Saturday, 10 August 2013
Homochirality- Left-Handed Life
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.