AMINO-ACID COSMOGEOCHEMISTRY

Amino acids are ubiquitous components of living organisms and as a result they are widely distributed on the surface of the Earth. Whereas only 20 amino acids are found in proteins, a much more diverse mixture of amino acids has been detected in carbonaceous meteorites. Amino acids in living organisms consist exclusively of the L-enantiomers, but in meteorites, amino acids with chiral carbons are present as racemic mixtures. Protein amino acids undergo a variety of diagenetic reactions that produce some other amino acids but not the unique amino acids present in meteorites. Nevertheless, trace quantities of meteoritic amino acids may occur on the Earth, either as a result of bolide impact or from the capture of cosmic dust particles. The ensemble of amino acids present on the early Earth before life existed was probably similar to those in prebiotic experiments and meteorites. This generates a question about why the L-amino acids on which life is based were selected.