1 / 16

Chirality and Stereoisomers

Chirality and Stereoisomers. Isomers and Stereoisomers. Isomer : different compound with same molecular formula Structural or Constriutional Isomers : different structures. Propan-1-ol Propan-2-ol Methyl ethyl ether.

minowa
Download Presentation

Chirality and Stereoisomers

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Chirality and Stereoisomers

  2. Isomers and Stereoisomers • Isomer: different compound with same molecular formula • Structural or Constriutional Isomers: different structures Propan-1-ol Propan-2-ol Methyl ethyl ether • Stereoisomers: same bonding structure, but different geometrical positioning • “Spatial” isomers • Divided into two classes • Enantiomers: non-superimposiblemirror images • Diastereomers: NOT mirror image Lactic acid

  3. Two classes of Diastereomers • Cis-trans isomers • Caused by rigid structure of molecule (double bond) cis-but-2-ene trans-but-2-ene • Confomers or conformal isomers • Change form by rotation about a single bond • Energy Barrier to rotation • Molecule can get “locked” in one structural form • Also called “Rotamers” Butane: Newman projection

  4. “Chiral” molecule: one with non-superimposible mirror image: • An enantiomer • Usually due to an sp3 hybridized carbon (asymmetric carbon) • Four different groups attached • Only other atoms that are chiral: certain metals (octahedral complex) • Diastereomers: different physical and chemical properties • Enantiomers: very similar properties • Differ only in 1) how they rotate polarized light • Rotate in clockwise or counterclockwise directions • Also differ in 2) how they interact with other chiral molecules • Significant effect in biology

  5. Properties of Enantiomers • Exhibit “optical activity” • Rotates plane of polarization of light • Not all compounds can do this, i.e. optically inactive • “Racemic” mixture of enantiomers also optically inactive • Racemic: same amounts of both isomers • (-) isomer rotates in counter-clockwise direction: levorotary • (+) iomer rotates in clockwise direction: dextrorotary • Separation of racemic mixtures • Only be done with other optically active compound • Enantiomeric excess or “ee”: measure of excess of one isomer vs. the other • 40% ee in one isomer (R): 60% of mixture is racemic • Total amount of R is 70%

  6. Nomenclature • R and S designation • Each of 4 groups given a priority • Group having atom of higher atomic number has higher priority • If tie, consider next atom from the stereocenter, then 3rd atom, etc. • Point lowest priority group away • If 1-2-3 ordering is clockwise: R (latin: rectus) • If 1-2-3 ordering is counter-clockwise: S (latin: sinister) • Not mapped into (+), (-) or L/D

  7. (+) and (-) designation: optical characterization • If light traveling towards viewer, clockwise rotation is + or d • If light traveling towards viewer, counterclockwise is – or l • D/L designation: based on glyceraldehyde • Labeled according to which form of glyceraldehyde its derived • D-glyceraldehyde has + (dextrorotary) rotation • L-glyceraldehyde has - (levorotary)rotation • If molecule looks like D (L)-glyceraldehyde, then D (L) form • If amino acid, look at COOH, -R, NH2 and H • H atom pointed away, COOH, R, amine go clockwise: D • COOH, R, amine go counter-clockwise: L Glyceraldehyde L- Glyceraldehyde L-alanine

  8. D-glyceraldehyde L-glyceraldehyde L-alanine

  9. Importance in Biology • Biological systems: amino acids exclusively L-form in Proteins • Sugars are exclusively D-form in RNA, DNA • Source of homochirality: UNKNOWN • Biological systems sense difference • Spearmint leaves contain L-carvone • Caraway seeds D-carvone • Caraway and spearmint taste differently • Left and Right-handed proteins: L and D amino acids • Enzymes have specific active sites • Molecule upon which an enzyme acts: substrate • Only one chiral form of substrate “fits” • Only one chiral form generated by enzyme • Muscle tissue converts pyruvic acid: (+)-lactic acid only

  10. Active Site: cleft or pocket lined by amino acid residues • Pocket defined by hydrogen bonds, hydrophobic interactions, • temporary covalent interactions (van der Waals) • Amino acids recognize substrate and can participate in enzyme reaction COOH O=C CH2 Pyruvic acid (+) lactic acid

  11. Origin of Chirality in Biological Systems • Subject of great debate • Result of polarized light in interstellar space • Interaction of magnetic fields • Clay surfaces with pockets for reactions that favor R or S • Quantum mechanical basis ? • Where else are chiral compounds found ? • Cannot distinguish with molecular spectroscopy • Use polarized light or chemical analysis • Difficult for remote sensing • Have observed ee in meteorites • Compounds extracted from several • carbonaceous chondrites • ee found in 8 amino acids, one hydroxy acid (Pizzarello and Groy 2011)

  12. MN = Murchison MY = Murray OR = Orgiel • Excesses 3 –12% • L-lactic acid has highest

  13. Is Technique Reliable ? • GCMS = Gas chromatography with Mass Spectroscopy • Inert carrier gas: helium, nitrogen • Inject liquid sample into gas • Gas carriers mixture through a packed column • Column heated: liquid remains in gas phase • Separation of compounds: detected with mass spectrometer

  14. Source of EE in Meteorites ?? • Asymmetric photolysis of amino acids by circularly-polarized UV light • Experiments show this possible, but much lower ee • Inorganic matrices in meteorite surface might favor one form • Synthesize a chiral compound in the lab: racemic mixture • Non-racemic mixtures will racemize over time • Measurements in meteorites only findings of ee outside biosphere • Suggest connection of life to interstellar space

More Related