An amino acid is a compound that has both an amine group and a carboxyl group. The structures of three amino acids, P, Q and R, are shown in Figure 9.1.

P	Q	R
NH $_{2}$ CH(CH $_{3}$ )COOH	NH $_{2}$ C(CH $_{3}$ ) $=$ CHCOOH	NH $_{2}$ CH $_{2}$ COOH

Two of the amino acids in Figure 9.1 show stereoisomerism. Draw the two stereoisomers of each of these amino acids in Figure 9.2. Name the type of stereoisomerism shown for each pair of stereoisomers. Use three-dimensional bonds where appropriate.

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9(b)(i)Amino Acids Proteins EnzymesEasy1 mark

The structures of amino acids T and U and their isoelectric points are shown in Table 9.1.

Table 9.1: T is proline, a five-membered ring with an N-H in the ring bonded to a ring carbon that carries a COOH group, isoelectric point 6.3; U is phenylalanine, a benzene ring joined by a CH2 group to a carbon bearing both an NH2 group and a COOH group, isoelectric point 5.5

Explain what is meant by isoelectric point.

Answer

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9(b)(ii)Amino Acids Proteins EnzymesHard3 marks

Complete Table 9.2 to show the predominant species formed by amino acids T and U at the pH values shown.

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9(c)(i)Amino Acids Proteins EnzymesMedium2 marks

Amino acids P, NH

_{2}

CH(CH

_{3}

)COOH, and R, NH

_{2}

_{2}

COOH, can form tripeptide W, C

_{8}

_{15}

_{3}

_{4}

. The structure of tripeptide W can be represented as P

-

-

Draw the structure of W. The peptide functional groups formed should be displayed.

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9(c)(ii)Amino Acids Proteins EnzymesEasy1 mark

W reacts under suitable conditions to form two molecules of P and one molecule of R. Name the type of reaction.

9(c)(iii)Condensation PolymersEasy1 mark

Amino acids P and R can also form a number of polymers. One of these polymers, X, has a structure that can be represented as

- [P - R -]_{n} -

. Name the type of polymerisation involved in the formation of X.