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