Isoleucine stereoisomers on the Earth

Abstract

The amino acid L-isoleucine (L-Ile) undergoes epimerization at the α-carbon producing D-allo-isoleucine (D-aIle), an amino acid not generally found in living organisms. In fossils and oceanic sediments, this reaction has a half-life, that is, the time required to reach D-aIle/L-Ile ∼0.4, of 10⁵–10⁷ yr (ref. 1). Two other possible isoleucine stereoisomers, D-isoleucine (D-Ile) and L-allo-isoleucine (L-aIle), have not been previously detected in fossils or sediments. These amino acids could be produced by epimerization of L-Ile and D-aIle at the β-carbon². However, the β-carbon is insulated from the amino and carboxy groups which by electron-withdrawing effects and resonance stabilization promote epimerization at the α-position³. Thus, the β-epimerization rates of the isoleucine stereoisomers are predicted to be extremely slow in comparison to their α-epimerization rates. However, no kinetic studies have been conducted. To assess whether the D-Ile and L-aIle could be produced on the Earth from β-epimerization of L-Ile and D-aIle, we have measured the β-epimerization rate at 250°C in aqueous solution, and determined the isoleucine stereoisomeric composition in fossil molluscs of various geological ages.