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Chapter 27: Amino Acids, Peptides and Proteins |
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Amino Acid Stereochemistry
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Stereochemistry was introduced in Chapter
7 and most recently revisited for carbohydrates
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Here we will look at Fischer projections,
the D-, L- notation of amino acids
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It's a good idea to review the basics of these topics if you do remember
them before continuing.
Fischer projections are commonly used to represent amino acids.
Recall
that Fischer projections are typically drawn with the longest chain oriented
vertically
and with the more
highly
oxidised C at the top.
For the 20 α-amino acids that occur naturally
in proteins, if we focus on the α-center, a
chirality center, and draw the Fischer projection putting the -CO2H
group up, then the ammonium group, NH3+, will be on the left,
making it like L-glyceraldehyde where the -OH is on the left (review
?).
Hence, we have the L-amino acids. You can check this out
using the 3D-JMOL images given below:
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alanine
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tryptophan
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S-(-)-glyceraldehyde
or
L-glyceraldehyde
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QUESTIONS
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Which of the common amino acids is achiral ? ANSWER
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R- and S- nomenclature is also used to
define configuration at chirality centers. Are the L-amino acids
R- or S- ? ANSWER
NOTE : Although only the L-amino acid series are incorporated into
natural proteins, the D-amino acids also occur naturally.
