Spectroscopy of hydrothermal reactions, part 26: Kinetics of decarboxylation of aliphatic amino acids and comparison with the rates of racemization

The kinetics of decarboxylation of six α‐amino acids (glycine, alanine, aminobutyric acid, valine, leucine, and isoleucine) and β‐aminobutyric acid were studied in aqueous solution at 310–330ˆC and 275 bar over the pH 25 range 1.5–8.5 by using an in situ FT‐IR spectroscopy flow reactor. Based on the rate of formation of CO 2 , the first‐order or pseudo‐first‐order rate constants were obtained along with the Arrhenius parameters. The decarboxylation rates of amino acids follow the order Gly > Leu ≈ Ile ≈ Val > Ala > α‐Aib > β‐Aib. Differences in the concentration between 0.05 and 0.5 m had only a minor effect on the decarboxylation rate. The effect of the position of the amino group on the decarboxylation rate was investigated for α‐, β‐, and γ‐aminobutyric acid and the order was found to be α > β ≫ γ. Although the pH dependence is complex, the decarboxylation rates of α‐amino acids qualitatively have the inverse trend of the racemization rates.