SYNTHESIS, CHARACTERIZATION, AND THERMAL INVESTIGATION OF METAL PHOSPHITES AND POTENTIAL IMPLICATIONS FOR ASTROBIOLOGY
Date of Award
Master of Science in Chemical Sciences (MSCB)
The role of phosphorus in biochemistry is well understood. However, the route by which phosphorus was incorporated into early biomolecules on the prebiotic Earth is uncertain. Phosphate, the most prevalent species of phosphorus found in Earth’s geological record, is insoluble and unreactive with organics in aqueous environments. While the most abundant biogenic elements (C, N, H, O, and S) can be found in a volatile phase under terrestrial conditions, phosphorus cannot, suggesting that minerals must have been the main sources of phosphorus on the early Earth. One possible explanation is that phosphite was a major source of reduced, reactive phosphorus on the early Earth and facilitated phosphorylation of biomolecules. Plausible sources of phosphite include meteoritic corrosion products and iron redox geochemistry in Archean oceans. The presence of phosphite in geological records is scarce and could be a result of oxidative geochemical processes. We present the synthesis, characterization, and thermal investigation of four metal phosphites with prebiotically plausible cations: Ca2+, Mg2+, Fe2+, and Fe3+. Structural characterization was conducted using nuclear magnetic resonance of phosphorus nuclei, infrared spectroscopy, thermogravimetric analysis, and X-ray diffraction. Metal phosphites were heated from 200 to 600°C in stages of 100°C to facilitate oxidation and the formation of oligomerization products, which were identified with phosphorus nuclear magnetic resonance, and infrared spectroscopy. This study demonstrates concurrent chemical processes that complicate geochemical modeling of phosphite minerals. Interstitial water was revealed to be the primary oxidant of metal phosphites and was retained in samples to high temperatures (>500°C). These results could explain the low abundance of phosphite in rock records over geologic timescales. Potential redox buffering in samples containing ferrous iron was also evident, suggesting that ferrous iron may preserve reduced phosphorus at high temperatures.