Oxidative decarboxylation of L-alanine on hematite: Implications for prebiotic chemistry in planetary environments

Iron oxides, such as hematite (α-Fe₂O₃), likely served as key mineral constituents in the sedimentary formations of primordial geothermal environments and planetary surfaces. This study investigates the stability and reactivity of the l-alanine/hematite system under plausible primitive environmental conditions, varying contact time (0.5 - 90 min), pH (2, 6, 9), and redox potential (oxygenic vs. anoxic atmospheres). The aqueous phase was analyzed using Gas Chromatography-Mass Spectrometry (GC-MS) and Electrospray Ionization High-Performance Liquid Chromatography-Mass Spectrometry (ESI-HPLC-MS), while the solid phase was characterized by X-ray Diffraction (XRD) and Attenuated Total Reflectance Fourier-transform Infrared Spectroscopy (ATR-FTIR). Results indicate that pH and oxygen availability critically influence both adsorption and reactivity. Maximum adsorption (67%) occurred at pH 6 (near the Point of Zero Charge) under oxygenic conditions, compared to only 15% under anoxygenic conditions. Significant variations were also observed at pH 9 (49% vs. 11%) and pH 2 (33.5% vs. negligible). Besides adsorption, the main chemical change observed was the decarboxylation of l-alanine to ethylamine, confirmed in all heterogeneous experiments via GC-MS. We conclude that surface Fe³⁺ sites promote this transformation through a mechanism of heterogeneous oxidative catalysis, which is dependent on the presence of an oxidizing agent (electron acceptor) to regenerate active sites. These findings suggest that iron-rich planetary regoliths (e.g., on Mars or early Earth) can act as active chemical reactors, transforming amino acids into ethylamine, a relevant precursor for more complex nitrogen-containing compounds in chemical evolution.