German Edition: DOI: 10.1002/ange.201608001 Prebiotic Chemistry Very Important Paper International Edition: DOI: 10.1002/anie.201608001 Evaporite Borate-Containing Mineral Ensembles Make Phosphate Available and Regiospecifically Phosphorylate Ribonucleosides: Borate as a Multifaceted Problem Solver in Prebiotic Chemistry Hyo-Joong Kim, Yoshihiro Furukawa, Takeshi Kakegawa, Andrei Bita, Romulus Scorei, and Steven A. Benner* Abstract: RNA is currently thought to have been the first biopolymer to support Darwinian natural selection on Earth. However, the phosphate esters in RNA and its precursors, and the many sites at which phosphorylation might occur in ribonucleosides under conditions that make it possible, chal- lenge prebiotic chemists. Moreover, free inorganic phosphate may have been scarce on early Earth owing to its sequestration by calcium in the unreactive mineral hydroxyapatite. Herein, it is shown that these problems can be mitigated by a particular geological environment that contains borate, magnesium, sulfate, calcium, and phosphate in evaporite deposits. Actual geological environments, reproduced here, show that Mg 2+ and borate sequester phosphate from calcium to form the mineral lüneburgite. Ribonucleosides stabilized by borate mobilize borate and phosphate from lüneburgite, and are then regio- specifically phosphorylated by the mineral. Thus, in addition to guiding carbohydrate pre-metabolism, borate minerals in evaporite geoorganic contexts offer a solution to the phosphate problem in the “RNA first” model for the origins of life. Nearly all prebiotic chemistry is done in glassware, from the classical work of Stanley Miller [1] to the most promising recent efforts. [2, 3] However, as the first Darwinian biopolymers (RNA is today considered the most likely) [4, 5] almost certainly did not arise in Pyrex, it becomes important to seek actual geological and mineralogical contexts that are conducive to managing the well-known problems in prebiotic chemistry, especially those found within the “RNA first” model for the origin of life. [6, 7] These include problems arising from the well- documented instability of RNA, [8] a molecule that some in the field have called “a prebiotic chemists nightmare”. [9] Such problems include: 1) The tar problem, reflecting the propensity of organic molecules to devolve into complex mixtures, especially ribose and its carbohydrate precursors; [10–12] 2) the water problem, reflecting the thermodynamic insta- bility of many bonds in RNA with respect to hydrolysis in water; 3) the phosphate problem, which reflects the need for available phosphorus for the RNA backbone; [13] and 4) the concentration problem, which reflects the need to have substantial amounts of RNA precursor monomers. Of course, it would be preferable to find geological conditions in which all of these problems might be mitigated at the same time. For example, desert environments, inter- mittently exposed to water that then evaporates to dryness, might manage the hydrolytic instability of many bonds in RNA. [14] Furthermore, if exposed to aqueous runoff from basalts (as in the highlands of Mars, a model for early Earth), [15] such environments, would collect alkaline borate (from serpenti- nization of basaltic olivine and the erosion of its igneous tourmalines [16] ). Borate stabilizes ribose [17, 18] under the alka- line conditions needed to make it from the formaldehyde that almost certainly rained out from prebiotic atmospheres. [19] These borate minerals accumulate in evaporites such as borax, colemanite, and ulexite, among others. Moreover, after the pH of the basaltic runoff drops under a CO 2 atmosphere, [ * ] sterile desert environments [ ** ] will accumulate formamide and urea, formed, respectively by the hydrolysis of hydrogen cyanide and cyanamide, both of which are also generated in prebiotic atmospheres. [20] Formamide and urea are suitable media for the formation of hydrolyti- cally unstable phosphate esters from inorganic phos- phate; [21, 22] formamide is also an excellent precusor for nucleobases. [23, 24] Missing here, however, is a discussion of the phosphate problem. Free phosphate seems unable to be abundant in any prebiotic environment rich in calcium, the fifth most abun- [*] Dr. H.-J. Kim, Prof. S. A. Benner Firebird Biomolecular Sciences LLC 13709 Progress Blvd., Alachua, FL 32615 (USA) E-mail: sbenner@firebirdbio.com Prof. Y. Furukawa, Prof. T. Kakegawa Department of Earth Science, Tohoku University Sendai (Japan) Dr. A. Bita University of Medicine and Pharmacy of Craiova Craiova (Romania) Prof. R. Scorei University of Craiova Craiova (Romania) Supporting information, including experimental details, for this article can be found under: http://dx.doi.org/10.1002/anie.201608001. [*] Lowering the pH further will eventually release phosphate from calcium phosphate rock. However, as phosphate ester formation requires evaporation, if CO 2 is the acidification reagent, the pH will rise again as the water evaporates. [**] Natural desert environments are not sterile; therefore, such experiments cannot be run in any natural desert on Earth. A ngewandte Chemie Communications 15816  2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2016, 55, 15816 –15820