Biological Chemistry - Amino Acids (9.1)

Question 1

Why are amino acids important in life?

Answer 1

They are central to many chemical processes within the human body

Question 2

What are alpha amino acids?

Answer 2

They are amino acids that have -NH2 group & -COOH group attached to the same carbon

Question 3

What are essential amino acids?

Answer 3

They are amino acids that the body cannot synthesise itself - so they have to be obtained from food

Question 4

Name at least 3 properties of amino acids

Answer 4

• Are amphoteric
• Form zwitterions in the solid state
• Have high m.p & b.p
• Generally soluble in water
• Have an isoelectric point
• Usually contain at least 1 chiral centre

Question 5

What does it mean when a molecule is ‘amphoteric’?

Answer 5

An amphoteric molecule can act as both an acid and a base

Question 6

Why are amino acids amphoteric?

Answer 6

They contain acidic & basic functional groups

Question 7

Describe what happens when you add an alkaline solution to amino acids

Answer 7

• The carboxylic acid group can act as an amino acid

- The -COOH group donates H+ ions to form the carboxylate ion -COO^-

Question 8

Describe what happens when you add an acidic solution to amino acids

Answer 8

• The amine group can act as a base

- The lone pair on the N can accept a H+ ion

Question 9

What is the effect of amophoteric properties on amino acids? (What does this allow to happen?)

Answer 9

• In the solid state, amino acid molecules react in acid-base reactions w/ neighbouring molecules
• One amino acid donates H+ to form -COO^- group & the other amino acid can accept it to form -NH3+

(One acts as an acid, one acts as a base)

Question 10

What do amino acids form in the solid state?

Answer 10

Zwitterions

Question 11

What is a zwitterion?

Answer 11

An ion that contains a region of positive & negative charge in the same molecule (overall neutral)

Question 12

Explain why amino acids have high melting & boiling points

Answer 12

• There are strong electrostatic forces of attraction between molecules (zwitterions)
• These intermolecular forces require a large amount of energy to be broken

Question 13

What is meant by the ‘isoelectric point’?

Answer 13

The pH at which the amino acid exists only as species w/ zero net charge

Question 14

What does an amino acid at the isoelectric point look like?

Answer 14

A zwitterion

Question 15

In what conditions are amino acids below the isoelectric point? What happens to the zwitterion (amino acid)?

Answer 15

• In acidic conditions

- The H+ protonates -COO^- to -COOH

Question 16

In what conditions are amino acids above the isoelectric point? What happens to the zwitterion (amino acid)?

Answer 16

• In alkaline conditions

- H+ is taken away from -NH3+ to form -NH2

Question 17

Why are amino acids generally soluble in water?

Answer 17

• They form zwitterions in solution & can form strong ion-dipole bonds w/ water molecules

Question 18

What affects the degree of solubility of amino acids?

Answer 18

• The identity/type of side chain

- Properties of the side chains

Question 19

What types of group decrease the solubility of an amino acid?

Answer 19

• Uncharged groups

- Non-polar side chains

Question 20

Wha types of group increase the solubility of an amino acid?

Answer 20

• Polar side chains

- Charged side chains

Question 21

What makes an amino acid more soluble? A polar side chain or a charged side chain?

Answer 21

A charged side chain - can form stronger intermolecular forces with oppositely charged groups

Question 22

How does the length of the amino acid affect its solubility?

Answer 22

If two amino acids were exactly the same in terms of the types of side chains but one was longer, the longer amino acid would be less soluble

Question 23

Why are longer, non-polar amino acids less soluble?

Answer 23

It is energetically unfavourable for water molecules to form London forces w/ non-polar parts of the molecule (more energetically favourable for water molecules to form H bonds with one another)