nature and origin of life lecture 7 chirality and stereoisomers

Nature and Origin of Life Lecture 7

Chirality and Stereoisomers

http://en.wikipedia.org/wiki/File:Murchison_crop.jpg


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-superimposible mirror images

• Diastereomers: NOT mirror image Lactic acid



• 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



• “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



• 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%

Properties of Enantiomers



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



• (+) 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

GlyceraldehydeL- Glyceraldehyde L-alanine



D-glyceraldehydeL-glyceraldehyde

L-alanine



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



• 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

O=CCH2

COOH

Pyruvic acid (+) lactic acid



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)



MN = MurchisonMY = MurrayOR = Orgiel

• Excesses 3 –12%

• L-lactic acid has

highest



• 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

Is Technique Reliable ?



• 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

Source of EE in Meteorites ??


nature and origin of life lecture 7 chirality and stereoisomers

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