Prebiotic Photoredox Synthesis from Carbon Dioxide and Sulfite

Carbon dioxide (CO2) is the major carbonaceous component of many planetary atmospheres including the Earth throughout its history, and prebiological chemistry that reduces this C1 feedstock to organics has accordingly been sought. Carbon fixation chemistry utilizing hydrogen as stoichiometric reductant tends to require high pressures and temperatures, and yields of products of potential use to nascent biology are low1 . Here we demonstrate efficient ultraviolet (UV) photoredox chemistry between CO2 and sulfite (SO3 2–) that generates organics and sulfate (SO4 2– ). The chemistry is initiated by electron photodetachment from SO3 2– giving sulfite radicals and hydrated electrons, which reduce CO2 to its radical anion. By subjecting individual products and putative intermediates to the reaction conditions and analyzing the resultant mixtures, a network of ensuing reactions that can rationalize the products was revealed. In this way it was further discovered that citrate, malate, succinate, and tartrate can be generated by irradiation of glycolate in the presence of SO3 2– . The simplicity of this carboxysulfitic chemistry and the widespread occurrence and abundance of its feedstocks suggest that it could have readily taken place on the early Earth as well as on the surfaces of many rocky planets. The environmental availability of the carboxylate products on Earth could have driven the development of central carbon metabolism before the advent of biological CO2 fixation.