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The synthesis of Dilantin also involves imines (expt 7): + H Ph + H OH H2N O + + H O H2N O Ph Ph H N O Ph OH N H Note the two + C's are adjacent + H H N Ph Ph Ph O N H Dilantin N H OH H N Ph O Ph N H Ph O N O + HO Ph N O Ph N Imines in putative prebiotic synthesis of histidine: N H O H 2 NH3 + + H O H2N N H H O H HOOC + H O H O + H H NH H NH 2 NH3 O H OH NH + N N H + H N N H N N tautomerize NH N N - Imidazole (aromatic) N N H O ALDOL- like + H O H OH OH H H H N dehydration H O O O N N N H N H N + H NH3 H H N HOOC NH2 N H N NC NH2 N H Histidine Strecker synthesis H N NH N H • Interestingly, AA’s 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 AA’s (isovaline → most abundant) • Components found in Urey-Miller Exp’t!! – 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 AA’s were racemic, but some did show slight enantiomeric excess (L) (1-15%) – there is enrichment! There had to be a natural process that separated & concentrated one enantiomer over the other → chiral selection Mechanism of enrichment? a) Circularly polarized light from stars: • • • b) 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” • Hazen exp’t: – Immersed large crystal of calcite in a dilute solution of 50:50 D,Laspartic acid – GC analysis found that calcite absorbs different enantiomers on different surfaces • 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) i. Serine octamer • • • • • • forms a non-covalent homochiral octamer in a mass spectrometer via electrospray ionization Octamer was found to be chiroselective—formed 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 AA—providing 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 Serine Octamers ii. Enrichment by sublimation • • • • Took mixtures of AA’s (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