The synthesis of Dilantin also involves imines (expt 7):

1

• The synthesis of Dilantin also involves imines
  (expt 7)

2
Imines in putative prebiotic synthesis of
histidine
3
Strecker synthesis
4

• Interestingly, AAs have been detected in space
  Murchison Meteorite
• Murchison, Australia (1969)
• Contained noble gases insoluble material such
  as graphite silicates
• Also contained several organics
• Dicarboxylic acids, alkanes amino acids
• Contained gly, ala, glu non-proteinegenic AAs
  (isovaline ? most abundant)
• Components found in Urey-Miller Expt!!
• Origin in space?
• Isotopic distribution indicates amino acids were
  extraterrestrial in origin
• i.e., Natural abundance of 15N is 0.37,
  however, meteorites were found to have 50 to
  93
• Majority of AAs were racemic, but some did show
  slight enantiomeric excess (L) (1-15)
• ? there is enrichment!

5

• There had to be a natural process that separated
  concentrated one enantiomer over the other ?
  chiral selection
• Mechanism of enrichment?
• Circularly polarized light from stars
• This CPL is in the UV IR range is chiral
• ? CPL can form or destroy the two enantiomers of
  an AA at different rates ? asymmetric photolysis
• Could have led to enrichment of L-amino acids in
  meteorite
• Selection by crystal faces
• Most minerals are centric ? do not display
  handedness
• Calcite, CaCO3, (exception) displays surfaces
  that have a mirror relationship ? chiral-like

6

• Hazen expt
• Immersed large crystal of calcite in a dilute
  solution of 5050 D,L-aspartic acid
• GC analysis found that calcite absorbs different
  enantiomers on different surfaces

7

• Enantioenrichment?
• If one face proceeds forward, while the other is
  chemically inert, then we get enantioenrichment
• i.e., one face is exposed to light or one face
  is immersed (by chance) in water
• Does calcite promote amino acid chain
  formation?
• Whatever the origin of homo-chirality, the ee was
  likely low
• However, once one AA is present in excess, then
  enantioenrichment can occur
• Via Serine octamer
• (Cooks et al,. Angew. Chem. Int. Ed., 2003, 42,
  3521)
• Enrichment by sublimation
• (Feringa et al,. Chem. Commun., 2007, 2578)

8

• Serine octamer
• forms a non-covalent homochiral octamer in a mass
  spectrometer via electrospray ionization
• Octamer was found to be chiroselectiveformed
  from enantiopure samples, but not racemic ones!
• ? one L-serine selects to bind with 7
  more L-enantiomers
• Also found that they could incorporate more than
  one type of AAproviding that all of the amino
  acids had the same chirality
• Additionally, octamer forms adducts
  enantioselectively with D-glyceraldehyde ? could
  help explain relationship between L-amino acids
  in proteins D-sugars as the dominant species in
  nature!
• Serine cluster also catalyzed dimerization of
  glyceraldehyde giving a C6 sugar
• Cluster also found to bind to PO43- and some
  metals

9
Serine Octamers
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• Enrichment by sublimation
• Took mixtures of AAs (leu, ala, phe, etc.) with
  low ee (9) partially sublimed sample
• Results showed that in each case there was
  enrichment of the enantiomer (20-80)!
• Indicates that a heat source may suffice for
  enantiomeric enrichment
• Meteorites could be subjected to high
  temperatures that could result in enrichment