Phosphorus in prebiotic chemistry

Abstract

The prebiotic synthesis of phosphorus-containing compounds-such as nucleotides and polynucleotides-would require both a geologically plausible source of the element and pathways for its incorporation into chemical systems on the primitive Earth. The mineral apatite, which is the only significant source of phosphate on Earth, has long been thought to be problematical in this respect due to its low solubility and reactivity. However, in the last decade or so, at least two pathways have been demonstrated which would circumvent these perceived problems. In addition, recent results would seem to suggest an additional, extraterrestrial source of reactive phosphorus. It appears that the 'phosphorus problem' is no longer the stumbling block which it was once thought to be.

Figure 1

(a) Balanced, idealized reaction equation for the production of condensed phosphates by the heating of ammonium phosphate at elevated temperatures (greater than 125°C). In an actual experiment, of course, the reaction may not be complete. (b) Reaction of an alcohol or nucleoside with pyrophosphoric acid (the simplest, but least reactive example of a condensed phosphate) to produce a phosphate monoester with elimination of phosphoric acid.

Figure 2

The three oxyacids of phosphorus. The salts are referred to as phosphates, phosphites and hypophosphites, respectively.

Figure 3

Apparatus used to study the effect of electric discharge on mineral samples. Samples of apatite, mixed with a clay mineral to provide improved adhesion, were deposited onto the ends of an open tungsten loop, which formed a gap of a few millimetres. Pyrex caps (not shown) were fitted over the ends of the loop to prevent direct interaction of the sample with the metal. The chamber was evacuated and back-filled with the gas mixture to be tested. A spark was induced between the ends of the loop by exposing the apparatus to a microwave field. (Illustration from Glindemann et al. 1999; reproduced with kind permission of Springer Science and Business Media.)

Figure 4

The conversion of apatite to phosphite by spark discharge in a model atmosphere containing initially 60% CO₂ and 40% N₂, but with increasing additions of H₂ and CO (the abscissa shows the sum of the contents of H₂ and CO, which were present in equal concentrations). Data points (solid squares) are averages of three to five replicate analyses and the vertical lines show the average deviation of the mean of each set of measurements (De Graaf & Schwartz 2000; reproduced with kind permission of Springer Science and Business Media).

Figure 5

Yield of uridine-5′-phosphite by reaction of uridine with ammonium phosphite (1 : 2 molar ratio) at 60°C. Solid squares, four sets of reactions giving the yields in the absence of urea; solid circles (upper series of data points), yields in the presence of urea (uridine : ammonium phosphate : urea=1 : 2 : 4). The initial time-lag observed for 5 h and less was probably due to the evaporation of water from the reaction. Under identical conditions as those shown here, ammonium phosphate produced no products (De Graaf & Schwartz 2005; reproduced with kind permission of Springer Science and Business Media).

Figure 6

A hypothetical pathway starting with a nucleoside phosphite, leading to the formation of nucleoside-3′-5′ H-phosphonate linkages and, finally, to 3′-5′ phosphodiester linkages. While oxidation of a nucleoside H-phosphonate (monomer) is a difficult reaction, once incorporated into an H-phosphonate diester, oxidation proceeds readily with mild agents (De Graaf & Schwartz 2005). The final step of oxidation to a phosphodiester linkage could theoretically occur on the primitive Earth by reaction with Fe³⁺ or HOOH. Other linkage types (such as 5′-5′, 2′-5′ and so forth) are, of course, also possible.

Figure 7

Phosphonic acids identified in the Murchison meteorite. Note that they are all derivable from phosphorous acid in figure 2 by replacing the hydrogen atom attached to phosphorus with a methyl-, ethyl- or propyl-group.

Figure 8

Possible formation of pyrophosphate from hypophosphate suggested by Pasek & Lauretta (2005). Since hypophosphate is known to be a relatively stable compound under most conditions and has been synthesized by UV irradiation of phosphite, this is a rather unexpected consequence of the reaction of Fe₃P with water (Schwartz & van der Veen 1973).