Amino Acids Organic compounds containing both the amine -NH 2 and carboxyl -COOH functional groups. Amine e.g. CH 3 CH 2 NH 2 ethylamine Carboxylic acid e.g. CH 3 COOH ethanoic acid. Simplest amino acid: aminoethanoic acid- glycine if you are a biologist.

Week 7 © Pearson Education Ltd 2009 This document may have been altered from the original State the general formula for an α-amino acid as RCH(NH 2 )COOH. State that an amino acid exists as a zwitterion at a pH value called the isoelectric point. State that different R– groups in α-amino acids may result in different isoelectric points. Describe the acid–base properties of α-amino acids at different pH values.

Amino Acids Naturally occurring amino acids are all α amino acids. This means that both the carboxyl and the amino functional groups are on the SAME carbon atom. This leads to the general formula on the next slide. These compounds are BIFUNCTIONAL since both functional groups act independently of one another.

Week 7 © Pearson Education Ltd 2009 This document may have been altered from the original General formula of an α-amino acid R means any organic side chain.

Week 7 © Pearson Education Ltd 2009 This document may have been altered from the original Structures of some amino acids

Physical Properties White solids With relatively high melting points glycine (the simplest) has a melting point of 235°C. Normally readily soluble in water Almost totally insoluble in non-polar solvents Soluble in both acids and bases (alkalis).

Reactions The carboxylic acid group is a proton donor: CO 2 H  COO - + H + The amine group is a proton acceptor (a base): -NH 2 + H +  –NH 3 + The amino acids tend to exist as ZWITTERIONS. These are formed when the carboxyl group and the amine group have undergone an internal acid-base reaction. A proton is transferred FROM the acid TO the amine group so that both ends are charged. The overall charge is zero because the positive and negative charges cancel each other out.

Week 7 © Pearson Education Ltd 2009 This document may have been altered from the original Formation of a zwitterion from glycine

Isoelectric Point pH value at which the amino acid exists as a zwitterion. Varies from amino acid to amino acid since the inductive effects of the side chains affects the acid and base strengths of different amino acids differently. The isoelectric point has an impact on the acid-base behaviour of the amino acids.

Amphoteric Behaviour Amphoteric means can react with both acid and base. pH below isoelectric point: The amino acid is a base and accepts a proton from the the acid. The amino acid is a positively charged ion. pH above isoelectric point: The amino acid is an acid and donates a proton to the base. The amino acid forms a negatively charged ion.

Leucine Isoelectric point pH Draw the displayed formula of leucine. 2. Show the structure of leucine at pH 5.98, 2.0 and 7.

Answers Nearly full displayed formula. Skeletal zwitterionic form at pH At pH7 behaves as an acid At pH2 behaves as a base H+H+ -

Week 7 © Pearson Education Ltd 2009 This document may have been altered from the original Acid–base reactions of an amino acid

Week 7 © Pearson Education Ltd 2009 This document may have been altered from the original Explain the formation of a peptide (amide) linkage between α-amino acids to form polypeptides and proteins. Describe the acid and alkaline hydrolysis of proteins and peptides to form α- amino acids or carboxylates.

Peptides and Polypeptides A peptide linkage is the –CONH- link This is formed in a CONDENSATION reaction between two amino acids with the loss of a water molecule. A peptide is a compound containing amino acids linked by peptide bonds. Dipeptide = 2 amino acids Tripeptide = 3 amino acids etc.

Week 7 © Pearson Education Ltd 2009 This document may have been altered from the original Formation of a dipeptide between glycine and alanine

Week 7 © Pearson Education Ltd 2009 This document may have been altered from the original Alternative reaction of alanine and glycine

Peptides and Polypeptides A polypeptide is a long chain of amino acids joined by peptide links. A protein is a long polypeptide chain with more than 60 amino acid units. A polypeptide with 4 different amino acids.

Hydrolysis The breaking of a bond by reaction with water. Can be acid or base catalysed. Acid hydrolysis: Generally refluxed with aqueous (6mol dm -3 ) HCl for 24 hours. The product is the protonated form of the constituent amino acids. Alkaline hydrolysis Reflux with aqueous sodium hydroxide. Sodium salt formed.

Week 7 © Pearson Education Ltd 2009 This document may have been altered from the original Acid hydrolysis of a dipeptide

Week 7 © Pearson Education Ltd 2009 This document may have been altered from the original Alkaline hydrolysis of a polypeptide chain

Isomerism Structural Isomers Stereo Isomers E/Z (cis- trans) Isomers Optical Isomers Stereo isomerism –same structural formula but different arrangements of groups in space.

Week 7 © Pearson Education Ltd 2009 This document may have been altered from the original Describe optical isomers as non-superimposable mirror images about an organic chiral centre. Identify chiral centres in a molecule of given structural formula. Explain that optical isomerism and E/Z isomerism are types of stereoisomerism.

24 All molecules have a mirror image – but for most molecules it is the same molecule.

25 For some molecules the mirror image is a different molecule (the mirror image is non-superimposable). When this happens the 2 forms of the molecule are described as OPTICAL ISOMERS. Optical isomerism is a form of stereoisomerism.

Molecules that are optical isomers are called enantiomers. Enantiomers have identical chemical and physical properties, except: Their effect on plane polarised light; Their reaction with other chiral molecules. Many natural molecules are chiral and most natural reactions are affected by optical isomerism. In nature, only one optical isomer occurs (e.g. all natural amino acids are rotate polarised light to the left).

Light is a form of electromagnetic radiation.

The wave vibrations are perpendicular to the direction of travel of the wave.

Optical isomers rotate the plane of plane polarised light.

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Left and right hands are an example of non- superimposable mirror images. Optical Isomerism

OPTICAL ISOMERISM Occurrence another form of stereoisomerism occurs when compounds have non-superimposable mirror images Isomers the two different forms are known as optical isomers or enantiomers they occur when molecules have a chiral centre a chiral centre contains an asymmetric carbon atom an asymmetric carbon has four different atoms (or groups) arranged tetrahedrally around it.

OPTICAL ISOMERISM Occurrence another form of stereoisomerism occurs when compounds have non-superimposable mirror images Isomers the two different forms are known as optical isomers or enantiomers they occur when molecules have a chiral centre a chiral centre contains an asymmetric carbon atom an asymmetric carbon has four different atoms (or groups) arranged tetrahedrally around it. There are four different colours arranged tetrahedrally about the carbon atom 2-chlorobutane exhibits optical isomerism because the second carbon atom has four different atoms/groups attached CHIRAL CENTRES

34 Most α amino acids are optically active – having non superimposable mirror image isomers. Exception?

Week 7 © Pearson Education Ltd 2009 This document may have been altered from the original The butan-2-ol molecule has a chiral carbon

Week 7 © Pearson Education Ltd 2009 This document may have been altered from the original Optical isomers of CH 3 CH 2 CH(NH 2 )CH 3

Week 7 © Pearson Education Ltd 2009 This document may have been altered from the original Simplified three-dimensional representation of two optical isomers

TASK Which of the following molecules are optically active? 1)propan-2-ol 2)2-chlorobutane 3)1-chlorobutane 4)3-methylhexane 5)butanone 6)2-methylbutanoic acid 7)butan-2-ol 8)1-chloro-3-methylpentane

propan-2-ol NOT OPTICALLY ACTIVE

2-chlorobutane OPTICALLY ACTIVE

1-chlorobutane NOT OPTICALLY ACTIVE

3-methylhexane OPTICALLY ACTIVE

butanone NOT OPTICALLY ACTIVE

propan-2-ol NOT OPTICALLY ACTIVE

2-methylbutanoic acid OPTICALLY ACTIVE

butan-2-ol OPTICALLY ACTIVE

1-chloro-3-methylpentane OPTICALLY ACTIVE

S carvone (caraway seed)R carvone (spearmint) Caraway Seed has a warm, pungent, slightly bitter flavour with aniseed overtones.

S limonene (lemons)R limonene (oranges)

Week 7 © Pearson Education Ltd 2009 This document may have been altered from the original E and Z isomers of but-2-ene