Low-lying energy levels of amino acids and its implications for origin of life

A major challenge in understanding the origins of life is to elucidate how nature selects a small part of molecules from a large number of candidates as building blocks of life, e.g. amino acids and proteins. Inspired by the thermodynamic principle in protein-structure selection, we attempted to explore whether there exists a thermodynamic criterion to differentiate amino acids from their isomers. In this letter, the energies of 20 natural amino acid and corresponding isomers are calculated by quantum chemical methods. It is revealed that the energy levels of amino acids (especially small ones) are not distributed randomly but located at the bottom of the energy diagrams of their isomers, suggesting that thermodynamic factor plays an important role in selecting the basic building blocks of life.