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

Abstract

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 pH₂₅ range 1.5–8.5 by using an in situ FT-IR spectroscopy flow reactor. Based on the rate of formation of CO₂, 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. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 602–610, 2003