Transcript Left Right - The Institute of Chemistry

On Left and Right:
Chirality from molecules to galaxies to Rembrandt
David Avnir
Institute of Chemistry
The Hebrew University of Jerusalem
WSCI, Jerusalem, 20.8.15
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1. Properties of chirality and its origin
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Kelvin's definition:
"I call any geometrical figure, or group of points,
chiral, and say it has chirality, if its image in a plane
mirror, ideally realized, cannot be brought to
coincide with itself.”
(Lord Kelvin, 1904, The Baltimore Lectures)
“Chiral”, from Greek χειρ (kheir), "hand",
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A positive definition
Chirality: The property of having for the same object a
left-form and a right-form
This left and right forms are mirror-images of each other;
they are termed enantiomers
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* Enantiomers are different objects,
but they look very similar
The similarity is because they are mirror-images of each other
The difference is that they cannot coincide with each other
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Pairs of chiral helical enantiomers which cannot coincide
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Chiral crystals - quartz
32- Left Helix
31- Right Helix
SiO4
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Labeling the left and right enantiomers
Common labels:
Left – Right
S - R
L - D
The enantiomers of amino acids
On our planet only L exists naturally – why?
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The enantiomeric pair of the same design
Left
Alexander McQueen
Right
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The two enantiomers of the Guggenheim Museum,
Bilbao, Frank Gehry
Which one is the left enantiomer and which one is the right one?
While chirality is an inherent structural property,
handedness is an arbitrary label
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Molecules and crystals that are not chiral - achiral
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These achiral structures have a common feature:
reflection symmetry
or mirror symmetry, or bilateral symmetry
Zeolite
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If there is mirror symmetry, the object coincides
with its mirror image (no enantiomers):
the object is achiral
No pair of enantiomers
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So why is it that for chirality we need to avoid mirror symmetry,
while for achirality we need that symmetry?
To understand it, let us first see why is this symmetry called “mirror
symmetry”:
Objects with that symmetry are built by reflection
Combining an object with its mirror reflection produces mirror
symmetry
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Another example:
Building an achiral object from (chiral) components in 2D:
a mirror line
An object
Its enantiomer
The combined pair
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So why does the reflection of an achiral object results in the same?
Because reflection of mirror symmetry preserves itself.
2D chirality – a mirror line
Slide one picture over the other
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2. Diastereomeric interactions
DS Interactions
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In general:
For chirality to be expressed the chiral object must interact with
another chiral object
If the other object is achiral – the chirality is not expressed.
Example:
Adsorption of the chiral
enantiomers of a chiral molecule
on an achiral gold nanoparticle
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Chirality is expressed when a chiral object interacts with another
chiral object:
Example A right hand is detecting the difference between a
right glove and a left glove
The two different interactions:
Rh-Rg
Rh-Lg
Comfortable vs. Very awkward
This difference is called diastereomerism (DS)
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Extreme case of DS: Bolts and nuts
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In the life-sciences chiral DS interactions are highly important
Reason: All biological receptors are chiral – they are built
from left-handed amino acids only!; therefore:
The interaction: Left-molecule
receptor
and the interaction: Right-molecule
receptor
are different
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Therefore: left-handed and right-handed molecules:
* Taste differently
* Smell differently
* Can heal or kill (Thalidomide)
Carvone
(R): Spearmint
(S): Caraway22
(kummel)
Left-handed and right-handed molecules:
* Can heal or kill (Thalidomide)
(R) – teratogenic
(S) - sedative
* Or can have different pharmaceutical indications
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“How would you like to live in a Lookingglass House, Kitty? I wonder if they'd give
you milk, there? Perhaps Looking-glass
milk isn't good to drink“
“Through the Looking Glasss, and What Alice
Found There”, Lewis Carrol (1871)
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Chiral perception interactions with the brain
* The left and right
hemispheres of the brain are
very unequal
* Therefore, no mirror
symmetry – the brain is chiral
Specifically: the brain is a •
chiral information receptor
Therefore, left and right objects must be perceived
differently by the brain
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Psychology of aesthetic perception
“When some pictures are mirror reversed, aesthetic evaluations
of them change dramatically.”
“When a painting is viewed in a mirror… even the meaning can
change…”
“ The first major finding… was that paintings containing left-toright directional cues were preferred…”
A. M. Mead and J. P. McLaughlin, Brain and Cognition, 20, 300 (1992)
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Rembrandt’s 2D-chiral preferences
N. Konstom,
“Rembrandt’s use
of models and
mirrors”,
Burlington
Magazine, 99, 94
(1977)
Saskia van Uylenburgh
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The big question:
Why does biology avoid DS interactions and works with only ONE
enantiomer building blocks (homochirality)?
Life is based only on left-handed amino acids. Why, and why not
the opposite?
And, when we make contact with aliens on distant planets,
will they have the same handedness?
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Temporary answers:
Answer 1.
Life can select its handedness (either left or right), but
it must be homochiral because of efficiency of lifeinvolved operations – DS adds complexity
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Temporary answer 2. The observed handedness on Earth is a
chance-event which selected one option by a positive feedback loop mechanism of initial fluctuations
One starts with a racemic mixture
A process
of partial
righthanded
enrichment
Racemic mixture
of gloves
Homochiral
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Temporary answer 3. The observed handedness on Earth is the
result of a chiral influence on the synthesis of the building blocks
Right-handed
and left-handed light
Neutron star
Black-hole
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3. Chirality and randomness
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A chiral object with random features
* What is its enantiomer?
* What is the handedness of that tree?
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Diffusion limited aggregates (DLAs)
A random walker
(drunken walker)
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DLAs are chiral (in 2D)
New concepts are needed to treat this type of chirality
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The chirality of a DLA is incidental
Nothing in its construction is associated with left or right
handedness
Inherent chirality
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The enantiomer of a DLA is virtual
It can never be constructed by repeating the process
The original DLA
and its virtual enantiomer
Real enantiomers
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Assignment of handedness:
The functional chirality solution
(a)“Right” and (b) “left” DLAs
Based on the differences in the diastereomeric interactions with a
small L and the mirror image-L
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Incidental and inherent chiralities can appear in the same object:
Spiral chiral DLAs
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The concept of real near enantiomers
Left
Right (real)
Right
(virtual)
Right (real)
Two(!) real right-handed near enantiomers
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A chiral object may have an infinite number of near
counter-enantiomers
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4. The measurement of chirality
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Different degrees of chirality
Low degree
and higher degree of chirality
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Different degree of chirality in design
Various degrees of chirality:
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Gradual changing 2D chirality in aggregates
O. Katzenelson
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How do you build a measurement tool?
The continuous symmetry measure
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S(G)  100  min
Nd 2
N
Q
k 1
k
ˆ
Q
k
2
Q k : The original structure
‫ צ‬: The nearest G-symmetric srtucture
Q
k
N : Number of vertices
d : Size normalization factor
* The scale is 0 - 1 (0 - 100):
The larger S(G) is, the higher is the deviation from G-symmetry
H. Zabrodsky Hel-Or
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The Continuous Chirality Measure
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Is there a relation between the degree of chirality
of a molecule and its ability to interact with a
receptor?
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Inhibition of acetylcholine esterase by
chiral organophosphates
S. Keinan
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Symmetry of handaxes of early man
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Symmetry of handaxes of early man
S(s) = 1.84
S(s) = 0.77
S(s) = 0.29
I. Saragusti
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Conclusion
Where does it all leave us?
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Be careful: One day you may encounter your own
enantiomer, and she/he may claim to be the real thing!
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