OCR A Jun 2016 Paper 4 Q3

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The building blocks of peptides and proteins are -amino acids. A tripeptide is hydrolysed to form a mixture of three different -amino acids. (a) The first step of an incomplete mechanism for the alkaline hydrolysis of the tripeptide is shown below. Add curly arrows and relevant dipoles to the diagram to suggest how the hydroxide ion takes part in the first step of this mechanism. O OH [2] (b) The tripeptide is hydrolysed and the resulting mixture containing the three amino acids is neutralised. A student tries to separate and identify the three amino acids in the mixture using thin-layer chromatography (TLC). The diagram below shows the apparatus for the experiment and the chromatogram produced. watch glass TLC plate pencil line solvent front 0 0 pencil line solvent (cid:38)(cid:75)(cid:85)(cid:82)(cid:80)(cid:68)(cid:87)(cid:82)(cid:74)(cid:85)(cid:68)(cid:80) Explain how the chromatogram can be used to identify amino acids. The student thinks that there should be three spots on the chromatogram. Suggest why there are only two spots.[3]OCR 2016<br />
 (c) The three -amino acids in the tripeptide are aspartic acid, glycine and isoleucine. The general formula for an -amino acid is RCH(NH2)COOH. -amino acid aspartic acid glycine isoleucine R-group CH2COOH H CH(CH3)CH2CH3 (i) Aspartic acid has an isoelectric point of 2.77. What is meant by the term isoelectric point ? In your answer you should use the appropriate technical terms spelled correctly.[1] (ii) Draw the structure of aspartic acid when it is dissolved in a solution with a high pH. (iii) Suggest a structure for the tripeptide. On your structure, mark each chiral centre with an asterisk (*). [1]OCR 2016 [2] [Total: 9] Turn over<br />

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