Saturday, 10 August 2013

Homochirality- Left-Handed Life

Life is anything but ambidextrous. And the problem of life's origins is compounded by a basic configuration of amino acids and sugars. Amino acids are molecules consisting of both an amino group (NH2) and a carboxylic group (COOH); the alpha amino acids have the carbon atom in the centre attached to both groups. The infamous Miller-Urey experiment showed how at least 22 amino acids could be produced in a spark discharge tube simulating a prebiotic environment containing water (H2O), methane (CH4), ammonia (NH3), molecular hydrogen (H2) and very little oxygen (O2). But an anomaly arises when trying to reconcile such a result with the chirality of the amino acids; just as your left and right hand can't be translated into each other, they can in principle when you look at the mirror image of one hand. Hence, hands possess a mirror symmetry. Chirality can also be explained in terms of the direction of rotation of circularly polarised light, rightly-polarised and leftly-polarised light behave very differently when they pass via a medium consisting of molecules that have selected chirality. All amino acids naturally occurring on earth are left-handed (except glycine which is non-chiral) but the Miller-Urey experiment produced racemics (equal numbers of left and right handed amino acids), so how did the amino acids get left handed? Such homochirality is critical to protein function, if proteins made of L-amino acids had random incorporations of their D-enantiomers, they would have varying conformations. Sugars also possess homochirality, they are classified as D-sugars based on the arrangement of the chiral centre furthest away from the carbonyl group of the sugar; so sugars are essentially right handed. But why? The Murchison meteorite that landed in Australia in 1969 had five alpha-methyl amino acids and an excess of L-enantiomers; these translate as S-enantiomers (a configuration where the methyl group is attached where the hydrogen atom would normally be in the L-amino acids). So why such an excess? The discovery that our place in the interstellar neighbourhood contains more right-circularly polarised light than anywhere else in the universe hints many possible explanations for homochirality. Among them is processing by ultraviolet photons in outer space, such as their polarisation is highest when they pass via a dense nebulae to leave stars (allowing scattering) and would thereby destroy molecules of one chirality while preserving the others.

The weak interaction of beta decay is the only force with the potential of producing a chirality due its parity violation. Conservation of parity means that the mirror image of an object has to be identical as the object itself, hence the weak interaction could distort the balance between right and left-handed molecules. One way this could be achieved is by electrons produced via beta decay, which have antiparallel spins to the direction of motion (longitudinal polarisation); more energetic, relativistic electrons are entirely longitudinally polarised and would produce Bremsstrahlung photons which interact with molecules to cause chiral discrimination. A similar hypothesis involves amplification via catalytic reactions: an agent that could act as a catalyst for its own synthesis and an inhibitor for the synthesis of the chiral opposite. Imagine a left handed molecule L and a right handed molecule R (both are made of the constituents A and B); once synthesised they trigger 'autocatalysis' where they can drive the synthesis of new molecules of their identical handedness from A and B. Finally merging to form molecule B', which leads to the destruction of one R and one L molecule.

An approach from astrobiology involves the interplay between neutrinos, amino acids and supernovae. 14N (nitrogen-14) is a constituent common to all amino acids and has a non-zero spin. The recently described 'Buckingham effect' occurs when the interaction of a nuclear magnetic moment with the magnetic moment possessed by electrons (produced by the Faraday effect), would behave in a different manner in a right-handed molecule than in a left-handed molecule. So the non-zero spin of the 14N nucleus, coupled with a strong magnetic field could allow a mechanism for chiral discrimination. The SNAAP (Supernova Neutrino Amino Acid Processing) model proposes that supernovae produce carbon, nitrogen, oxygen and a racemic assortment of amino acids (which synthesise in supernova nebulae).  Neutrinos from other supernovae, together with the magnetic field from a neutron star or black hole, make the racemic mixture enantiomeric by selectively destroying one type of chirality of 14N based molecules. Subsequently, chemical evolution quickly amplifies the enantiomers and more L-amino acids are produced as the galaxy is permeated with molecular clouds.


  1. Do you write these yourself or do you take these articles from external sources such as journals and magazines. Also, I have heard that you disregard Adam as being the first man, yet you call yourself a Muslim. i would appreciate it if I could discuss with you. Don't worry. I won't be like those Neanderthals that are your classmate and call you an atheist.

    1. Ha ha Neanderthals! Thanks for commenting, I actually write all my articles by myself (and base them on prior research). As for Adam, I understand why evolution is a fact (based on the evidence of molecular genetics, comparative biology, etc): moreover,we know very well there was no first person; since our genes coalesce to different times and ancestors, there is simply too much genetic variation to bottleneck the whole human race to a primordial couple. You would have to invoke an impossibly high rate of mutation. And since evolution happens in populations and so gradually, the notion of a 'first person' looses meaning.

  2. I acknowledge the process of natural selection as being correct. There are many examples of it occurring around us as we speak. But you do realise that the chances of evolution occurring beginning all the way from biochemical evolution (Haldane and Oparin's experiement with 22 amino acids, atmospheric gases such as water vapour and methane and electricity) is ridiculously low. I'm talking near to 0. The chances of occurring beginning from Adam and Eve, and genetic variation in the human population as a result of mutations made to their DNA is far more likely, don't you agree? Also, when I make my arguments, I will not include religion in them, rather explain religious facts with science. It would be far more convincing to you that way.

  3. Even if Adam and Eve did have the needed genes, we would be able to see it in reduced population bottlenecks (which we don't). Evolution is an explanation of the diversity of life not the origin of life like abiogenesis; these are two different theories. I don't agree the chances of abiogenesis is near 0, the calculation of odds assumes the creation of life in its present form. The first life would have been very much simpler.

    In addition, humans are products of a long process of evolution. We are bipedal apes and share a common ancestor with chimpanzees. Evidence such as endogenous retroviral insertions, chromosome fusion and pseudogenes attest to it. I consider Adam and Eve as a metaphor and not a literal account. The function of a Holy Book is not to explain science but to answer the 'why' questions. :)

  4. Remember that each of our genes goes back to a different ancestor, indicating (as genetics predicts),our genetic legacy derives from several different individuals. It doesn't go back to merely two individuals, regardless of when they lived. The most recent paternal ancestor of all modern humans (Mitochondrial Eve)lived around 150,000 years ago while the most recent maternal ancestor of all modern humans (Y-Chromosomal Adam) lived about 60,000 years ago.

    Modern genetics shows that any 'Adam and Eve' (in terms of mtDNA and Y-Chromosome) must have lived many thousands of years apart from each other. Note that Mitochondrial eve and Y-chromosomal Adam are 'most recent ancestors', they were no the only humans around during their time.