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27.1 Classification of Amino Acids

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Classification of Amino Acids 27.2 Stereochemistry of a-Amino Acids 27.1 Classification of Amino Acids 27.2 Stereochemistry of a-Amino Acids Classification of Amino ... – PowerPoint PPT presentation

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Title: 27.1 Classification of Amino Acids


1
27.1 Classification of Amino Acids
2
Classification of Amino Acids
amino acids are classified as ?, ?, ?, etc. to
indicate where the nitrogen atom is relative to
the carboxylic acid
3
Selected Amino Acids
1-Aminocyclopropanecarboxylic Acid an ?-amino
acid that is an intermediate in the
biosynthesis of ethylene
3-Aminopropanoic Acid a ?-amino acid that is
one of the structural units present in coenzyme A
4-Aminobutanoic Acid a ?-amino acid involved
in the transmission of nerve impulses
4
a-Amino Acids are Most Ubiquitous in Nature
5
Revision Amines are Brønsted Bases
Amines are electron-rich and have a reactive
lone pair, which can form a covalent bond to a
proton to form an ammonium ion.
6
Amino Acids Exist as Zwitterions
While their name implies that amino acids are
compounds that contain an amine (NH2) and a
carboxylic acid (-CO2H), these groups are
actually present as their conjugate acid (NH3)
and conjugate base (CO2), respectively.
7
Amino Acids Exist as Zwitterions
Resonance-Stabilized Zwitterion
8
Zwitterions Defined
Zwitterionic Compounds/Zwitterions neutral
compounds having formal unit electrical charges
of opposite sign. Some chemists restrict the term
to compounds with the charges on non-adjacent
atoms. Sometimes referred to as inner salts,
dipolar ions (a misnomer), e.g. H3N-CH2CO2-
ammonioacetate (glycine). IUPAC Compendium of
Chemical Terminology
9
Amino Acids Exist as Zwitterions
What evidence do we have for this behavior
10
Properties of Glycine Reflect its Zwitterionic
Structure
High Water Solubility glycine is soluble in
water but not in non-polar solvents. High
Melting Point when heated to 233C it decomposes
before it melts.
11
Properties of Glycine Reflect its Zwitterionic
Structure
The physical properties of glycine are
consistent with this structure
12
27.2 Stereochemistry of a-Amino Acids
13
Non-Polar Side Chains Alanine
Alanine (Ala, A)
Alanine, valine, leucine, and isoleucine have
alkyl groups as side chains, which are non-polar
and hydrophobic
14
Non-Polar Side Chains Valine
Valine (Val, V)
15
Non-Polar Side Chains Phenylalanine
Phenylalanine (Phe, F)
The side chain in phenylalanine (a non-polar
amino acid) is a benzyl group.
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