A phenomenon familiar to volcanologists is the occurrence of lightning discharges in eruption clouds. Gilbert & Lane (1994)
, for example, in a specialist report related to the topic of aviation safety, cite multiple cases of lightning in volcanic clouds, which were observed during eruptions at 11 different volcanoes, while Navarro-Gonzalez et al. (1996)
list an additional six documented examples and yet more can be found on the Internet. Two features of these events make them interesting from the point of view of prebiotic chemistry: (i) the presence of reduced gases in volcanic eruptions and (ii) the presence of abundant dust in the clouds. It was pointed out by Russian workers in the 1970s that these conditions were suitable for prebiotic syntheses such as the Miller experiment and a number of kinds of organic compounds, including amino acids, were identified (chiefly by paper and thin-layer chromatography) in volcanic deposits (e.g. Markhinin & Podkletnov 1977
). Today, we would require more definitive molecular characterization of compounds of potentially biological significance than were then applied, as well as evidence that they were not introduced by contamination. Nevertheless, conditions such as those described earlier also raise the possibility of reduction of phosphate to phosphite. In a simulation of the process, we have found that reduction is surprisingly effective, but strongly dependent on the reducing nature of the gas-mixture employed ( and ).
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 (more ...)
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). (more ...)
The interest of phosphite as a possible prebiotic phosphorus source is not only due to the higher solubility of its calcium salt compared to apatite (approx. 1000 times higher),1
but also to its greater reactivity as a phosphorylating agent. It has been established that ammonium phosphite, for example, readily reacts with nucleosides to yield nucleoside-phosphites (nucleoside H-phosphonates) under conditions in which ammonium phosphate is unreactive ().
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 (more ...)
We have shown that nucleoside H-phosphonates are easily synthesized. Preliminary results also indicate that they are thermally condensed to form dimers linked by H-phosphonate diester bonds. While the monomers are resistant to oxidation, once incorporated into a dimer, the linkage is easily oxidized to a phosphodiester. It is, therefore, conceivable that they might act as intermediates in a pathway towards the formation of polynucleotides, as illustrated in .
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 (more ...)
Phosphite, incidentally, also provides a possible link to the presence of phosphonic acids in carbonaceous chondrites. These compounds, which are the only phosphorus-containing organic compounds to be identified in meteorites, were reported by Cooper et al. (1992)
in investigations of the Murchison meteorite ().
Phosphonic acids identified in the Murchison meteorite. Note that they are all derivable from phosphorous acid in by replacing the hydrogen atom attached to phosphorus with a methyl-, ethyl- or propyl-group.
An interesting possibility concerning the mechanism of synthesis of these compounds in meteorites is that they might derive from reactions of the PO3
radical di-anion and thus, ultimately, from a source such as phosphorous acid or phosphites. For example, recombination of ·
with a methyl radical has been shown to produce methyl phosphonic acid, the most abundant phosphorus-containing compound in Murchison. Similarly, a suite of other phosphonic acids has been synthesized via ultraviolet irradiation of phosphite in the presence of formaldehyde or simple alcohols (De Graaf et al. 1995