Adenine Synthesis in Interstellar Space: Mechanisms of Prebiotic Pyrimidine-Ring Formation of Monocyclic HCN-Pentamers
Glaser, Rainer; Hodgen, Brian; Farrelly, Dean; McKee, Elliot
Astrobiology, Volume 7, Issue 3, pp. 455-470. 10.1089/ast.2006.0112
So after getting hold of this paper via interlibrary loan (I get up to 40 a year for a nominal fee of £1 each as a research student), I figured I might as well write a short review of it. It’s good practice, and it kinda helps it all sink in. Adenine, incidentally, is the molecule pictured to the right here, and one of the 4 nucleobases in DNA. Astrochemistry meets astrobiology…
On reading this paper, the first thing that struck me was what appears to be a glaring error right in the very title. A couple actually. On closer inspection, I can see what they’re trying to say, but it’s phrased rather ambiguously. Adenine (as the abstract continues to explain) is formally a HCN pentamer. It’s a purine ring, which is actually bicyclic; an imidazole fused to a pyrimidine. The paper itself spends most of it’s time discussing the cyclisation reaction of the latter, but nonetheless, it gets a little irksome throughout most of the sections where the auther is speaking about pyrimidines, when he plainly means purines. The abstract, together with the discussion and conclusion sections make it much clearer what chemical they’re actually talking about forming. If I’m perfectly honest, it almost seemed at first, as if someone had read about nucleobases and gotten their purines and pyrimidines mixed up!
Pressing on though, this paper has a well composed abstract, explaining the commonly accepted mechanism for adenine formation; via the HCN tetramer, AICN (aminoimidazole carbonitrile). It also explains the tautomerism required for the formation of adenine itself, as well as proposing a proton catalysed mechanism under photolytic conditions. Interesting, as this sounds like exactly the sort of conditions that are likely to be found in interstellar space. My interest was piqued even further, however, by the statement that there is no sizeable activation barrier for the actual cyclisation reaction.
The introduction is very comprehensive, including a brief literature review, going all the way back to the now famous Miller-Urey experiment. Building up what is, in effect, a brief history of the study of prebiotic chemistry, from the proposal of chemical reactions in eutectic ices (including how it was proven by Levy et al in 2000) to the measurement of HCN in interstellar dust cores. The paper then continues with the introduction of a mechanism for adenine formation, starting with DAMN (diamino maleonitrile); the uncyclised alter-ego of AICN. However, this is quite a large oligomer to be starting with. I must admit, I’d have liked to see a mechanism for the formation of DAMN from simple HCN molecules, though this was probably dealt with in a previous paper.
After explaining concisely the methods used to model and compute their findings, the paper dives into a detailed discussion of the results. Once again, unfortunately, the first thing to see is “pyrimidine ring formation”, when to be honest, “purine” would be proper nomenclature. Three mechanisms are proposed and intensely critiqued. Ring formation via proton-catalysed nitrile aminolysis; uncatalysed cyclisation; and photoactivation. The discussion is fair and balanced, and includes both pros and cons for each hypothesis, discussing activation energies and the effectiveness of thepossible mechanisms involved (for instance, how molecular vibrations assist the transfer of H atoms in tautomerisation). Interestingly, for the first hypothesis, while the stability of the respective isomers seems to favour a cyclic molecule, the transformation between the two forms (imino and amino) of adenine apparently requires more energy than would be available in interstellar space. A crucial intermediate step requires an endothermic change, and out in space, endothermic reactions are rather unlikely to happen. In the discussion of the uncatalysed reaction, it goes on to explain that interesting statement from the abstract about the lack of any activation barrier. The key, it seems, is in dative bonding between nitrogen and carbon atoms. This allows σ-bonds to form without any charge separation and thus, minimal movement of electrons.
The section on photoactivation starts with a critique of the previous two mechanisms and their shortfalls (the proton catalysed reaction is frustrated by an energy barrier, while the uncatalysed reaction contains an indermediate, inaccessible due to the lack of any thermal pathway. Essentially, both paths can be photoactivated by ultraviolet photons in the UV-C range. Curiously, the paper at this point begins to go off on a tangent about the conditions on primeval Earth. This is a little confusing at first, seeing as it’s supposedly about interstellar chemistry, not planetary chemistry. The point it seems to prove is that for adenine formation for commence (or at least for it to conclude), a circumstellar environment is required.
Finally, the discussion section draws reference to potential energy surfaces (diagramised in the paper) which were constructed using experimental data. The key points made are that the protonated form is generally the more stable form, no direct route exists to form adenine from amidine and that the switch from one isomer to another isessentially unreachable in interstellar space (requiring 30 kcal mol-1 of energy). No real discussion is given on the consequences of these isomers or what they might eventually form. Essentially, the formation of adenine most likely requires a photolytic reaction. Driven by starlight, it’s an entirely plausible reaction pathway.
One thing is seemingly a given though; adenine is a very stable molecule, and stable molecules, once formed tend to stay formed. To quote directly from the paper: ‘Adenine is the “thermodynamic sink” for (HCN)5, and the same is true for the protonated system H+(HCN)5.’