Nitrogen Compounds

1. Nitrogen Compounds
Amines/ Amides/ Amino Acids/ Proteins

2. How do I assign pK
a
i.e which –COOH is stronger?
pK
a1
= 2.23
pK
a2
= 4.42
pK
a3
= 9.95
Amino Acid

3. pK
a3
= 9.95
pK
a1
= 2.23
pK
a2
= 4.42
Amino Acid
–COOH closer to NH
3
+
is the stronger acid.

4. Reason:
-Recall factors which affects the acidity of org compounds
 stability of conjugate base
-–NH
3
+
is an electron withdrawing group; – COOH closer to
it will form a more stable conjugate base since the –ve
charge the O atom will be more dispersed.
Amino Acid

5. Amino Acid
How do I assign pK
a
i.e which –NH
3
+
is stronger?
pK
a1
= 2.04
pK
a2
= 9.08
pK
a3
= 10.69

6. NH
3
+
closer to –COOH is the stronger acid.
pK
a3
= 9.95
pK
a1
= 2.23
pK
a2
= 4.42
Amino Acid

7. Reason:
–COOH is an electron withdrawing group; –NH
2
closer will
be a weaker base since its lone pairs are less available for
dative bonding to H
+
 stronger acid
Amino Acid

8. -Definition of 1⁰/2⁰/3⁰/4⁰ structure
-Drawing of α-helix and β-pleat
-Protein sequencing
-Account for denaturation
Proteins

9. 1⁰/2⁰/3⁰/4⁰ structure
1.Description
2.… bonds between ____ and ____
2. is often missed out in students’ answers
Definition

10. Enzyme cleavage
-Selective
-No overlapping fragments
-Need 2 sets of data fr
different enzymes
Acid/ Base hydrolysis
- Non selective
- Overlapping fragments
1 2
Digestion of Proteins

11. Sequencing Approach
Enzyme cleavage
- determine terminal
residues; common in 2 sets
of data
- spot repetition between
the sets of data.
Acid/ Base hydrolysis
- spot repetition of residues
in fragments
1 2
Digestion of Proteins

12. Note:
To digest a protein, reflux with aq HCl or H
2
SO
4
or NaOH
for a few hours.
Digestion of Proteins