (c)(vii)Identify with an asterisk (*) the chiral centre in proline. [Figure 7.10][1]
14. (d)Part of the structure of gelatin is shown. [Figure 7.11] Identify the number of amino acid units in the structure shown.[1]
15. (e)(i)At pH 6.5, proline exists in aqueous solution as a zwitterion. Draw the structure of the zwitterion of proline. Explain how the zwitterion of proline forms.[2]
16. (e)(ii)The isoelectric point of an amino acid is the pH at which it exists as a zwitterion. Three of the amino acids in gelatin are proline, alanine and glutamic acid. Their isoelectric points are shown. [Figure 7.12] A mixture of these amino acids was analysed by electrophoresis using a buffer solution at pH 4.0. Draw and label three spots on the diagram of the electropherogram to indicate the likely position of each of these three species after electrophoresis. Explain your answer. [Figure 7.13][4]
17. (f)The weak acid ACES is a compound that can be used to make a buffer solution for electrophoresis experiments. [Figure 7.14] The anion of the sodium salt of ACES, C₄H₉N₂O₄SNa, is a strong base. A buffer solution is prepared by the following steps. • 3.50 g of C₄H₉N₂O₄SNa is dissolved in 100 cm³ of distilled water. • 50.0 cm³ of 0.200 mol dm⁻³ dilute hydrochloric acid is added to the solution. • The resulting mixture is transferred to a 250.0 cm³ volumetric flask, and the solution made up to the mark. C₄H₉N₂O₄SNa reacts with HCl with a 1:1 stoichiometry. The pKa of ACES is 6.88 at 298 K. Calculate the pH of the buffer solution formed at 298 K. [Mr: C₄H₉N₂O₄SNa, 204.1] pH = ..........................[4]

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