lecture 6
DESCRIPTION
BIC1B Lecture 6TRANSCRIPT
• All objects have mirror images
• Some mirror images are IDENTICAL to the object (superimposable) – e.g. mugs
• Some mirror images are DIFFERENT form the object (non-superimposable) –e.g hands
Some molecules are non-superimposable mirror images of one another:
• They are called STEREOISOMERS
• They have the same covalent structure
• But different properties
• and different 3D structures
How do we know when a structure represents two molecules?
How can we represent a specific stereoisomer?
RULE: If a molecule contains a carbon atom with 4 different ‘substituents’, it will have a
non-superimposable mirror image
Such a carbon is called
• an asymmetric carbon
• a chiral carbon
• a chiral centre
GLYCINE ALANINE
R R
NO STEREOISOMERS 2 STEREOISOMERS
Chiral centre
Except for Glycine, ALL amino acids have 2 stereoisomers referred to as
D- and L-
How can we represent the L- or the D-form of an amino acid?
Write the structure in ‘Fischer projection’
The amino acids that we found in proteins are L-amino acids!
The properties of individual amino acids are determined by the side-chain (R)
You have to know the structures of all the amino acids!
When drawing amino acid structures, always make sure:
1.That each carbon is involved in 4 bonds
2.That each nitrogen is involved in 3 bonds (except for –NH3
+)
3. ‘Fill up’ with Hs
R
Carboxyl (–COOH or COO-)
Amino (-NH2 or NH3+)
(Primary amine)
Alpha carbon (C)
Beta carbon (C)
Gamma carbon (C)
Delta carbon (C)
Epsilon carbon (C)
Methyl group
(CH3)
GROUP 1: Non-polar, hydrophobic
amino acids
(aliphatic)
Indole ring
Phenol ring or benzene ring
GROUP 1: Non-polar, hydrophobic (S-containing and aromatic)
Carbonyl (C=O)
Hydroxyl (-OH)
GROUP 2: Polar, hydrophilic (uncharged)
Thiol or sulfhydryl
(-SH)
GROUP 2: Polar, hydrophilic (uncharged)
GROUP 3: Acidic,
Negatively charged
Imidazole ring
Guanidino group
GROUP 4: Basic,
Positively charged
Uncommon amino acids
Found in only a few proteins in connective tissue, e.g. collagen
Uncommon amino acids
Hormone produced by the thyroid gland