Spectroscopy of hydrothermal reactions, 19: pH and salt dependence of decarboxylation of α‐alanine at 280–330°C in an FT‐IR spectroscopy flow reactor

Abstract

The spontaneous decarboxylation of 0.5 m aqueous α-alanine solutions as a function of pH (1-9 at 320 • C, where neutrality is approximately 6) was determined with a flow reactor at 280-330 • C and 275 bar by FT-IR spectroscopy. The kinetics for the cationic and anionic forms have not been previously reported. The rate constants for the cationic form [CH 3 (NH 3 + )CHCO 2 H], the anionic form [CH 3 (NH 2 )CHCO 2 − ], and the zwitterion form [CH 3 (NH 3 + )CHCO 2 − ] were obtained and followed the first-order rate law. The rate of decarboxylation of the zwitterion is three times greater than that of the cationic and anionic forms in the temperature and pH ranges of study. The corresponding Arrhenius parameters were determined and compared with previously reported data. The addition of KCl (1 and 2 m) at the natural pH of α-alanine resulted in a reduction of the decarboxylation rate, suggesting that the transition state is less polar than the zwitterion and/or that the activity of the zwitterion has been reduced. The α-alanine solution is therefore somewhat more robust in solutions of high ionic strength, such as seawater, than it is in pure water.

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

“…Moreover, other operating conditions such as the materials used to construct the reactor [52] or the addition of salts like KCl [51] or K 2 HPO 4 [53] may also affect the susceptibility of the decarboxylation and deamination reactions of amino acids under hydrothermal conditions. CP and SP both contain different types and quantities of amino acids.…”

Thermal stability of amino acids in biogenic sediments and aqueous solutions at seafloor hydrothermal temperatures

“…The concentrations of neutral AAs, except SER, were higher in alkaline solution at temperatures greater than 200°C (Yamaoka et al, 2007), and the anionic and cationic forms of neutral AAs were more stable than zwitterions (Snider and Wolfenden, 2000;Li et al, 2002). Thus, the thermal stability of neutral AAs is significantly enhanced in alkaline solution at temperatures greater than 200°C.…”

The reaction kinetics of alanine and glycine under hydrothermal conditions

“…It follows that even small amino acids are potentially more stable to breakdown than previously understood, and that polymerization may play an important role under a range of hydrothermal conditions. ln (k, s -1 ) k obs (ala), Abelson (1956) k obs (ala), Andersson & Holm (2000) k obs (ala), Bada et al (1995) k obs (ala), Conway & Libby (1958) k obs (ala), Li et al (2002), pH 6 k obs (ala), Li & Brill (2003) k obs (ala), Qian et al (1993) k obs (ala), Sato et al (2004) k obs (ala), Snider & Wolfenden (2000) k obs (ala), Vallentyne (1964) k hydr (ala2), White/ Bernhardt (1984) k hydr (ala2), Kawamura (2005) k hydr (ala3), Kawamura (2005) k hydr (ala4), Kawamura (2005) k hydr (ala5), Kawamura (2005) k cycl (ala2), this study k cycl (ala2), Kawamura & Yukioka (2001) 1000 T -1 / K -1…”

“…glass vessels y, + CO 2 added radio-labelled CO 2 detection of radiolabelled CO 2 exchanged Alanine disappearance modeled by first-order decarboxylation. Li et al (2002) 280-330 275 1-9 Grade 2 Ti reactor, flow-through, sapphire windows y, Ar none in situ observation of CO 2 production in flow-through mode by IR Alanine breakdown modeled by first-order decarboxylation. Li and Brill (2003a) 310-330 275 nat.…”