Prebiotic photoredox synthesis from carbon dioxide and sulfite

Carbon dioxide (CO(2)) is the major carbonaceous component of many planetary atmospheres, including the Earth throughout its history. Carbon fixation chemistry— that reduces CO(2) to organics—utilizing hydrogen as stoichiometric reductant usually requires high pressures and temperatures, and yields of products of potential use to nascent biology are low. Here we demonstrate efficient ultraviolet photoredox chemistry between CO(2) and sulfite that generates organics and sulfate. The chemistry is initiated by electron photodetachment from sulfite giving sulfite radicals and hydrated electrons, which reduce CO(2) to its radical anion. A network of reactions—generating citrate, malate, succinate and tartrate by irradiation of glycolate in the presence of sulfite—was revealed. 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 surfaces of rocky planets. The availability of the carboxylate products on early Earth could have driven the development of central carbon metabolism before the advent of biological CO(2) fixation.