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6.2.2 Amino Acids and Chirality

General structure of an α amino acid

NH2 CH CO2H

RThe R group can be a variety ofdifferent things depending on whatamino acid it is.

The simplest amino acid isglycine, where the R is an H

NH2 CH2 CO2H

Some amino acids have an extracarboxylic acid or an aminegroup on the R group. These areclassed as acidic or basic(respectively) amino acids

NH2 C CO2HCH2

H

CO2H

Aspartic acid

Naming amino acids

You do not need to know anycommon names for the 20essential amino acids. Weshould, however, be able toname given amino acids usingIUPAC organic naming

2-aminobutanedioic acid

NH2 CH2 CO2H(2-)aminoethanoic acid

NH2 C CO2HCH2

H

OH

2-amino-3-hydroxypropanoic acid

H2N

C

CO2H

(CH2)4H NH2Lycine (basic)2,6-diaminohexanoic acid

NH2 C CO2HCH2

H

CO2H

Zwitterions

The no charge form of an amino acid never occurs.The amino acid exists as a dipolar zwitterion.

CNH2 CO2H

H

R

CNH3+

CO2-

H

R

Amino acids are often solids

The ionic interaction between zwitterions explains the relatively high melting points of amino acids asopposed to the weaker hydrogen bonding that would occur in the no charge form.

Zwitterion

Acidity and BasicityThe amine group is basic and the carboxylic acid group is acidic.

CNH3+

CO2-

H

R

CNH2 CO2-

H

R

CNH3+

CO2H

H

ROH-

H+ OH-

H+

+NH3-CH2-CO2- + HCl Cl- NH3

+-CH2-CO2H+NH3-CH2-CO2

- + NaOH NH2-CH2-CO2-Na+ +H2O

Amino acids act as weak buffers and willonly gradually change pH if smallamounts of acid or alkali are added to theamino acids.

Species inalkaline solutionHigh pH

Species inneutral solution

Species inacidic solutionLow pH

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The alpha in ‘α’ amino acid meansboth NH2 and COOH groups arejoined to the same C.

An amino acid exists as a zwitterion at a pH value called the isoelectric point

If the side R group of an amino acid containsan acidic of basic group then pH value of theisoelectric point will be different

An amine group on the R group may make theisoelectric point be pH > 10An carboxylic acid group on the R group may makethe isoelectric point be pH < 3

The extra carboxylic acid or amine groupson the R group will also react and changeform in alkaline and acid conditions NH2 C COO

-CH2

H

COO-

Aspartic acid in high pH

2

Other reactions of amino acidsThe carboxylic acid group and amine group in amino acids can undergo the usual reactions of these functional groups metin earlier topics.

CNH2 CO2H

H

CH3

+ CH3OHH+

e.g. Esterification reaction

CNH3+

C

H

CH3 O

O CH3 + H2O

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Amides

Add –amide to the stemname CH3 C

O

NH2

ethanamide

Secondary and tertiary amidesare named differently to show thetwo (or three) carbon chains.The smaller alkyl group ispreceded by an –N which playsthe same role as a number inpositioning a side alkyl chain

CH3 CH2 C

O

NH CH3

N-methylpropanamide

CH3 CH2 C

O

N

CH3

CH3

N,N-dimethylpropanamideCH3 CH C

O

N

CH3

CH3

CH3

N,N,2-trimethylpropanamide

H2N

C

CO2H

(CH2)4H NH2 + 2CH3COCl NH

C

CO2H

(CH2)4H NH C

O

CH3

C O

CH3

If the R group containsa amine or carboxylicacid then these will dothe same reactions asthe α amine andcarboxylic groups

If proteins are heated with dilute acid or alkali they can behydrolysed and split back in to their constituent amino acids.

The composition of the proteinmolecule may then be deduced byusing TLC chromatography

Hydrolysis of di-peptides/proteins

N CH CO

O

CH2

H

CHCH3 CH3

HCH COCH3

N

H

H CH CO

O

CH3

NH3+

HNH3

+CH C

OO

CH2

CHCH3 CH3

H

H+

+

N CH C

O

O

H2C

H

CHCH3 CH3

HCH C

OCH2

N

H

H

C OH

O

NaOH

CH C

OCH2

NH2

C O-

O

O-

+NH2 CH C

O

O-

H2C

CHCH3 CH3

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Optical Isomerism

Optical isomerism occurs in carbon compounds with 4different groups of atoms attached to a carbon (calledan asymmetric carbon). C

H

C

H

C

H

H

H

C

H H

H

O

H

H

A carbon atom that hasfour different groupsattached is called achiral (asymmetric)carbon atom

These four groups are arrangedtetrahedrally around the carbon.

CH5C2

CH3H

OH

CC2H5

CH3H

OH This causes two differentisomers that are notsuperimposable to beformed. They are mirrorimages

Two compounds that are optical isomers ofeach other are called enantiomers.

Many naturally occurringmolecules contain chiral C atoms,but are usually found in nature asa pure enantiomer

Optical isomers have similar physical and chemical properties,but they rotate plane polarised light in different directions.

One enantiomer rotates it in one direction and the otherenantiomer rotates it by the same amount in the oppositedirection.

Different systems of nomenclature are isexistence for optical isomers. D/L or +/- arecommonly used, but both have beensuperseded by the more useful and informativeR/S system (this is not on the syllabus – forinformation only).

One optical isomer will rotate light clockwise (+)(calleddextrorotatory). The other will rotate it anticlockwise(-)(called laevorotatory).

-ve enantiomerAnticlockwiserotation

+ve enantiomerclockwiserotation

Optical ActivityAll amino acids, exceptglycine, are chiral becausethere are four different groupsaround the C NH2

CCO2H

CH3

H

NH2

CHO2C

CH3

H

Optical isomerism and EIZ isomerism are different typesof stereoisomerism, which is defined as the samestructural formula but a different spatial arrangement ofatoms