Prebiotic phosphorylation enabled by microdroplets.

The significant acceleration of reactions in confined environments has recently been reported in the literature (1–7). The reaction environments investigated span a diverse range, including charged microdroplets (1–3), microdiameter emulsions (5), inverted micelles (8), and the surfaces of aerosol particles (9). These experimental results have made the case that the surface of aqueous drops provides a special and unique reaction environment with qualitatively different thermodynamic and kinetic properties from bulk aqueous solutions. In PNAS, Nam et al. (7) report on a compelling application of accelerated reactions in aqueous microdroplets to chemistry that may have occurred prebiotically, in the absence of enzymes. They specifically address the abiotic production of sugar phosphates and uridine ribonucleoside inmicrodroplets, reactions that do not occur in bulk aqueous solutions. In biology, these reactions are enzyme-catalyzed. Investigations of reactions in confined aqueous environments are especially compelling in prebiotic chemistry, where nonenzymatic pathways must be found to explain the abiotic formation of biopolymers that can be used as building blocks of life (10–12). It has long been appreciated that phosphorus is important in life (13), yet abiotic phosphorylation remains an area of intense recent activity (14). The contribution of Nam et al. (7) provides a possible route for the formation of prebiotically plausible sugar phosphate in aqueous microdrops. The experiments reported in the literature have used laboratory model systems to show different chemistry at aqueous interfaces than in bulk solutions; however, these model systems have counterparts in the natural environment (10, 15). Therefore, the thermodynamic and kinetic arguments emerging from laboratory model studies can be generalized. On the contemporary and early Earth, aqueous microdroplets and water– air interfaces are ubiquitous, as illustrated in Fig. 1. On any rotating planet with a liquid ocean, wind action at the ocean surface generates sea spray and aqueous aerosol. The most probable size for aerosol particles is given by the competition between coagulation of small particles and fast settling out of larger particles under the force of gravity. Atmospheric aerosols on Earth are commonly particles of a few micrometers in diameter (15). Some of these aerosol particles nucleate clouds and fogs, which contain aqueous drops with diameters of a few to tens of microns. Water–air interfaces are found in a number of natural environments, including the surface of oceans, lakes, and atmospheric aerosols on both the modern and early Earth. The collective surface area of aqueous aerosol particles is several orders of magnitude larger than the surface area of oceans. More than 90% of the early Earth was covered by oceans; therefore, aqueous atmospheric particles would have been present in even greater numbers than today. Their aqueous surfaces concentrate and align reagents, providing a unique hydrophobic reaction environment. During their lifetime, atmospheric aerosols experience fluctuations in temperature, relative humidity, pH, salinity, and exposure to radiation and gas phase species, as well as undergoing coagulation, hydration, and dehydration. Water–air interfaces on oceans, lakes, aerosols, and cloud and fog droplets Clouds