Amino acids, amides and chirality

Question 1

What is an amino acid?

Answer 1

An organic compound contain both amine NH2 and carboxylic acid COOH functional groups

Question 2

How many different amino acids are there in the bodying what are all these amino acids?

Answer 2

• The body has 2- common amino acids that can be built into proteins
• These amino acids are all alpha amino acids in which the amine groups is attached the alpha carbon atom (the second carbon atom, next to the carbonyl group)
• These 20 amino acids differ by the side chain, R attached the the same alpha carbon atom

Question 3

Describe the structure of an alpha amino acid

Answer 3

All amino acids contain a basic amine group and an acidic carboxylic group and each amino acid has a different R group

Question 4

What is the general formula of an alpha amino acid?

Answer 4

RCH(NH2)COOH

Question 5

Are there other types of amino acids?

Answer 5

There are also less common amino acids in which the amine group is connected to the beta carbon atom (the third carbon) and the gamma carbon atom (the fourth carbon atom)

Question 6

To what is amino acids have similar reactions to and why?

Answer 6

-Since amino acids have both acidic COOH and basic NH2 functional groups, amino acids age similar reactions to both carboxylic acids and amines

Question 7

Describe the reaction of the amine group in amino acids

Answer 7

• The amine group is basic and reacts with acids to form salts
• The amino acid alanine (2-aminopropanoic acid) react with HCL acid to form an ammonium salt

Question 8

Describe the reaction of the carboxylic acid group in amino acids

Answer 8

The carboxylic acid group can react with alkalis to form salts and with alcohols to form esters

Question 9

Describe the reaction of an amino acid with aqueous alkali

Answer 9

• An amino cid reacts with an aqueous alkali such as sodium or potassium hydroxide to from a salt and water
• Glycine (aminoethanoic acid) reacts with sodium hydroxide to form a sodium salt

Question 10

Describe the esterification of amino acids with alcohols

Answer 10

• Amino acids, like carboxylic acids are easily esterified by heating with an alcohol in the presence of concentrated sulphuric acid
• The alpha amino acid serine is reacted with excess ethanol and a small amount of sulphuric acid
• The carboxylic acid is esterfied producing an ester

Question 11

What does the acidic condition in the esterifcaiton of amino acids with alcohol do?

Answer 11

The acidic conditions protonate the basic amine group of the ester

Question 12

What is a zwitterion?

Answer 12

Within the structure of the amino acid, the basic amine group can accept a proton from the carboxylic acid group to form an ion containing both a positive and negative charge and this ion is known as a zwitterion

Question 13

What charge do zwitterions have and why?

Answer 13

Zwitterions have no overall charge because the positive and negative charges cancel out

Question 14

What is the isoelectric point?

Answer 14

It is the pH at which the zwitterion is formed and each amino cid has its own unique isoelectric point

Question 15

What happens if an amino acid is added to a solution with a pH greater than its isoelectric point?

Answer 15

The amino acid behaves as an acid and loses a proton

Question 16

What happens if an amino acid is added to a solution with a pH lower than its isoelectric point?

Answer 16

The amino acid acts as a base and gains a proton

Question 17

What are amides?

Answer 17

• Amides are the products of reactions of acyl chlorides with ammonia and amines
• Amide groups are common in nature e.g. in proteins the amine and carboxylic groups of amino acids are bonded together to form amide groups

Question 18

How are amides classed?

Answer 18

1. Primary: one carbon atom bonded to N (e.g. propanamide)
2. Secondary: two carbon atoms bonded to N (e.g. N-methylethanamide)
3. Tertiary: three carbon atoms bonded to N (e.g. N,N-dimethylmethanamide)

Question 19

What are stereoisomers?

Answer 19

Compounds with eh stem structural formula but. different arrangement of atoms (another type of stereoisomerism is optical isomerism)

Question 20

What is an optical isomerism?

Answer 20

It is found in molecules that contain chiral centres

Question 21

What is the chiral centre?

Answer 21

A carbon atom that is attached to four different atoms or group of atoms

Question 22

What does the presence of a chiral carbon atom lead to?

Answer 22

• The presence of a chiral carbon atom in a molecule leads to the existence of two non-superimposable micro image structure, these two molecules are known as optical isomers or enantiomers
• For each chiral carbon atom, there is always one pair of optical isomers

Question 23

Describe chirality in alpha amino acids

Answer 23

• Chiral carbon atoms exist widely in naturally occurring organic molecules e.g. all sugars, proteins and nucleic acids contain chiral carbon atoms
• With the exception of glycine H2NCH2COOH, all alpha amino acids RCH(NH2)COOH contain a chiral carbon atom, with he alpha carbon atom show labelled with an asterisk

Question 24

Is chirality only for carbon atoms?

Answer 24

• Chirality is not just reserved for carbon atoms, and the term applies to any centre that holds attachments that can be arranged as two non-superimposable mirror image forms
• Like a pair of hands, optical isomers can be considered as right and let handed forms and one optical isomer chant be superimposed Upton the other

Question 25

How do you draw optical isomers?

Answer 25

• Optical isomers are drawn to show the 3D tetrahedral arrangement of the four different groups around the central chiral carbon atom
• Once one isomer has been drawn, the other isomer is drawn as mirror image, reflecting the first structure

Question 26

What is alpha amino acid?

Answer 26

Both NH2 and COOH are joined to the same Carbon