Chemical, Thermal, and Radiation Resistance of an Iron Porphyrin: A Model Study of Biosignature Stability

Abstract

Metal complexes of porphyrins and porphyrin-type compounds are ubiquitous in all three domains of life, with hemes and chlorophylls being the best-known examples. Their diagenetic transformation products are found as geoporphyrins, in which the characteristic porphyrin core structure is retained and which can be up to 1.1 billion years old. Because of this, and their relative ease of detection, metalloporphyrins appear attractive as chemical biosignatures in the search for extraterrestrial life. In this study, we investigated the stability of solid chlorido(2,3,7,8,12,13,17,18-octaethylporphyrinato)iron(III) [FeCl(oep)], which served as a model for heme-like molecules and iron geoporphyrins. [FeCl(oep)] was exposed to a variety of astrobiologically relevant extreme conditions, namely: aqueous acids and bases, oxidants, heat, and radiation. Key results are: (1) the [Fe(oep)] + core is stable over the pH range 0.0–13.5 even at 80°C; (2) the oxidizing power follows the order ClO − > H 2 O 2 > ClO 3 − > HNO 3 > ClO 4 − ; (3) in an inert atmosphere, the iron porphyrin is thermally stable to near 250°C; (4) at high temperatures, carbon dioxide gas is not inert but acts as an oxidant, forming carbon monoxide; (5) a decomposition layer is formed on ultraviolet irradiation and protects the [FeCl(oep)] underneath; (6) an NaCl/NaHCO 3 salt mixture has a protective effect against X-rays; and (7) no such effect is observed when [FeCl(oep)] is exposed to iron ion particle radiation. The relevance to potential iron porphyrin biosignatures on Mars, Europa, and Enceladus is discussed.

Heat-induced changes in molecular biosignatures and the influence of Mars-relevant minerals

“…(2) Haemin showed well-defined decomposition behaviour between ∼240 and 260°C where two processes occurred, namely decarboxylation and formation of HCl. The iron porphyrin core, which may serve as an important biosignature (Pleyer et al, 2022), remained intact at 260°C. However, it decomposed at higher temperatures.…”

“…1(a). Iron porphyrins such as haemin are attractive biosignatures in the search for extraterrestrial life (Pleyer et al, 2022). In crystals of haemin the molecules are linked pairwise by hydrogen bonds between propionic acid side chains (-CH 2 CH 2 COOH) (Koenig, 1965;Kaduk et al, 2016).…”

Porphyrin-Based Molecules in the Fossil Record Shed Light on the Evolution of Life

“…The formation of a metalloporphyrin complex with a metal cation further stabilizes the macrocyclic structure against geochemical degradation [33,43,44]. Heme and heme-like porphyrins have been described as resistant to changes in environmental pH and remain relatively unchanged at both low (e.g., pH = 0) and high pH (e.g., pH = 13.5) [180]. These molecules have also demonstrated resistance to degradation by high temperatures (330 • C) [180][181][182].…”

Optimization and Enhancement of the Peroxidase-like Activity of Hemin in Aqueous Solutions of Sodium Dodecylsulfate

“…Although there are several reports on the potentially prebiotic synthesis of porphyrins, − porphyrins are considered as biomarkers (“biosignatures”), , i.e., if detected on other planets than the Earth in the solar system or on extrasolar planets would be taken as sign of extraterrestrial life forms. If porphyrins formed on the early Earth, before life originated, metalation of the porphyrins would probably have happened relatively easily, , i.e., iron porphyrins similar to heme b would have formed due to the large abundance of iron ions on the prebiotic Earth.…”

“…Although probably not abundant in the early atmosphere, H 2 O 2 and O 2 might have been formed on the surface of the early Earth, 149 arguing that not only the chemical composition of the atmosphere should be considered but also the likely heterogeneous distribution of chemical compounds on the Earth's surface. 144 Various studies have shown that H 2 O 2 easily forms from H 2 O on the surface of minerals, for example, pyrite (FeS 2 ) 150,151 or silicates (SiO 2 ), 152 or on the surface of micrometer-sized water Although there are several reports on the potentially prebiotic synthesis of porphyrins, 154−156 porphyrins are considered as biomarkers ("biosignatures"), 157,158 i.e., if detected on other planets than the Earth in the solar system or on extrasolar planets would be taken as sign of extraterrestrial life forms. If porphyrins formed on the early Earth, before life originated, metalation of the porphyrins would probably have happened relatively easily, 159,160 i.e., iron porphyrins similar to heme b would have formed due to the large abundance of iron ions on the prebiotic Earth.…”