The effects of temperature, pH and redox state on the stability of glutamic acid in hydrothermal fluids

Natural hydrothermal vent environments cover a wide range of physicochemical conditions involving temperature, pH and redox state. The stability of simple biomolecules such as amino acids in such environments is of interest in various fields of study from the origin of life to the metabolism of microbes at the present day. Numerous previous experimental studies have suggested that amino acids are unstable under hydrothermal conditions and decompose rapidly. However, previous studies have not effectively controlled the redox state of the hydrothermal fluids. Here we studied the stability of glutamate with and without reducing hydrothermal conditions imposed by 13 mM aqueous H2 at temperatures of 150, 200 and 250 °C and initial (25 °C) pH values of 6 and 10 in a flow-through hydrothermal reactor with reaction times from 3 to 36 min. We combined the experimental measurements with theoretical calculations to model the in situ aqueous speciation and pH values. As previously observed under hydrothermal conditions, the main reaction involves glutamate cyclizing to pyroglutamate through a simple dehydration reaction. However, the amounts of decomposition products of the glutamate detected, including succinate, formate, carbon dioxide and ammonia depend on the temperature, the pH and particularly the redox state of the fluid. In the absence of dissolved H2, glutamate decomposes in the sequence glutamate, glutaconate, α-hydroxyglutarate, ketoglutarate, formate and succinate, and ultimately to CO2 and micromolar quantities of H2(aq). Model speciation calculations indicate the CO2, formate and H2(aq) are not in metastable thermodynamic equilibrium. However, with 13 mM H2(aq) concentrations, the amounts of decomposition products are suppressed at all temperatures and pH values investigated. The small amounts of CO2 and formate present are calculated to be in metastable equilibrium with the H2. It is further proposed that there is a metastable equilibrium between glutamate, glutaconate, α-hydroxyglutarate, ketoglutarate and H2. The key redox-sensitive step is the reaction of α-hydroxyglutarate to α-ketoglutarate, which is effectively inhibited by the elevated H2 concentrations, which in turn dramatically lowers the amounts of all decomposition products including ammonia. Theoretical calculations of the metastable thermodynamic equilibrium between glutamate and ketoglutarate are consistent with the experimentally determined effects of reducing conditions. These findings establish that redox state is as important a variable as temperature and pH in affecting the stability of amino acids under hydrothermal conditions. It is suggested that when natural hydrothermal fluids contain enough dissolved H2, the stability of amino acids may be enhanced in fluids at least on short time scales. In turn, this result suggests that reducing hydrothermal environments may have been favorable for assembling the building blocks of biomolecules in the origin of life. Furthermore, in present day hydrothermal vents the microbial ecosystems may in part be supported by the availability of metastable amino acids through heterotrophic metabolism.