THE SIGNIFICANCE OF PROTEIN AMINO ACIDS IN CARBONACEOUS METEORITES

Approximately twenty amino acids are considered the building blocks of proteins in all organisms on Earth. As best as can be determined from the fossil record, this amino acid pool for protein synthesis has never varied. Why these twenty amino acids were selected from the larger pool of amino acids presumably available prior to life's origin is unknown. Similarly, whilst there are obvious advantages for homochirality, processes responsible for the prebiotic synthesis and/or preferential selection of L-amino acids (a necessary precondition for life) are also unknown. Some of Glenn Goodfriend's last studies focused on this problem. Numerous attempts have been made to simulate the prebiotic synthesis of amino acids on the early Earth. However, given the preponderance of impact events during the Earth's early history and the fact that carbonaceous meteorites contain amino acids, the exogenous delivery of life's precursors to Earth is a reasonable alternative. Carbonaceous meteorites contain eight of the protein amino acids that, with the exception of glycine, exhibit the L-enantiomer excess that is characteristic of life. Assuming that these compounds were life's precursors, the question is where did the other twelve protein amino acids come from? It is possible that most of these amino acids were also present in carbonaceous meteorites, but diagenetic processes and analytical uncertainties have effectively limited their detection.