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 310-330 • C 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. C 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 602-610, 2003

Ionization constants of aqueous amino acids at temperatures up to 250°C using hydrothermal pH indicators and UV-visible spectroscopy: Glycine, α-alanine, and proline

“…Calculations using the rate data of Li and Brill (2003) suggest that the degree of decomposition of alanine at 250°C in our system is less than 1%, whereas the degree of decomposition of glycine may be as high as 3% at 225°C and 20% at 250°C. Because the surface to volume ratio in our equipment is lower than that used by Li and Brill, we believe these are upper limits.…”

“…After these measurements were completed, work was reported by other laboratories which indicates that titanium may catalyze the decomposition of amino acids by decarboxylation and other mechanisms (Cox, 2004;Li and Brill, 2003). Our earlier studies in a platinum vibrating tube densitometer and flow microcalorimeter, which used very similar injection systems constructed of platinum (Clarke and Tremaine, 1999;, showed no detectable decomposition of glycine at 225°C, or of ␣-alanine and proline at 250°C.…”

An investigation of reaction pathways of hydrothermal liquefaction using Chlorella pyrenoidosa and Spirulina platensis

“…The position of amino group position and the alkyl side chain may affect the rate constants for the two reactions. For example, Li and Brill [48] studied the decarboxylation rate of aminobutyric acid with three different positions of the amino group (a, b, and c). It was observed that the decarboxylation rate of b-aminobutyric acid was about two times slower than that of a-aminobutyric acid and the rate of c-aminobutyric acid was the slowest due to the reduction of inductive and electrostatic effects.…”

“…They reported that increasing the reactant concentration of alanine promoted the decarboxylation reaction rather than deamination. Li and Brill [48] investigated the effect of pH of the solution on the decarboxylation kinetics of all forms of a-alanine.…”

Kinetics of COx formation in the homogeneous metal/bromide‐catalyzed aerobic oxidation of p‐xylene

“…The reaction mechanism of decarboxylation can be complicated. Generally, the decarboxylation of aliphatic carboxylic acids is strongly dependent on the group attached and the bond type at the β-position [12,13]. For aromatic acids, the aromatic ring acts as an electron sink that dissipates the developing negative charge on the α-carbon atom [13].…”