Schoonen, Martin - Reactive Oxygen Species Generation by Lunar Simulants

Abstract: 
With the eventual deployment of human explorers to near-Earth, airless planetary bodies and the establishment of a long-term manned research site on the Moon or other Target Bodies, it is inevitable that humans will be exposed to local mineral dust. Repetitive inhalation exposure to mineral dust in industrial settings is considered an occupational health risk, which can lead to various lung ailments. While daily and lifetime exposures for human explorers are expected to be far less than for those working a lifetime in industrial settings, the mineral dust that human explorers will likely be exposed to is expected to be highly reactive due to the presence of unsatisfied surface bonds and nanophase metallic iron.  Mineral inhalation exposure can lead to inflammation, cytotoxicity (i.e., toxicity to cells), genotoxicity (i.e, damage to DNA), and fibrosis.  One of the factors possibly contributing to the toxicity of a material is its ability to generate Reactive Oxygen Species (ROS). 
  ROS are oxygen-containing species are chemically reactive molecules containing molecular oxygen.  In vitro, the step-wise reduction of molecular oxygen leads to the formation of superoxide radical (O2*-), hydrogen peroxide (H2O2) and hydroxyl radical (OH*).  Hydroxyl radical is by far the most damaging to biomolecules.  In vivo, the immune system triggers the formation H2O2 when challenged with a foreign substance.  Cellularly-derived H2O2 can react with transition metal-containing minerals to generate OH* via the Fenton reaction. 
In this contribution, we report results of an ongoing study to determine the spontaneous generation of OH* upon dispersion of lunar simulants in water as well as through conversion of H2O2.  The OH* formation was quantified using both Electron Spin Resonance (ESR) spectroscopy and molecular probes.  ESR combined with spin trapping using 5,5-Dimethyl-1-pyrroline N-oxide (DMPO) provides an assessment of the initial formation of OH*, while the molecular probe provides insights into the formation of OH* over periods up to 12 days.  A suite of lunar simulants, including JSC-1A and NU-LHT-2M , were investigated.  We have evaluated the spontaneous formation of OH* as a function of mechanical stress by hand grinding as well as the formation of OH* through the interaction between H2O2 and simulant. We also plan to evaluate the effects of UV irradiation and dehydroxylation on these simulants as well as to determine the formation of OH radicals upon the dispersion of treated and untreated simulants in  simulated lung fluid. 
  ESR spin trap results thus far indicate a modest increase in the amount of OH* released as a result of mechanical stress.  This is likely associated with the formation of broken bonds at mineral surfaces.  Additional studies are needed to determine if this treatment imparts reactivity that lasts for more than a few minutes.   Experiments with untreated simulants and H2O2 using  a molecular probe showed no formation of OH* within 286 hours.