|
Mass spectrometry and X-ray diffraction may have helped uncoil one of the most intriguing questions in biology - why is life handed? Results from an international collaboration have revealed differences between the way enantiomers of peptides pack in crystals that could help explain the origins of life's handedness - why most living things use left-handed amino acids and why the double helix of DNA generally corkscrews in one direction. Natural systems are 'handed', or chiral from the Greek. The spiralling of snail shells, the entwining of honeysuckle stems and many other biological examples hint at an underlying asymmetry in the living world around us. In chemistry, chirality is all important. Limonene, for instance, is the smell of orange or lemon depending on which chiral form you are sniffing. Carvone smells like spearmint on the one hand while its enantiomer has the scent of caraway. (-)-Cocaine is psychoactive while the (+)-enantiomer is not. The L,L version of the artificial sweetener aspartame is 200 times sweeter than sugar, while the L,D enantiomer is actually rather bitter tasting. In the pharmaceutical world thalidomide in one form cures the symptoms of morning sickness drug, but its enantiomer is a tragic teratogen, although is racemised in the body so could never be safe for women of child-bearing age. But, where does this bias originate? There have been all kinds of theories from radioactive beta decay and the underlying physics of symmetry breaking, synchrotron radiation from exploding stars, and simple off the cuff arguments about asymmetric reaction kinetics. The most seemingly plausible explanation simply suggests that a bias was somehow introduced during the primordial reactions from which life emerged in which an enantioselective reaction starting from mixed enantiomers between amino acid and nucleic acid precursors found itself heading down one of the two possible handed results as these fledgling biopolymers grew, purely by chance. Once the path was chosen natural selection pushed it further down one route and not the other so that only the fittest reactions then survived. Now, an Israeli-Danish-French team have observed chiral amplification of short peptide chains as they form two-dimensional crystalline self-assemblies on water. Meir Lahav and colleagues at the Weizmann Institute of Science in Rehovot, Israel are working with Krisian Kjaer and Torben Jensen and co-workers at Riso National Laboratory in Roskilde, Denmark and Gérard Bolbach of the Pierre and Marie Curie University in Paris, France, together with Leslie Leiserowitz and Isabelle Weissbuch and their collaborators H. Zepik, Edna Shavit and M. Tang. They say that matrix-assisted laser desorption time-of-flight mass spectrometry (MALDI-TOF MS) using enantio-labeling coupled with X-ray diffraction has revealed a possible origin of the handedness of nature. The overall process comprises the following steps. Racemic amphiphilic molecules (composed of a hydrophilic amino acid derivative head group and a hydrophobic hydrocarbon chain), self-organize into racemic two-dimensional crystallites when deposited on the water surface. Injection of a catalyst into the water sub-phase induces polymerisation of the molecules at the air-water interface. The distribution of oligomers seems to be dictated by the organization of the molecules within the 2-D crystallites. D, L mixtures containing an enantiomeric imbalance (i.e. D and L of unequal quantity) undergo a separation of phases at the interface. The D,L fraction assembles into one crystallite phase whereas the enantiomer in excess separates into another phase. The products generated in the racemic crystallites yield oligopeptides of D,L composition whereas the enantiomer in excess forms homochiral oligopeptides of a single handedness. "The present study does not allude to the specific question of why left or why right oligo-peptides are formed," explains Lahav, however. What it does address is the question of how one may generate homochiral oligo-peptides from racemic mixtures of amino acids, and homochiral oligo-peptides of a single handedness starting with mixtures of monomer enriched with one of the enantiomers. "Amino acids found in meteorites have been found to be optically active with one of the amino acids in excess," points out Lahav, "This could imply that those amino acids that reached Earth from elsewhere in the universe may already have been partially optically enriched." The team used grazing incidence X-ray diffraction (GIXD) at the synchrotron in Hamburg, Germany, to reveal the crystalline structures of the monomer phase which controls the polymerisation reaction while their diastereomeric composition was determined by MALDI-TOF MS by labelling the hydrocarbon chains of one of the enantiomers with deuterium atoms. "We used perdeuterated hydrocarbon chains - 35 hydrogens by 35 deuteriums - thus introducing a difference of 35 (or 37) mass units between left- and right-handed molecules in the polymeric chains. This difference in mass allowed us to determine the number of D and L units in the different oligopeptides," explains Lahav. The observed reactivity of the ordered clusters at the interface between air and water and of reactivity within organized self-assemblies might have played an important role in the generation of homochiral biopolymers; a legacy we observe to this day in every living thing. Related links: |
|
