ABSTRACT
The “phosphate problem” refers to a theoretical difficulty caused by the low solubility of the calcium phosphate mineral apatite, of which the most common forms on Earth are hydroxylapatite and fluorapatite. How could insoluble and relatively unreactive mineral phosphate have entered into prebiotic pathways, leading to the precursors of the first biochemical systems? In recent years, a number of possible solutions have been suggested in the literature and are reviewed and updated in this chapter. Topics discussed or reviewed include the following:
Reduction of phosphate to phosphite by lightning discharges.
Phosphorylation involving apatite in the presence of urea and ammonium chloride or ammonium oxalate.
Phosphorylation involving the magnesium ammonium phosphate mineral struvite.
Hydrolysis reactions in water, involving the meteoritic iron phosphide mineral schreibersite, leading to phosphorylation.
Phosphorylation in deep eutectic solvent systems containing urea and choline chloride.
Phosphorylation in deep eutectic solvent systems containing urea, ammonium formate, and water (UAFW).
Finally, the question of the plausibility and the probable interrelationships of key prebiotic chemical pathways involving ammonia and hydrogen cyanide is discussed.
