esters - brianna lush...preparation of esters - esterification • esters are derived from a...

ESTERS ORGANIC AND BIOLOGICAL CHEMISTRY

• Esters are organic compounds which contain the functional

group –COO–

• Properties:

– Volatile – they are liquids that become vapours easily

– Distinctive smells, commonly sweet and ‘fruity’

Ester Functional Group

Preparation of Esters - Esterification

• Esters are derived from a carboxylic acid and an alcohol

CARBOXYLIC ACID ALCOHOL



• Esters are formed by reacting a carboxylic acid and an

alcohol under reflux conditions in the presence of an acid

catalyst to produce an ester and water.

– Reactions that produce water in this way are called

condensation reactions.

heat



• Esters are formed by reacting a carboxylic acid and an

alcohol under reflux conditions in the presence of an acid

catalyst to produce an ester and water.

– Reactions that produce water in this way are called

condensation reactions.

heat

FROM THE CARBOXYLIC ACID

FROM THE ALCOHOL

WATER

You need to be able to:

1. Draw the ester that is prepared from a carboxylic acid

and an alcohol

2. Draw the structure of the carboxylic acid and alcohol

which are used to prepare an ester

Drawing Esters

heat

FROM THE CARBOXYLIC ACID FROM THE ALCOHOL

1. Draw the ester that is prepared from a carboxylic acid and an alcohol

Drawing Esters

CARBOXYLIC

ACID ALCOHOL


Drawing Esters

CARBOXYLIC

ACID ALCOHOL

WATER


Drawing Esters

CARBOXYLIC

ACID ALCOHOL

WATER

ESTER


Drawing Esters

CARBOXYLIC

ACID ALCOHOL

WATER

ESTER

FROM THE

CARBOXYLIC

ACID FROM THE

ALCOHOL


Drawing Esters

CARBOXYLIC

ACID ALCOHOL


Drawing Esters

CARBOXYLIC

ACID ALCOHOL

WATER


Drawing Esters

CARBOXYLIC

ACID ALCOHOL

WATER

ESTER

2. Draw the structure of the carboxylic acid and alcohol which are used to

prepare an ester

Drawing Esters

ESTER


prepare an ester

Drawing Esters

ESTER

SPLIT APART Where does the

carbonyl group

come from? The

carboxylic acid or

the alcohol?

REMEMBER:

Water is the

molecule that is

lost.


prepare an ester

Drawing Esters

ESTER


carbonyl group

come from? The

carboxylic acid or

the alcohol?

REMEMBER:

Water is the

molecule that is

lost. CARBOXYLIC ACID ALCOHOL


prepare an ester

Drawing Esters

ESTER


prepare an ester

Drawing Esters

ESTER


carbonyl group

come from? The

carboxylic acid or

the alcohol?

REMEMBER:

Water is the

molecule that is

lost.

• Esters are systematically named based upon the

carboxylic acid and alcohol from which they are prepared

Systematic Naming of Esters


The R group from the carboxylic acid forms the base

name of the ester.

The –ic ending is changed to –oate. (eg. If the carboxylic acid name is ethanoic acid, the contribution to the

systematic ester name is ethanoate)

The R group from the alcohol is named as an alkyl

group with the ending –yl. (eg. If the alcohol name is methanol, the contribution to the systematic ester

name is methyl)


CARBOXYLIC

ACID ALCOHOL

The systematic name of the ester is written with the

contribution from the alcohol first, followed by the

contribution from the carboxylic acid. (eg. Contribution from the alcohol is methyl and contribution from the

carboxylic acid is ethanoate, so the systematic ester name is

methyl ethanoate)


CARBOXYLIC

ACID ALCOHOL


CARBOXYLIC

ACID ALCOHOL

This ester is prepared from the following carboxylic acid and alcohol:

BUTANOIC ACID METHANOL


CARBOXYLIC

ACID ALCOHOL



Contribution to ester name: butanoate Contribution to ester name: methyl


CARBOXYLIC

ACID ALCOHOL



Contribution to ester name: butanoate Contribution to ester name: methyl

The systematic name of the ester is

methyl butanoate


CARBOXYLIC

ACID ALCOHOL



CARBOXYLIC

ACID ALCOHOL


PENTANOIC ACID BUTANOL

Contribution to ester name: pentanoate Contribution to ester name: butyl


butyl pentanoate

• After learning how to draw esters from a carboxylic acid

and alcohol, and vice versa, you should be able to

quickly identify which parts of the molecule come from

which reactant.

• Using this knowledge and what you already know about

the IUPAC naming rules, you have the skills to be able to

name an ester without specifically knowing the names of

the carboxylic acid or alcohol.


CARBOXYLIC

ACID ALCOHOL


CARBOXYLIC

ACID ALCOHOL

Determining the systematic name of an ester without knowing the names

of the carboxylic acid or alcohol from which it is derived


CARBOXYLIC

ACID ALCOHOL



This part of the molecule comes from the carboxylic acid, so it

will contribute __oate to the ester name.


CARBOXYLIC

ACID ALCOHOL





This section is named

in a similar way to

naming a normal

carboxylic acid. The

longest carbon chain

in this section of the

molecule contains two

carbons with single

bonds only. So

according to IUPAC

rules, the base name

for the carboxylic acid

would be ethane.


CARBOXYLIC

ACID ALCOHOL





This section is named

in a similar way to

naming a normal

carboxylic acid. The

longest carbon chain

in this section of the


carbons with single

bonds only. So

according to IUPAC



would be ethane.

The carboxylic acid contribution to the

systematic ester name is ethanoate


CARBOXYLIC

ACID ALCOHOL



ETHANOATE

This part of the molecule comes from the alcohol, so it will

contribute __yl to the ester name.


CARBOXYLIC

ACID ALCOHOL



ETHANOATE This section is named

in a similar way to

naming a normal

alcohol. The longest

carbon chain in this

section of the


carbons with single

bonds only. So

according to IUPAC



would be ethane.




CARBOXYLIC

ACID ALCOHOL



ETHANOATE This section is named

in a similar way to

naming a normal

alcohol. The longest

carbon chain in this

section of the


carbons with single

bonds only. So

according to IUPAC



would be ethane.

The alcohol contribution to the

systematic ester name is ethyl




CARBOXYLIC

ACID ALCOHOL



ETHANOATE ETHYL


ethyl ethanoate


CARBOXYLIC

ACID ALCOHOL




CARBOXYLIC

ACID ALCOHOL



From the alcohol = _yl

Three carbons in chain with single bonds = propane

Contribution to systematic ester name = propyl

From the carboxylic acid = _oate

One carbon in chain with single bonds = methane

Contribution to systematic ester name = methanoate


propyl methanoate

• Polyesters are molecules that are made up of a

chain of repeating monomers that are held

together by ester groups (links).

• Ester links are created through a condensation

reaction between the monomers.

• Polyesters are very strong and have a range of

uses depending on how they are processed.

Polyesters

• Polyesters are prepared through a condensation

reaction between diol and dicarboxylic acid

monomers

Polyesters

DICARBOXYLIC ACID DIOL

Polyesters


Drawing polyesters

Polyesters


Drawing polyesters

Water

Polyesters


Drawing polyesters

Water

ESTER LINK

Polyesters

DICARBOXYLIC ACID

Drawing polyesters

EXISTING POLYMER CHAIN WITH

FREE ALCOHOL GROUP

Polyesters

DICARBOXYLIC ACID

Drawing polyesters


FREE ALCOHOL GROUP

Water

Polyesters

DICARBOXYLIC ACID

Drawing polyesters

ESTER LINK


FREE ALCOHOL GROUP

Water

Polyesters

DIOL

Drawing polyesters


FREE CARBOXYLIC ACID

Polyesters

DIOL

Drawing polyesters



Water

Polyesters

DIOL

Drawing polyesters



Water

ESTER LINK

… and these condensation reactions continue to form a long polymer chain.

• Drawing the monomer units which make up a

polyester can be done in the same way to that

for single ester compounds

Polyesters




Polyesters

FROM THE

DICARBOXYLIC ACID FROM THE DIOL




Polyesters

FROM THE

DICARBOXYLIC ACID FROM THE DIOL

SPLIT APART

DIOL MONOMER DICARBOXYLIC ACID

MONOMER

• The polyester that has been shown here is

Polyethene terephthalate (PET)

• The repeating unit of the polyester can be

written in square brackets as shown below:

Polyesters

Polyethene terephthalate (PET)

• Polyethene terephthalate (PET) is commonly

used in:

– Fibres for clothing

– Containers for liquids and foods

– Thermoforming in manufacturing (heating a plastic

sheet which is formed into a desired shape using a

mold)

Polyesters

Esterification

• Successful esterification

reactions require

– Reflux conditions

– Presence of an acid

catalyst

Esterification

Reflux Conditions

• Reflux: a process whereby a reaction mixture is heated

for a prescribed period of time. Involves the use of a

heat source and a condenser.

• Esterification reactions are quite slow, so to achieve a

satisfactory yield in a reasonable time, the reaction

requires an extended period of heating

• Heating – increases reaction rate.

• Condenser – prevents the loss of volatile reactant and

product vapours. Upon cooling the vapours condense

and return to the reaction mixture.

Esterification

Presence of an Acid Catalyst

• Concentrated sulfuric acid is most commonly used.

• Role of the catalyst:

– Speeds up the reaction

– Acts as a dehydrating agent, forcing the equilibrium to the

product side which results in a greater yield of the ester

• Only small amounts of acid catalyst are required.

Ester Hydrolysis

• Esters undergo hydrolysis when refluxed with

aqueous acid or base

• Hydrolysis can be considered as the reverse of

esterification – water is consumed as a reactant

in a hydrolysis reaction

Ester Hydrolysis

Acidic Conditions

• Products of acidic hydrolysis are a carboxylic acid and

an alcohol

• Reaction is catalysed by acid

+

H+

reflux

+


Ester Hydrolysis

Basic Conditions

• Products of basic hydrolysis are a carboxylate salt and

an alcohol

• Basic conditions created by using a sodium hydroxide

solution

+ reflux

+

CARBOXYLATE

SALT

ALCOHOL

• We will be completing an esterification practical

in the laboratory.

• We will be preparing a quantity of the ester ethyl

ethanoate. Three processes will be used during

the practical:

– Esterification (preparation of the ester)

– Distillation (purification of the ester)

– Isolation of the ester (separating the ester

from other compounds)

Summative Esterification Practical

• Esterification – Preparation of the ester

– The reaction mixture will be heated under reflux for 10 minutes and then cooled.

• Distillation – The ester is separated from other components in the

reaction mixture due to differences in boiling point. This leads to greater purity in the ester product.

• Separation – During the distillation, some unreacted alcohol will distil

with the ester.

– Using a separating funnel, the ester can be separated from the alcohol using sodium carbonate – ethanol is very soluble in sodium carbonate, whereas ethyl ethanoate is only slightly soluble. This creates two layers in the funnel, and the layer containing the ester can be obtained.


• The write-up for the practical will be due one week from the day of completion of the practical.

• The write-up should include:

– An introduction with relevant chemistry concepts explained

– Hypothesis

– Variables

– Materials list

– Procedure

– Any relevant safety considerations

– Results

– A discussion, including analysis of results, evaluation of procedures and indentification of sources of error

– A conclusion with justification

• The report should be a maximum of 1500 words (NOTE: materials list, procedure, safety considerations and results ARE NOT included in the word count)


• As per usual, safety glasses, a lab coat and

closed-in shoes are required for the entire

duration of the practical. Long hair must also be

tied back.

• There is some pre-lab work which must be

completed and checked before beginning the

practical. No extra time will be given if this work

has not been completed prior to the lesson.


AMIDES ORGANIC AND BIOLOGICAL CHEMISTRY

Amide Functional Group

• Amides are organic compounds with the functional group –CON–

• The most common example of amides is proteins, where the polypeptide chain is joined together with amide linkages.

• The R’ and R” groups could be hydrogen atoms or other carbon-based groups.

Preparation of Amides- Esterification

• Amides are derived from a carboxylic acid and an amine

CARBOXYLIC ACID AMINE



• Amides are formed through a condensation reaction, by

reacting a carboxylic acid and amine under reflux conditions

+ reflux

+



• Amides are formed through a condensation reaction, by

reacting a carboxylic acid and amine under reflux conditions

+ reflux

+

Water


+ reflux

+

Water

+ reflux

+

Water


+ reflux

+

Water

+ reflux

+

Water

+ reflux

+

Water

Drawing Amides

You need to be able to:

1. Draw the amide that is prepared from a carboxylic acid

and an amine

2. Draw the structure of the carboxylic acid and amine

which are used to prepare an amide

FROM THE CARBOXYLIC ACID FROM THE AMINE

+ reflux

+

1. Draw the amide that is prepared from a carboxylic acid and an amine

Drawing Amides

CARBOXYLIC

ACID AMINE


Drawing Amides

CARBOXYLIC

ACID AMINE

Water


Drawing Amides

CARBOXYLIC

ACID AMINE

Water

AMIDE


Drawing Amides

CARBOXYLIC

ACID AMINE

Water

AMIDE

FROM THE

CARBOXYLIC

ACID

FROM THE

AMINE


Drawing Amides

CARBOXYLIC

ACID AMINE


Drawing Amides

CARBOXYLIC

ACID AMINE

Water


Drawing Amides

CARBOXYLIC

ACID AMINE

Water

AMIDE

2. Draw the structure of the carboxylic acid and amine which are used to

prepare an amide

Drawing Amides

AMIDE


prepare an amide

Drawing Amides

AMIDE

SPLIT APART


prepare an amide

Drawing Amides

AMIDE


prepare an amide

Drawing Amides

AMIDE

SPLIT APART

Polyamides

• Polyamides are molecules that are made up of a

chain of repeating monomers that are held

together by amide groups (links).

• Amide links are created through a condensation

reaction between the monomers.

Polyamides

• Polyamides are prepared through a

condensation reaction between diamine and

dicarboxylic acid monomers

DICARBOXYLIC ACID DIAMINE

Polyamides

Drawing polyamides


Polyamides

Drawing polyamides


Water

Polyamides

Drawing polyamides


Water

AMIDE LINK

Polyamides

Drawing polyamides


FREE AMINE GROUP DICARBOXYLIC ACID

Polyamides

Drawing polyamides



Water

Polyamides

Drawing polyamides



Water

AMIDE LINK

Polyamides

Drawing polyamides

EXISTING POLYMER CHAIN WITH FREE

CARBOXLYIC ACID GROUP DIAMINE

Polyamides

Drawing polyamides



Water

Polyamides

Drawing polyamides



Water

AMIDE LINK

… and these condensation reactions continue to form a long polymer chain.

Polyamides

• Polyamides can also be prepared through a

condensation reaction between

aminocarboxylic acid monomers

AMINOCARBOXYLIC ACID

Polyamides

AMINOCARBOXYLIC ACID AMINOCARBOXYLIC ACID

Polyamides


Water

Polyamides


Water

AMIDE LINK

Polyamides


polyamide can be done in the same way to that

for single amide compounds

Polyamides




FROM THE

DICARBOXYLIC ACID FROM THE DIAMINE

Polyamides




FROM THE

DICARBOXYLIC ACID FROM THE DIAMINE SPLIT APART

DIAMINE MONOMER DICARBOXYLIC ACID

MONOMER

Bonding Between Polyamide Chains

• Polyamides such as nylon and kevlar are strong due to

the interactions that are occurring between polymer

chains.

• Hydrogen bonding – the hydrogen atom bonded to the

nitrogen atom forms a hydrogen bond with the carbonyl

group in the amide link on the adjacent chain.

Amide Hydrolysis

• Amides undergo hydrolysis when refluxed for an

extended period of time under strongly acidic

or basic conditions

• Hydrolysis of amide linkages in proteins occurs

more readily when catalysed by enzymes

Amide Hydrolysis

Acidic Conditions

• Products of acidic hydrolysis are a ammonium or

substituted ammonium salt and a carboxylic acid.

• The reaction uses concentrated hydrochloric acid.

+ reflux

+

+ reflux

+

AMMONIUM SALT CARBOYXLYIC ACID

AMMONIUM CARBOYXLYIC ACID

Amide Hydrolysis

Basic Conditions

• Products of acidic hydrolysis are a ammonia or an amine

and a carboxylate salt.

• The reaction uses concentrated sodium hydroxide

solution.

+ reflux

+

+ reflux

+

AMINE CARBOXYLATE SALT

AMMONIA CARBOXYLATE SALT

TRIGLYCERIDES ORGANIC AND BIOLOGICAL CHEMISTRY

Triglycerides

• Triglycerides are edible fats and oils derived from

plants and animals.

• Triglycerides are triesters – they are made up of a

propane-1,2,3-triol backbone (common name

glycerol) and three long, straight chain carboxylic

acids which are attached to the glycerol backbone with

ester linkages.

– The straight chain carboxylic acids are commonly referred to as

“fatty acids”

Triglycerides

+

• The ester linkages between the fatty acid chains

and glycerol are prepared through a

condensation reaction

ALCOHOL

(GLYCEROL)

CARBOXYLIC ACID

(FATTY ACIDS) TRIGLYCERIDE

Triglycerides

+




ALCOHOL

(GLYCEROL)

CARBOXYLIC ACID

(FATTY ACIDS)

H2O

H2O

H2O

TRIGLYCERIDE

Triglycerides

+




ALCOHOL

(GLYCEROL)

CARBOXYLIC ACID

(FATTY ACIDS)

H2O

H2O

H2O

Glycerol backbone

TRIGLYCERIDE

Ester links

Triglycerides

• The carboxylic acid chains almost always

contain an even number of carbon atoms,

including the carbon atom in the carbonyl group.

• These chains can be saturated (single bonds

only) or unsaturated (contains at least one

double bond)

– If more than one double bond is present in the carbon

chain, it is described as polyunsaturated

Drawing Triglycerides

Drawing triglycerides given the structural formula of the carboxylic

acid(s) from which it is derived

Drawing Triglycerides

Drawing triglycerides given the structural formula of the carboxylic

acid(s) from which it is derived

H2O

H2O

H2O

Triglycerides Hydrolysis

• Fats and oils are highly concentrated stores of energy.

Triglyceride hydrolysis is required to release this energy

for use in the body.

• Triglyceride hydrolysis is catalysed by enzymes called

lipases. Without these enzymes, hydrolysis would only

be able to occur under severe conditions.

• Water is also required for triglyceride hydrolysis.

• The products of triglyceride hydrolysis is glycerol and

three fatty acid molecules.


General equation for hydrolysis

+ +

GLYCEROL 3 FATTY ACID MOLECULES TRIGLYCERIDE


General equation for hydrolysis

+ +

GLYCEROL 3 FATTY ACID MOLECULES TRIGLYCERIDE

NOTICE: 3 molecules of water are required


Drawing hydrolysis products



FROM

GLYCEROL

FROM THE FATTY ACID MOLECULES



FROM

GLYCEROL

FROM THE FATTY ACID MOLECULES

SPLIT APART



GLYCEROL THREE FATTY ACID MOLECULES

Saturated and Unsaturated Triglycerides

• Triglycerides can be classified as saturated or

unsaturated depending on the fatty acid chains

that they are made of.

SATURATED UNSATURATED


SATURATED TRIGLYCERIDE

(SINGLE BONDS ONLY)

UNSATURATED TRIGLYCERIDE

(DOUBLE BONDS)


• The physical state of a fat or oil is determined by the degree

of unsaturation of the triglyceride.

• Saturated triglycerides

‒ Have an ordered and compact structure, allowing the molecules

to pack very closely together. Fats/oils with a high percentage of

saturated triglycerides are generally solid at room temperature.

• Unsaturated triglycerides

‒ Have a disordered and loose structure, causing the molecules to

not be able to pack closely together. Fats/oils with a high

percentage of unsaturated triglycerides are generally liquid at

room temperature.


• Edible fats are solids at room temperature and are

generally derived from animals.

• Edible oils are liquids at room temperature and are

generally derived from plants or fish.

• Animal fats contain a greater percentage of saturated

triglycerides than vegetable oils.

Melting Point of Fats/Oils

• Melting point increases as the length of the fatty acid

chains in the triglyceride increase. This is caused by

an increase in dispersion forces between triglyceride

molecules.

• Melting points decrease as the degree of

unsaturation (number of double bonds) in the fatty

acid chains increases. This is caused by disordered

and loose packing between molecules. Dispersion forces

are weaker as chains are further apart.

Addition Reactions Across C=C Groups

• Unsaturated molecules – contain C=C double bonds.

• C=C groups can undergo addition reactions with

diatomic molecules such as bromine (Br2)/iodine (I2) and

hydrogen (H2).


Addition Reactions Across C=C Groups with Bromine (Br2) and Iodine

(I2) = Determining The Degree Of Unsaturation

• The degree of unsaturation of a triglyceride (how many double

bonds in the molecule) can be determined by reacting a

triglyceride solution with bromine (Br2) or iodine (I2).

• After adding Br2 solution, the orange bromine colour

disappears, as the products of the addition reaction are

colourless.

• The degree of unsaturation is determined from the end point

(the point at which no more Br2 reacts with the triyglyceride)

• Addition of I2 across C=C double bonds does not occur as

readily as that for Br2. However, the degree of unsaturation is

usually quoted as an iodine number.


Addition Reactions Across C=C Groups with Bromine (Br2) and

Iodine (I2) = Determining The Degree Of Unsaturation

Conical flask

containing fat/oil

in cyclohexane

Burette containing

bromine in

cyclohexane

• Add solution of Br2 to known

volume of fat/oil in

cyclohexane

• End point = first sign of

permanent orange colour in

the flask

• The greater the amount of

bromine that reacts, the

greater the degree of

unsaturation




+




The iodine/bromine number of a fat or oil is the mass of iodine/bromine as I2 / Br2 that reacts with exactly with

100 grams of the fat or oil. The greater the value of the iodine/bromine number, the greater the

degree of unsaturation.


Addition Reactions Across C=C Groups with Hydrogen (H2)

= Hydrogenation

• Hydrogenation involves adding across H2 C=C double

bond

• The reaction involves heating a liquid oil with hydrogen

gas under pressure in the presence of a nickel catalyst.

• Using this process on unsaturated fats and oils to be

converted from liquids forms to solid forms.

– Adding H2 decreases the degree of unsaturation (less double

bonds), triglyceride molecules become more ordered and

become able to pack together very closely, hence becoming

solid.



= Hydrogenation

• Elevated temperature, high pressure and a nickel

catalyst are all used to increase the rate of addition

reaction between the unsaturated molecules and

hydrogen.

– Elevated temperature: increases the energy of colliding

reactant molecules, causing more collisions which lead to the

formation of products.

– High pressure: increases the concentration of the hydrogen

gas, leading to more productive collisions between reactants.

– Nickel catalyst: provides an alternative reaction pathway with a

lower activation energy.



= Hydrogenation

+

Triglyceride Hydrolysis

• Soap and detergents are able to be produced

when triglycerides are hydrolysed under

alkaline conditions.

• This process is referred to as saponification. – Animal fats or vegetable oils are boiled with concentrated

sodium hydroxide solution

– The products of alkaline conditions are glycerol and three long

chain carboxylate ions

Alkaline Hydrolysis of Triglycerides

+

GLYCEROL 3 CARBOXYLATE IONS

heat

Alkaline Hydrolysis of Triglycerides

CARBOXYLATE ION

• Carboxylate ions have hydrophilic (water loving)

and hydrophobic (water fearing) regions.

• Carboxylate ions in soap are able to move non-

polar substances through aqueous solutions

through micelle formation

Micelles

CARBOXYLATE ION

Polar head group,

hydrophilic

Non-polar hydrocarbon

chain, hydrophobic

• The hydrophobic, non-polar hydrocarbon chain attaches

to the non-polar dirt or grease.

• The hydrophilic, polar head interacts with the polar

water molecules.

• These interactions together create a globule of

dirt/grease.

Micelles

• With some agitation to the system, the globule dislodges

from the surface it is attached to and the carboxylate ions

come together to form spheres called micelles.

Micelles

• The centre of the micelle is hydrophobic and contains

the globules of dirt/grease.

• The ionic heads interact with the polar water because it

is hydrophilic. Since the ionic heads are negatively

charged, micelles repel each other, preventing the

dirt/grease globules from joining back together.

• The interactions between the ionic head and the water

allows the micelle to be soluble in water. As a result,

when the water is washed away, the micelle

(containing the dirt/grease) is also washed away,

leaving a clean surface behind.

PROTEINS ORGANIC AND BIOLOGICAL CHEMISTRY

Amino Acids

General structure of amino acids

AMINE GROUP

CENTRAL CARBON ATOM

ONE OTHER ATOMS OR

GROUP OF ATOMS

(REPRESENTED AS “R”)

CARBOXYL

GROUP

HYDROGEN ATOM

Amino Acids

• Amino acids are the building blocks of proteins. – Proteins are made up of one or more polypeptide chains.

These chains are made up of amino acids that are held together with peptide bonds.

– There are 500 different amino acids, however the human genetic code only directly encodes 20.

Amino Acids

ESSENTIAL amino acids:

must be obtained from the diet.

NON- ESSENTIAL amino acids:

can be synthesised in the body.

Amino Acids

• Amino acids can undergo self-ionisation in a

neutral environment (pH=7.0).

• The product of self-ionisation is a dipolar ion called

a zwitterion.

The amine group has a

pair of non-bonding

electrons, making it a

base capable of

accepting a proton

The carboxyl group

can lose a proton due

to the polarity of the

group

Amino Acids

• Amino acids can undergo self-ionisation in a

neutral environment (pH=7.0).

• The product of self-ionisation is a dipolar ion called

a zwitterion.

ZWITTERION

Amino Acids

Drawing products of self-ionisation (zwitterions)

Amino Acids

pH < 7 (acidic)

pH > 7 (basic)

pH = 7 ZWITTERION

Proteins

• Amino acids can undergo a condensation

reaction to form a polypeptide chain.

• The condensation reaction occurs between the

carboxyl group of one amino acid and the amine

group of another amino acid.

+

AMINO ACID 1 AMINO ACID 2

Proteins






+

AMINO ACID 1 AMINO ACID 2

H2O

Proteins






+

AMINO ACID 1 AMINO ACID 2 DIPEPTIDE

H2O

Proteins






+

AMINO ACID 1 AMINO ACID 2 DIPEPTIDE

H2O

Peptide link

or bond

Remember: proteins are the most common

example of an amide. You may notice that the

peptide bond is the same as an amide bond.

The condensation reaction between a carboxylic

acid and amine is very difficult to achieve in the

laboratory. However in nature this occurs very

readily.

• The continuation of condensation reactions involving many

amino acids forms a long molecule called a polypeptide,

which contains many peptide bonds.

Proteins

Drawing polypeptide chains

+




Proteins

H2O


+




Proteins


+

EXISTING DIPEPTIDE




Proteins

H2O


+

EXISTING DIPEPTIDE




Proteins


+

EXISTING POLYPEPTIDE




Proteins

H2O


+





Proteins

H2O


+

… and these condensation reactions continue to form a long polypeptide chain.


Protein Structure

• The structure of proteins is described at four levels: primary,

secondary, tertiary and quaternary.

SECONDARY

TERTIARY QUATERNARY

PRIMARY

Protein Structure

Interactions in polypeptide chains

• Peptide bonds are polar. This allows interactions to occur both

within a polypeptide chain and between polypeptide chains or

molecules.

– The secondary structure of a protein refers to the shape the

polypeptide folds or twists into.

– Two main folding patterns: α-helix and β-pleated sheet

– Polypeptide chains are held in these configurations by hydrogen

bonding.

Protein Structure

α-helix β

Protein Structure




molecules.

– The tertiary structure of a protein refers to the folding that the

α-helix and β-pleated sheet exhibit.

– The tertiary structure is stabilised by a number of interactions:

• Ionic bonding

• Hydrogen bonding

• Dispersion forces

• Covalent bonding

Protein Structure




molecules.

– The quaternary structure of a protein refers to the 3-

dimensional arrangement of more than one polypeptide chain.

– Dispersion forces between non-polar R groups stabilize the

quaternary structure.

Protein Structure

Hydrogen bonding in polypeptide chains

• Hydrogen bonding can also occur between polypeptide chains or

molecules.

Hydrogen

bonding

between chains

Protein Structure

Hydrogen bonding in polypeptide chains

• Hydrogen bonding can also occur between polypeptide chains or

molecules.

Hydrogen

bonding

between protein

chains and water molecules

Protein Denaturation

• The structure of a protein and its biological functions are closely

linked. If the secondary, tertiary or quaternary structure of a

protein is altered, the protein loses it capacity to perform

biological functions.

• Altered structure and hence loss of function is referred to as

denaturation.

• Two factors commonly cause denaturation: pH and temperature


Unravelling of a protein

to the primary structure

(simple polypeptide chain). Function is lost.


All enzymes function at optimum temperature and pH conditions.

Effect of pH

• Increase or decrease in pH effects ionic bonding interactions,

causing structure destabilisation.

Effect of temperature

• Increasing the temperature causes secondary interactions

(dispersion forces, dipole-dipole interactions and hydrogen bonds)

to break, causing the entire protein to unravel to the primary

structure. – Decreasing temperature also impacts on the rate of enzyme activity.


Unravelling of the protein structure results in its active site being no

longer available to catalyse reactions.

pH or temperature

Rate of enzyme

activity

Maximum rate of activity

occurs when the

optimum conditions are

met. The optimum

conditions are unique for each enzyme.

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