OCR A Jan 2010 Paper 4 Q3

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3 -Amino acids are found in human sweat. A student had read that chromatography could be used to separate and identify the amino acids present in human sweat. (a) The student used Thin-Layer Chromatography (TLC) to separate the -amino acids in a sample of human sweat and discovered that three different -amino acids were present. (i) Name the process by which TLC separates -amino acids.[1] (ii) The chromatogram was treated to show the positions of the separated -amino acids. Explain how the student could analyse the chromatogram to identify the three -amino acids that were present.[2] (iii) Several -amino acids have structures that are very similar. Suggest why this could cause problems when using TLC to analyse mixtures of -amino acids.[1] (b) Some of the -amino acids found in human sweat are shown in the table below. -amino acid glycine leucine isoleucine alanine valine lysine glutamic acid R group CH2CH(CH3)2 CH(CH3)CH2CH3 CH3 CH(CH3)2 (CH2)4NH2 (CH2)2COOH Table 1OCR 2010<br />
 (i) State the general formula of an -amino acid. (ii) There are four stereoisomers of isoleucine. One of the stereoisomers is shown below. HOOC H3C NH2 CH2CH3 Draw 3D diagrams for the other three stereoisomers of isoleucine. [1] [3]OCR 2010 Turn over<br />
 -amino acid glycine leucine isoleucine alanine valine lysine glutamic acid Table 1 R group CH2CH(CH3)2 CH(CH3)CH2CH3 CH3 CH(CH3)2 (CH2)4NH2 (CH2)2COOH (c) -Amino acids form different ions at different pH values. Zwitterions are formed when the pH is equal to the isoelectric point of the -amino acid. The isoelectric points of three -amino acids are given below: alanine, pH = 6.0 glutamic acid, pH = 3.2 lysine, pH = 9.7 Draw the structures of the ions formed by these -amino acids at the pH values below. Refer to Table 1 above. alanine at pH = 6.0 glutamic acid at pH = 10 lysine at pH = 2.0 [3]OCR 2010<br />
 (d) -Amino acids can react to form polypeptides. A short section of a polypeptide is shown below. Name the -amino acid sequence in this section of the polypeptide. Refer to Table 1.[1] (e) Synthetic polyamides, such as nylon, contain the same link as polypeptides. Nylon is the general name for a family of polyamides. A short section of a nylon polymer is shown below. (CH2)6 (CH2)8 (CH2)6 (CH2)8 Draw the structures of two monomers that could be used to make this nylon. [2] [Total: 14]OCR 2010 Turn over<br />

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