Saturday, July 18, 2009

Going Right-handed

Why we are made of only right-handed sugars? That’s long been one of the biggest puzzles in understanding how life began, and this origin of homochirality in sugars and amino acids have been intrigued researchers for decades. So for, convincing theory and experiment on the origin of homochirality are still lacking.

Armando Co´rdova and coworkers at Stockholm University, Sweden, used amino acid as catalyst for the formation of hexoses sugars with >99 ee. Hexose’s have been suggested as building blocks of ancient RNA. It may be an example of the theoretical basis for the evolution of homochirality of sugars(right handed configuration) in the prebiotic world.

Andrew Pohorille and Chenyu Wei at NASA Ames Research Center found that (JACS,2009) ribose permeates membranes an order of magnitude faster than its diastereomers. On this basis it was hypothesized that differences in membrane permeability to aldopentoses provide a basis for preferential delivery of ribose to primitive cells for subsequent selective incorporation into nucleotides and their polymers.

D ribose (right handed sugar) polynucleotide tend to form right handed helices, What is the neutral macroscopic cause that gave rise to preference of both right handed helical nucleic acids and proteins on earth?

Y J He et al suggested that a net natural chiral right-handed helical force field, produced by the Earth’s orbital chirality (EOC) could affect the stability of molecule helical enantiomers and make the right-handed helical enantiomers more stable than their left- handed enantiomers. So, terrestrial living systems must select both right-handed nucleic acids based on D-sugars and right-handed proteins based on L-amino acids.

This homochirality can be observed on the macroscopic scale, for example, in the helical chirality of snail shells (preferred right-handed) and the helical winding of some kind of plants.

May be full understanding of life and its evolution will never be possible. But this will not certainly stop scientist to seek the secret of the origin of life. Whitesides recently expressed the current state of understanding of origin of life in frank words
"Most chemists believe, as do I, that life emerged spontaneously from mixtures of molecules in the prebiotic Earth.How? I have no idea. Perhaps it was by the spontaneous emergence of “simple” autocatalytic cycles and then by their combination. On the basis of all the chemistry that I know, it seems to me astonishingly improbable"