Was photoresolution of amino acids the origin of optical activity in life?

Abstract

SINCE Pasteur showed that the amino acid asparagine in plants was purely one optical isomer, constant interest has centred around the question of optical activity in life. The quantities of D-amino acids in the biosphere are practically negligible and none has ever been found in any enzyme or other protein, whereas the abundance of L-amino acids is well known. Many of the molecules important to living processes can be synthesised¹ in conditions which may have prevailed on a primitive planet and it is easier to suggest a prebiotic synthesis of a racemate than to explain an enantiomeric selection. Whereas many workers claim the clue might be found in various stereo-selective mechanisms, likely to have operated already at the stage of the ‘chemical evolution^2–5’, no example of the selection of a biologically crucial compound by means of circularly polarised light has been previously reported. Apart from an effect due to an enantiomorphous relationship between matter and antimatter^6–8, this is the only mechanism with selection capacity in a large system (the Earth)⁹. We report here that tartaric acid, α-alanine and glutamic acid partially resolve under circularly polarised irradiation. The L-amino acid dominance in the biosphere can by analogy, be explained by a selection caused by circularly polarised interstellar ultraviolet radiation.