Peptides

Question 1

What are amides a product of?

Answer 1

• Amides are a product of a condensation reaction between amines and carboxylic acids.
• The other product is water.

Question 2

Amide bond structure

Lone pair

Answer 2

• The nitrogen lone pair is conjugated to the C=O double bond creating a partial double-bond between N and C.
• Due to orbital constraints, no free rotation about the C-N bond.
• The group is planar
• Amide nitrogens are not nucleophillic

Question 3

Amide synthesis

Answer 3

• Amides are synthesised from activated analogues of carboxlyic acids
• Amines act as nucleophiles, acid derivatives as electrophiles.

Question 4

How do you calculate the isoelectric point (pI) of a peptide?

Answer 4

1. Write out the pKas from highest to lowest
2. Choose a pH below the lowest one and calculate the charge (negative, neutral or positive)
3. Choose a pH between the first and second lowest and calculate charge
4. Repeat
5. pI is the average of the pKa either side of q = 0
6. q = charge

Question 5

Amino acids are linked…

Answer 5

Amino acids are linked by amide bonds which have a fixed planar geometry and are quite stable.