Astrobiology Science Conference 2010 (2010) 5104.pdf

Exploring the Mechanisms for Highly Reactive Oxygen Species (hROS) Formation in Oxide Particle Suspen- sions: Implications for the Origin and Evolution of Life at Mineral Surfaces. J. Xu1,2, S. Hylton3, M. A. Schoonen 3,4, N. Sahai 1,2, 1Department of Geoscience, University of Wisconsin, 1215 W. Dayton St., Madison, WI 53706, [email protected], [email protected], 2 Wisconsin Astrobiology Research Consortium. 3Department of Geosciences, Stony Brook University, Stony Brook, NY 11794-2100, [email protected], [email protected]. 4Astrobiology Biogeocatalysis Research Center, Montana State University.

Abstract: The broad goals of our study were to iden- play a role in the hROS formation. tify processes by which organic molecules may have The formation of hROS in suspen- been destroyed or altered on early Earth, thus affecting sions is likely generated via a pathway involving Fe3+ the total inventory of organics available for prebiotic and H2O2 in a sequence of reactions with the Fenton synthesis reactions. Previous studies in the literature reaction as the last step [6]. It is not clear why [1-4] have shown that certain minerals in aqueous sus- maghemite does not form ROS. The formation of H2O2 pensions generate highly Reactive Oxygen Species and OH radical in non-iron bearing suspension is (hROS) that can break down organic molecules by thought to depend on the presence of surface defects. oxidation or hydrolysis reactions. Formation mecha- The rutile-induced formation of significant amounts of nisms for these hROS, however, are not fully under- H2O2 suggests that a combination of rutile and hema- stood. Therefore, we designed a systematic study with tite (a likely mineral combination in the Hadean) may oxide particles to evaluate the hROS-forming ability of lead to significant amounts of OH radical on the pre- these particle suspensions and to explore the relevant biotic Earth. This hypothesis is currently being tested. formation pathways. The influence of irradiation by relatively low- We chose seven oxide phases presenting a energy UV wavelength (365 nm) compared to visible wide spectrum of composition, structure, and light on the formation of H2O2 to OH was also ex- surface electrostatics to test. Specifically, the phases plored in experiments with hematite, rutile and . were amorphous silica (SiO2), quartz (α-SiO2), anatase There was a slight decrease in OH radical formation (β-TiO2), rutile (α-TiO2), γ-alumina (γ-Al2O3), with visible light, but no significant difference in H2O2 maghemite (γ-Fe2O3), and hematite (α-Fe2O3). All ox- production for these three mineral. Additional experi- ide slurries were irradiated with visible light or low- ments to evaluate the role of irradiation are planned. energy UV light (365 nm) for one hour before experi- Although we report here only initial results of ments. a new collaborative study, the results suggest that We used a fluorescent probe 3'-(p- hROS generated at mineral surfaces expected to be Aminophenyl) fluorescein (APF) to quantify the hROS present on early Earth may have affected the concen- [5] and H2O2, respectively, in the oxide suspensions. tration and composition of organic molecules before hROS formation, normalized by particle mass loading, life emerged. decreased in the following sequence: hematite> ana-

tase > rutile > amorphous silica > quartz maghemite ≈ ≈ γ-alumina . However, when normalized on the basis References: [1] Schoonen M., Cohn C., et al. (2006) of surface area (BET) loading, the sequence became Reviews in Mineralogy & Geochemistry 64, 179-221. hematite> anatase > rutile > quartz > amorphous silica [2]Fubini B., Mollo L. (1995) Toxicology Letters 82, > maghemite ≈ γ-alumina. Adding Horse Radish Per- 951– 960. [3]Borda M., Elsetinow A., Schoonen M., oxidase (HRP) along with the APF probe allows for Strongin D.. (2001) Astrobiology 1, 283-288. [4]Cohn, the determination of the combined concentrations of C., Laffers R., et al. (2006). Environmental Science & H2O2 and OH radical (HRP converts H2O2 to OH radi- Technology 40, 2838-2843. [5] Cohn C., Pedigo C., cals [5]). All oxide slurries showed enhanced APF Hylton S., Simon S., Schoonen M. (2009) Geochemi- responses in the presence of HRP, indicating their abil- cal Transactions 1, 0-8. [6] Lee C., Sedlack L.(2009). ity to form H2O2. Rutile suspensions generated by far Journal of Molecular Catalysis A: Chemical 311, 1-6. the highest response. Thus, rutile appears to produce significant amounts of H2O2 but little OH radical. The results indicate that bulk chemical com- position is not a predictor of hROS concentrations. For example, hematite (α-Fe2O3) and maghemite (γ-Fe2O3) have the same chemical composition, but differed in both hROS and H2O2 levels. The same is true of rutile (α-TiO2) and anatase(β-TiO2). Thus, , surface defects and surface energey levels may also