Reactive oxygen species at the oxide/water interface: Formation mechanisms and implications for prebiotic chemistry and the origin of life

The goal of our study is to identify free radical formation pathways on mineral surfaces. Organic molecules on early Earth might have been modified or decomposed by such pathways, thus affecting the total organic inventory for prebiotic synthesis reactions. Specifically, we evaluated several common oxide minerals under a range of environmental conditions and combinations of conditions (pH, O2 level, UV-wavelength, and particle loading), for formation of highly reactive oxygen species (ROS) at the oxide surfaces by quantifying the generated [OH] and [H2O2]. We identified anatase/rutile (β-TiO2/α-TiO2) and hematite (α-Fe2O3) as active in ROS production and, importantly, found different dominant pathways for ROS formation on anatase/rutile versus hematite. Hydroxyl radicals (OH) in anatase and rutile suspensions were generated mainly through the oxidation of OH− by photo-generated holes and H2O2 was generated through the combination of an OH radical with an OH− and a hole. This pathway for the TiO2 phases did not require the presence of O2, and was not shut down under anaerobic conditions. In contrast, formation of H2O2 and OH in hematite suspensions involved reduction of O2 by electrons, which was inhibited under anaerobic conditions. The surface ROS as well as free radicals formed by reactions with other gases on early Earth atmosphere were capable of destroying molecules such as lipids and pre-RNA or RNA essential to assembly of protocells and survival of the earliest cells. At the same time, surface associated ROS and other free radicals may also have promoted aminoamide formation. Thus, the surface ROS would have affected prebiotic organic compound inventory and protocell/early life evolution.

► We study photocatalyzed formation of reactive oxygen species (ROS) on minerals. ► We examine a wide range of mineral physico-chemical properties and environments. ► We find distinct mechanisms for TiO2 and α-Fe2O3, resolving prior inconsistent reports. ► ROS likely affected prebiotic chemistry, organic inventory and protocell survival. ► Results also relevant to photocatalytic water-splitting and water/air disinfection.