1. Chirality, life, and the periodic table
On left and right:
Chirality, life, and the periodic table
David Avnir
Institute of Chemistry
The Hebrew University of Jerusalem, Israel
“150 Mendeleev”, the 4th International Conference on the Periodic Table
St. Petersburg, Russia, July 26-28, 2019

2. 1. The periodic table of life

4. The periodic table written in Tengwar
Created by JRR Tolkien (Lord of the Rings, The Hobbit)
Used by the Elves of Middle Earth

5. Elements which form life
Without water
C = 64%
O = 13%
N = 7%
H = 7%
Ca= 4%
P = 3%
All others = 2%

6. The dominance of carbon
# Complex life requires hundreds of thousands of molecules with a huge variability in structure, function, and size (from 10 to 1 million atoms).
# An amazing feature of the periodic table of our universe:
Only one element – carbon – is capable of forming such huge library of molecules
Pepsine
Alanine
And it turns out that the vast majority of biomolecules have a common structural property –
they are chiral

7. 2. Chirality

8. Chirality: Definitions and properties
# Chirality is the structural property of having for the same object a left-form and a right-form
# The pair of objects with opposite handedness are termed enantiomers
(from Greek “enantios” – opposite)
# The enantiomers are mirror-images of each other which do not coincide with each other when overlapped
χεῖρ (kheir)
hand in Greek

9. Enantiomers are similar and different at the same time
The similarity is because they are mirror-images of each other
The difference: they cannot coincide with each other
Regular right-handed screw
(on the right), and a
virtual left handed screw
Alexander McQueen

10. they do not have mirror symmetry
It is easy to identify chiral objects –
they do not have mirror symmetry
Achiral objects – they are mirror symmetric and they coincide with their reflection - No pair of enantiomers
In contrast: the enantiomers of an amino acid

11. Chiral objects – no plane of symmetry, no real enantiomers

12. 3. Molecular chirality and the life-essential elements

13. Molecular chirality: A structural signature of Life
* The majority of life-associated molecules are chiral
* Practically, all are based on carbon, with additional elements attached to it.
* Each of the two enantiomers functions differently
* The abundance of the two enantiomers is not equal – commonly only one prevails

14. The dominance of carbon based biomolecules
Carbon is so common that it usually not drawn explicitly
Testosterone – a hormone
Vitamin C
Left handed amino acids dominate
Limonene, the smell of lemon
Sucrose, the common sugar
Right handed sugars dominate
Taxol
a natural anticancer drug
All are chiral!

15. (R): Spearmint (S): Caraway
Left-handed and right-handed molecules:
* Taste differently
* Smell differently
* Can heal or kill (Thalidomide)
Carvone
(R): Spearmint (S): Caraway
(kummel)

16. The different activity of enantiomers: Chiral interactions
All biological receptors are chiral and of fixed handedness –
they are built from left-handed amino acids only!; therefore:
The interaction: Left-molecule with a chiral receptor
and the interaction: Right-molecule with the same receptor are different
And, each enantiomer may interact with different receptors
Glove:
The chiral receptor
Hands:
The left handed and right handed molecules

17. The periodic table beyond carbon:
Chirality associated with the other life essential elements
Phosphorous: DNA
Nitrogen: Morphine
Iodine: Thyroxine hormone
Sulphur:
Alliin - a garlic compound 17

18. The metallo-enzymes: Chiral environments
Metals are late arrivals to the periodic table
Zinc: Carbonic anhydrase, processing of CO2
Copper: Plastocyanin, involved in electron-transfers 18

19. 4. Chirality in non-essential elements which support life

20. Milk vs looking glass milk
“How would you like to live in a Looking-glass House, Kitty? I wonder if they'd give you milk, there? Perhaps Looking-glass milk isn't good to drink“
“Through the Looking Glass, and What Alice Found There”, Lewis Carrol (1871)
The natural right handed milk sugar
and its unnatural enantiomer

21. Left-handed and right-handed forms of a drug molecule may react differently
* Can heal or kill (Thalidomide)
(R) – teratogenic (S) - sedative
* Or can have different pharmaceutical indications

22. Most of the chemical processes from the two enantiomers
Chiral drugs which must be handedness-pure
Problem:
Most of the chemical processes from the two enantiomers
in equal proportions

23. Solution: Chiral catalysts with non-essential elements from the periodic table
Reactant molecules product molecules
Most chiral catalysts are based on
non-essential metallic elements
M = Ir, Rh
Each catalyst provides a chiral environment of specific handedness

24. Examples from the 2001 Nobel Prize in Chemistry
The Monsanto synthesis of L-Dopa (1968)
Chiral rhodium compound
William S. Knowles

25. Noyori's catalyst in synthesis of )S(-naproxen
Chiral ruthenium compound
Ryoji Noyori

26. 5. Three concluding questions

27. I. So why has chirality become so central in the evolution of life?
It is easier to form chiral molecules compared to achiral ones
Achiral molecules must have mirror symmetry;
that is a strong structural restriction on molecular shape.
Achiral molecules

28. The enigma of the dominance of one enantiomer over the other
So why has chirality become so central in the evolution of life?
B. Evolutionary prebiotic chemistry is characterized by increase in the complexity of molecules –
The more complex, the more rare it is to have mirror symmetry.
Equal amounts of the two enantiomers
(the similarity of enantiomers)
H-H
The enigma of the dominance of one enantiomer over the other

29. Perhaps we are less important and central than we think…
II. Why such a small proportion of the elements is used to form life?
Perhaps we are less important and central than we think…
The (weak) anthropic principle:
Only in a universe capable of eventually supporting life will there be living beings capable of observing and reflecting on the matter.

30. Carbon is an element in the periodic table that is aware of the periodic table, and is aware of itself.

31. Carbon is an element in the periodic table that is aware of the periodic table, and is aware of itself.
III. Is there in the universe life and intelligence which is not carbon-based?
Silicon based life
Julius Scheiner, astronomer, Berlin, 1891

32. a huge playground for the periodic table!
Hubble:
0.1 moon size
UV to near IR
10,000 galaxies
2014
The cosmos –
a huge playground for the periodic table!