L-cystine adsorption on a pyrite (100) surface exposed to O2 and CO2 atmospheres by near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS)

• Successful in-situ interaction of cystine on pyrite under simulated Early Earth conditions. • NAP-XPS identify cystine specific fingerprints on pre-treated oxidised pyrite at ALBA synchrotron. • Pyrite, a sink for accumulating organic molecules in O 2 and CO 2 atmospheres. • Equal molecular affinity for both oxidised substrates under O 2 and CO 2 atmospheres. • Cystine adsorption inhibits and protects oxidised pyrite substrate from further oxidation. Pyrite surfaces were exposed to oxygen or carbon dioxide atmospheres in a controlled manner at different pressures at the NAPP end station of the CIRCE beamline at the ALBA Synchrotron Light Source to simulate various primitive terrestrial environments, confirming the occurrence of surface oxidation and characterise the chemical species formed. After preliminary gas pretreatment to prepare the substrate, the dimeric amino acid molecule L-cystine was dosed onto the pyrite substrate under UHV conditions, representing the first time that an amino acid was successfully sublimated and adsorbed in situ on a surface at the NAPP-CIRCE beamline. The intense nitrogen feature and molecular disulphide bridge as a molecular fingerprint, confirmed the successful adsorption of L-cystine on the pretreated pyrite surfaces under both O 2 and CO 2 atmospheres, although the oxidation process differed. Our results using near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) at the ALBA synchrotron source help to clarify how different environments can favour or inhibit molecular adsorption and modify biomolecule-surface interactions. Furthermore, it could explain the high reactivity of pyrite surfaces to organic molecules, even after exposure to an oxidising atmosphere, thus inhibiting further reactions and provide insight relevant to the field of prebiotic chemistry.