Amino Acids

• Organic compounds containing both the amine -NH2 and carboxyl -COOH functional groups.

• Amine e.g. CH3CH2NH2 ethylamine

• Carboxylic acid e.g. CH3COOH ethanoic acid.

• Simplest amino acid: aminoethanoic acid- glycine if you are a biologist.

Week 7

© Pearson Education Ltd 2009This document may have been altered from the original

• State the general formula for an α-amino acid as RCH(NH2)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 2009This 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 2009This 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:

• CO2H COO- + H+

• The amine group is a proton acceptor (a base):

• -NH2 + H+ –NH3+

• 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 2009This 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 5.98

1. 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 5.98

-

At pH7 behaves as an acid At pH2 behaves as a base

H+

-

Week 7

© Pearson Education Ltd 2009This document may have been altered from the original

Acid–base reactions of an amino acid

Week 7

© Pearson Education Ltd 2009This 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 2009This document may have been altered from the original

Formation of a dipeptide between glycine and alanine

Week 7

© Pearson Education Ltd 2009This 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 2009This document may have been altered from the original

Acid hydrolysis of a dipeptide

Week 7

© Pearson Education Ltd 2009This document may have been altered from the original

Alkaline hydrolysis of a polypeptide chain

Isomerism

Stereo isomerism –same structural formula but different arrangements of groups in space.

Week 7

© Pearson Education Ltd 2009This 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.

fluoromethane

H

CH F

H

H

CHF

H

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.

OH

CH CH3

COOH

OH

CHH3C

HOOC

(-) lactic acid (+) lactic acidin sour milk in muscles

• 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.

normal light(waves vibrate in all directions)

plane-polarised light(vibrates in only one direction)

plane-polarised light after clockwise rotation

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

• Optical isomers rotate the plane of plane polarised light.

(-)-enantiomer(anticlockwise rotation)

(±)-racemate(no overall effect)

(+)-enantiomer(clockwise rotation)

• http://www.youtube.com/watch?v=3WZZXPOsPNI&feature=related

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

Optical Isomerism

OPTICAL ISOMERISMOPTICAL 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 ISOMERISMOPTICAL 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 2009This document may have been altered from the original

The butan-2-ol molecule has a chiral carbon

Week 7

© Pearson Education Ltd 2009This document may have been altered from the original

Optical isomers of CH3CH2CH(NH2)CH3

Week 7

© Pearson Education Ltd 2009This 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-ol2) 2-chlorobutane3) 1-chlorobutane4) 3-

methylhexane

5) butanone6) 2-methylbutanoic acid7) butan-2-ol8) 1-chloro-3-methylpentane

propan-2-ol

NOT OPTICALLY ACTIVE

CH3 CH CH3

OH

2-chlorobutane CH3 CH CH2 CH3

Cl

OPTICALLY ACTIVE

CH2CH3

CH Cl

CH3

CH2CH3

CHCl

H3C

1-chlorobutane

NOT OPTICALLY ACTIVE

CH2 CH2 CH2 CH3

Cl

3-methylhexane

OPTICALLY ACTIVE

CH2 CH CH2 CH2

CH3

CH3CH3

CH2CH2CH3

CH CH2CH3

CH3

CH2CH2CH3

CHCH3CH2

CH3

butanone

NOT OPTICALLY ACTIVE

C CH2 CH3CH3

O

propan-2-ol CH3 CH CH3

OH

NOT OPTICALLY ACTIVE

2-methylbutanoic acid

OPTICALLY ACTIVE

CH3 CH2 CH

CH3

C

O

OH

CH2CH3

CCH3 COOH

H

CH2CH3

CCH3

HHOOC

butan-2-ol

OPTICALLY ACTIVE

CH3 CH2 CH

OH

CH3

CH2CH3

CCH3 OH

H

CH2CH3

CCH3

HHO

1-chloro-3-methylpentane

OPTICALLY ACTIVE

CH3 CH2 CH

CH3

CH2 CH2

Cl

CH2CH3

CCH3 CH2CH2Cl

H

CH2CH3

CCH3

HCH2ClCH2

S carvone (caraway seed) R carvone (spearmint)

O

CH3

H C CH2

H3C

O

CH3

HCH2C

CH3

Caraway Seed has a warm, pungent, slightly bitter flavour with aniseed

overtones.

S limonene (lemons) R limonene (oranges)

CH3

HCCH2

CH3

CH3

H C CH2

H3C

Week 7

© Pearson Education Ltd 2009This document may have been altered from the original

E and Z isomers of but-2-ene