Stereoisomers and Molecular
Recognition
Isomerism
Same number, kind of atoms
Isomers
Different connectivity
Same connectivity but
different spatial arrangement
Stereoisomers
Structural
Isomers
Non-superimposable
mirror images
Geometric
Isomers
e.g., cis,trans
Optical
Isomers
Conformational
Isomers
Interconvert by sigma bond rotation
Optical Isomerism
Mirror Images
Mirror
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Can Mirror Images Be Superimposed?
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superimposable mirror images (not optical isomers)
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nonsuperimposable mirror images (optical isomers)
Optical Isomers Result When Center Carbon
Is Chiral (Has 4 Different Substituents)
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Same density, boiling point, and other physical properties.
BUT, different reactivities with other chiral molecules.
Lactic Acid
Optical Isomers of Lactic Acid
(Investigate This #5.62)
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Mirror Images, Non-Superimposable
Optical Isomer of Glucose?
(Check This #5.64)
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On one hexagon
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On second hexagon
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Put a colored dot at one apex to represent
ring O
Put a different colored dot at each apex
where -OH is oriented toward you
Put an “X” at apex with HO-CH2- group
Make mirror image of the first hexagon
Are the hexagons superimposable so that
all dots of same color match?
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Yes = same molecule
No = optical isomers
The Polarimeter
animation of polarimetry
Investigate This #5.59
#1
no sample
#2
no sample
#3
water
#4
corn syrup
filters
polarizing filters
positioned to allow
maximum light through
polarizing filters
polarizing filters
polarizing filters
positioned to allow ~no positioned to allow ~no positioned to allow ~no
light through
light through
light through (top filter
(no change from #2)
has been rotated from #2)
Molecular Recognition in Drug
Therapy
Dopamine in Neurotransmission
basal ganglia produce dopamine
Neuron
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No Dopamine = No Nerve Signals
Parkinson’s Disease:
Loss of Brain Dopamine
dopamine
receptors
normal
movement
movement
disorders
normal
movement
restored
normal
subject
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l-dopa therapy
replaces missing
dopamine
Parkinson
patient
Parkinson
patient
Parkinson’s Disease:
Loss of Brain Dopamine
dopamine
receptors
normal
movement
movement
disorders
normal
movement
restored
normal
subject
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L-dopa therapy
replaces missing
dopamine
Parkinson
patient
Parkinson
patient
Optical Isomerism in DOPA
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DOPA
L-DOPA
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D-DOPA
enzyme that
converts DOPA to
dopamine works
only with this
isomer
pharmaceutical
syntheses are
designed to avoid
making this
isomer
Optical Isomerism in DOPA
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Other Optical Isomers in Drug Therapy
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Penicillin
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Reacts with one optical isomer of alanine found in bacteria
Does not react with other optical isomer found in humans
Thalidomide
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One optical isomer effective against nausea (morning
sickness)
Other optical isomer causes birth defects
Many drugs are mixtures of optical isomers BOTH isomers must be tested for safety
Molecular Recognition in Vision
Cis-Trans Isomerism in Vision
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Rxn Occurs in Rhodopsin
Rhodopsin =
cis-retinal + opsin (protein)
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Cis-Trans Isomerism and Vision
photoisomerization of rhodopsin
photoisomerization of rhodopsin (same animation, alternate site)