Oligomers, prebiotic synthesis

A new method for forming 2, 3 -cyclic phosphates of ribonucleosides using very mild conditions has been described. It may also be used to introduce a terminal cyclic phosphate in protected oligonucleotides. When 2, 3 -cyclic AMP is evaporated from solution at alktiline pH in the presence of aliphatic diamines, and maintained dry at elevated temperatures, polymerization occurs. Oligomers up to the hexanucleotide are formed, preferentially with 3 ->5 internucleotidic links. This may have been an important process in prebiotic synthesis. 

The RNA world scenario has been developed to a greater extent than most of the other approaches to the origin of the first life. This does not mean it is correct but rather that the positive and negative aspects of this scenario are more clearly defined. Problems with this approach start with the absence of a plausible prebiotic synthesis of nucleosides from the RNA bases and ribose. After the nucleoside formation issues are solved, there is no obvious route to the activated monomers that are required for the formation of oligomers. Since the activated monomers undergo relatively rapid hydrolysis in aqueous solution, their rates of synthesis would have had to be faster than their rates of hydrolysis. Other unaddressed problems include formation of RNAs that catalyze the replication of other RNAs, and the catalysis of the ligation of these same RNA oligomers. 

Although the oligomerization of mononucleotides under the above conditions provides plausible models for the prebiotic synthesis of short-chain oligomers, it is doubtful that this type of condensation is applicable to the formation of long polymers. 

Four mechanisms have been advanced for the prebiotic formation of amino acids. The first involves a cyanohydrin (reaction 2) and a related route (reaction 3) can be invoked to account for the presence of hydroxy acids. These particular reactions have been studied in considerable detail both kinetically and in terms of thermodynamic quantities.347 An alternative route (4) involves the hydrolysis of a-aminonitriles, which are themselves formed directly in anhydrous CH4/NH3 mixtures.344 Cyanoacetylene, formed in CH4/N2 irradiations,349 yields significant amounts of asparagine and aspartic acids (reaction 5). Finally, a number of workers336,350"354 have proposed that HCN oligomers, especially the trimer aminoacetonitrile and the tetramer diaminomaleonitrile, could have been important precursors for amino acid synthesis. Reaction mixtures involving such species have yielded up to 12 amino acids. Table 11 indicates the range of amino acids produced in these kinds of sparking syntheses. Of some interest is the fact that close parallels between these kinds of experiments and amino acid contents of carbonaceous chondrite meteorites exist.331,355,356... 

Ferris PJ, Hill AR, Lui R et al. Synthesis of long prebiotic oligomers on mineral surfaces. Nature 1996 381 59. 

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