The question whether the nucleobases can be synthesized in interstellar space is of fundamental significance in considerations of the origin of life. Adenine is formally the HCN pentamer, and experiments have demonstrated that adenine is formed under certain conditions by HCN pentamerization in gas, liquid, and condensed phases. Most mechanistic proposals invoke the intermediacy of the HCN tetramer AICN (4), and it is thought that adenine synthesis is completed by addition of the 5(th) HCN to 4 to form amidine 5 and subsequent pyrimidine cyclization. In this context, we have been studying the mechanism for prebiotic pyrimidine-ring formation of monocyclic HCN-pentamers with ab initio electronic structure theory. The calculations model gas phase chemistry, and the results primarily inform discussions of adenine synthesis in interstellar space. Purine formation requires tautomerization of 5 to the conjugated amidine 6 (via hydrogen-tunneling, thermally with H(+) -catalysis, or by photolysis) or to keteneimine 7 (by photolysis). It was found that 5-(N'-formamidinyl)-1H-imidazole-4-carbonitrile (6) can serve as a substrate for proton-catalyzed purine formation under photolytic conditions and N-(4-(iminomethylene)-1H-imidazol-5(4H)-ylidene)formamidine (7) can serve as a substrate for uncatalyzed purine formation under photolytic conditions. The absence of any sizeable activation barrier for the cyclization of 7 to the (Z)-imino form of 9H-adenine (Z)-2 is quite remarkable, and it is this feature that allows for the formation of the purine skeleton from 7 without any further activation.

Metal ions form strong complexes with humic substances. When the metal ion is first complexed by humic material, it is bound in an 'exchangeable' mode. The metal ion in this fraction is strongly bound, however, if the metal-humic complex encounters a stronger binding site on a surface, then the metal ion may dissociate from the humic substance and be immobilised. However, over time, exchangeably-bound metal may transfer to a 'non-exchangeable' mode. Transfer into this mode and dissociation from it are slow, regardless of the strength of the competing sink, and so immobilisation may be hindered. A series of coupled chemical transport calculations has been performed to investigate the likely effects of non-exchangeable binding upon the transport of metal ions in the environment. The calculations show that metal in the non-exchangeable mode will have a significantly higher mobility than that in the exchangeable mode. The critical factor is the ratio of the non-exchangeable first-order dissociation rate constant and the residence time in the groundwater column, metal ion mobility increasing with decreasing rate constant. A second series of calculations has investigated the effect of the sorption to surfaces of humic/metal complexes on the transport of the non-exchangeably bound metal. It was found that such sorption may reduce mobility, depending upon the humic fraction to which the metal ion is bound. For the more weakly sorbing humic fractions, under ambient conditions (humic concentration etc.) the non-exchangeable fraction may still transport significantly. However, for the more strongly sorbed fractions, the non-exchangeable fraction has little effect upon mobility. In addition to direct retardation, sorption also increases the residence time of the non-exchangeable fraction, giving more time for dissociation and immobilisation. The non-exchangeable dissociation reaction, and the sorption reaction have been classified in terms of two Damkohler numbers, which can be used to determine the importance of chemical kinetics during transport calculations. These numbers have been used to develop a set of rules that determine when full chemical kinetic calculations are required for a reliable prediction, and when equilibrium may be assumed, or when the reactions are sufficiently slow that they may be ignored completely.

Previously, it has been suggested that metal ions complexed to humic acid in the environment might show slower dissociation than those added to humic substances in the laboratory, which has serious implications for the transport of radionuclides in the environment. The dissociation of lanthanide and anthropogenic actinide ions from humic substance complexes has been studied as a function of humic concentration and metal ion:humic concentration ratio. The results suggest that the apparently slower kinetics observed for metal ions complexed in the environment are probably due to the large humic concentrations that are used in those studies. Further, there is no evidence that the dissociation rate constant varies at very low metal ion concentrations. Although humic samples size-fractionated by ultrafiltration showed that more metal may be bound non-exchangeably, there was no evidence for different rate constants. Ultrafiltration of Eu(III)/humic acid mixtures did show a shift in Eu from smaller to larger fractions over a period of two days. Therefore, the results suggest that dissociation rate constants determined in the laboratory at metal ion concentrations higher than those expected in the environment may be used in predicting radionuclide mobility, provided that the humic acid concentration is in the range expected at the site.

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