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Highlights in Chemical Science

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Is water the answer to nature's handedness?


23 January 2006

Water molecules cause biological systems to prefer left-handed chiral centres, say scientists from Israel and the US. 

All amino acids found in living organisms have left-handed chirality, even though left- and right-handed amino acids should chemically be the same. Meir Shinitzky and colleagues at the Weizmann Institute of Science believe the humble water molecule to be responsible. 

 

                         in the hands of water


© iStockphotos

 

Shinitzky dissolved polymers of left- and right-handed amino acids in water, and measured the pH at which they denatured from helices to random coils. This was 0.2 to 0.3 units higher for the right-handed molecules, which implied they were less stable. The effect disappeared when heavy water (D2O) was used.

The group discounted undetectable impurities in the water as the cause. 'In most of our experiments with heavy water, where the differences are virtually eliminated, 20 per cent light water was still present,' said Shinitzky.

"The ultimate challenge is the possibility that the function of a mirror-image biological system would be inferior to its natural counterpart"
- Meir Shinitzky
 Water molecules exist in two forms, ortho-H2O and para-H2O. The interchange between these forms in liquid water is very slow, taking minutes, so an average form is not established. There is also a tiny difference in the magnetic fields of left-handed and right-handed chiral centres. 

Shinitzky's group suggests that the different interactions between the weak magnetic field of ortho-H2O and the magnetic fields of left- and right-handed amino acids is sufficient to account for the pH measurements. 

This is a controversial explanation, because the energy difference between molecule types is tiny yet the effect on denaturing is huge according to Jeanne Crassous at the University of Rennes, France. Crassous said she would like to see the results and the role of water impurities corroborated by another research group. 

Shinitzky's next step is to compare the structures and catalytic activities of left- and right-handed biological molecules. 'The ultimate challenge is the possibility that the function of a mirror-image biological system would be inferior to its natural counterpart,' he said. 

Colin R Batchelor

References

Y Scolnik, I Portnaya, U Cogan, S Tal, R Haimovitz, M Fridkin, A C Elitzur, D W Deamer and M Shinitzky, Phys. Chem. Chem. Phys., 2006, 8, 333 (DOI : 10.1039/b513974k)