Amino Acids & Chirality

Question 1

What is the general formula of an amino acid?

Answer 1

RCH(NH2)COOH

Question 2

What are alpha - amino acids?

Answer 2

Amino group and carboxylic acid group are attached to the same carbon atom.

Question 3

Amino acids are BIFUNCTIONAL. Explain what this means.

Answer 3

They have two functional groups present:

• NH2 (base)
• COOH (acid)

Question 4

What is an acid?

Answer 4

A proton donor (gives a H+ ion)

Question 5

What is a base?

Answer 5

A proton acceptor.

Question 6

What is a zwitterion?

Answer 6

An ‘internal’ salt of an amino acid, in which the -COOH group donates a proton to the -NH2 group forming NH3+ & COO-.

Question 7

What effect does a zwitterion have on the properties of an amino acid?

Answer 7

Ionic charges increase the attractive forces between the amino acid in the solid, so raising the melting point significantly.

Question 8

What is formed when amino acids react with acids or bases?

Answer 8

A salt.

Question 9

At a low pH, what is the charge of a zwitterion?

Answer 9

Positive.

Question 10

At a high pH, what is the charge of a zwitterion?

Answer 10

Negative.

Question 11

What is the definition of an isoelectric point?

Answer 11

When the solution of an amino acid has the maximum concentration of zwitterions.

Question 12

How are peptide bonds formed?

Answer 12

Via condensation reactions (condensation polymerisation).

Question 13

How an you represent a peptide link?

Answer 13

-CONH- or -NHCO-

-C=O -N-H
| |
N-H- -C=O

Question 14

What happens if you repeat condensation reactions many times?

Answer 14

You get polypeptides or proteins.

Question 15

What is hydrolysis?

Answer 15

The process of breaking the peptide link by a reaction with water.

Question 16

What is the catalyst for hydrolysis (living and lab)?

Answer 16

They are catalysed by enzymes in living organisms.

In the lab, they are catalysed by acids or alkalis.

Question 17

What does an a-amino acid and an excess alkali react to form?

Answer 17

Carboxylate salts e.g. In case of Sodium, on COOH group COO-Na+ formed.

Question 18

What is stereoisomerism?

Answer 18

When atoms have the same order of bonds and atoms but a different spatial arrangement of atoms.

Question 19

What is the difference between E and Z isomerism?

Answer 19

E - opposite sides

Z - same side

Question 20

What are optical isomers?

Answer 20

Molecules that are non-superimposable mirror images of each other.

Question 21

What is a chiral centre?

Answer 21

A (carbon) atom in a molecule attached to 4 different groups.

Question 22

How many optical isomers are formed and what are the differences between them?

Answer 22

2:
• 1 rotates plane polarised light to left
• the 2nd rotates plane polarised light to right

Question 23

What is a equal split of enantiomers called?

Answer 23

A racemic mixture.

Question 24

What can’t a racemic mixture do?

Answer 24

Deflect polarised light.

Question 25

How do drugs work?

Answer 25

Drugs work by binding to enzymes active sites or cell surface receptors causing a chemical reaction. Drugs must have the correct shape to fit into binding site - only one enantiomer will do. The other enantiomer may bind to another active site and have no effect, or cause a harmful side effect.

Question 26

Why would pharmaceutical companies make drugs with only one optical isomer?

Answer 26

* smaller doses can be used (only 1/2 amount) * fewer side effects * drug is more efficient and effective * reduces risk of litigation * this increases costs as optical isomers are difficult and expensive to seperate

Question 27

What methods can you use to produce single enantiomers?

Answer 27

* using natural enzymes or bacteria * using naturally occurring single optical isomers compounds as the starting material * using chemical chiral synthesis * using chiral catalysts - reusable but $$$

Question 28

Describe the properties of proteins.

Answer 28

* polypeptides with high molecular mass * chains can be lined up with each other * the C=O and N-H bonds are polar due to difference in electronegativity * hydrogen bonding exists between chains

Question 29

What are the reagents and conditions for a hydrolysis reaction?

Answer 29

Reagents: aqueous acid (H2SO4)/ alkali (NaOH) Conditions: warm/hot, reflux

Question 30

When you are given a chemical with chloride and you're asked for a reagent that would make it turn to an amino acid, what is this reagent?

Answer 30

(ethanolic) ammonia