Going Right-handed

Why are we made of only right-handed sugars? That has long been one of the biggest puzzles in understanding how life began, and this origin of homochirality in sugars and amino acids has 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 a 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 sugars' homochirality (right-handed configuration) in the prebiotic world.

Andrew Pohorille and Chenyu Wei at NASA Ames Research Center found that 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 tends to form right-handed helices; what is the neutral macroscopic cause that gave rise to the 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 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, the helical chirality of snail shells (preferred right-handed) and the helical winding of some kind of plants. 

Maybe a complete understanding of life and its evolution will never be possible. However, this will not certainly stop scientists from seeking the secret of the origin of life. Whitesides recently expressed the current state of understanding of the 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." 

.

Why are we made of only left-handed amino acids? A molecular concept of Life.

Most biomolecules are "chiral", that is to say, they exist in two left and right-handed mirror-image forms. However, biology only uses one hand, i.e., it is "homochiral". Life on Earth is made of left-handed amino acids, almost exclusively. One of the greatest puzzles in biophysics is the question of why life on Earth is based on left-handed (L) amino acids? Moreover, the synthesis of single enantiomers (ee ~100%) is one of the most critical industry demands.

Recently Prof. Pedro Cintas described that fractional sublimation of chiral organic compounds improves ee substantially.

The reported results provide a beneficial protocol for the resolution of racemates. Additionally, it suggests that improvement of optical activity by such a thermal process could be more efficient than or at least an alternative to spontaneous crystallization of suitable compounds.

He conclude that sublimation should be regarded as a reasonable mechanism for the formation of optically active crystals in the prebiotic world. Subsequent stochastic sorting of crystals of enantiopure compounds by natural agents might have generated highly enantiopure niches, which might be responsible for the prevalence of homochirality in nature.

Molecular evolution might lead to life, but it is not scientifically valid because life is a non-physical, non-chemical entity.

xDNA: A New Genetic System?

Professor Eric Kool and his co-workers have developed " eXpanded DNA " dubbed "xDNA". The synthetic xDNA is expanding by adding benzene molecule to the base on the nucleotides and is a bit longer than the regular 'B' form.


Unlike natural DNA, the expanded DNA is fluorescent and is considerably more stable when subjected to higher temperatures. Its unusual fluorescent properties could make it useful as a probe or diagnostic marker.

Is xDNA also capable of replicating? That is something Kool hopes to find out. Experiments with xDNA are expected to provide new insight into the behavior of natural DNA.

Ref: E. Kool, et al. Angew. Chem Int. Ed., 44, 3118-3122 (2005), JACS , 128, 9219-9230 (2006).

The solvent makes the difference

The solvent surrounding the chiral molecule creates a chiral shell (Chiral Imprint). The chiroptical properties can originate mainly from the chiral solvent shell rather than from the chiral solute. For example, (S)-methyloxirane has a positive optical rotation in water but has a relatively strong negative value in benzene (P. Mukhopadhyay et al. Angew Chem Int Ed 2007, 46, 6450-6452).

 

Computational experiments by J Neugebauer (Angew Chem Int Ed 2007, 46, 7738-7740) shows the presence of the solute imprints a chiral structure on the inner solvation shell.