Uracil in carbonaceous meteorites

Abstract

Previous analyses of carbonaceous chondrites have demonstrated the presence of the biologically significant purines adenine, guanine, hypoxanthine and xanthine^1–3. To date, however, no pyrimidine of biological importance has been reported. An earlier report⁴ of a fraction extracted from the Murray meteorite showing ‘cytosine-like’ spectral characteristics was later shown to be due to an artefact of the analytical method^5,6. Some unusual pyrimidines which have no known biological function were reported in extracts from the Murchison, Murray and Orgueil carbonaceous chondrites⁷. However, these results have not been replicated and are now thought to have been due to artefacts^2,3. Because of the reported synthesis of the pyrimidines uracil, thymine and cytosine in Fischer-Tropsch-type reactions^8,9 and the suggestion that such reactions may have been of significance in the production of organic material in meteorites^9,10, we have reinvestigated the possible occurrence of pyrimidines in extracts from the Murchison, Murray and Orgueil carbonaceous meteorites using specific fractionation techniques and high-sensitivity analysis. The pyrimidines uracil, thymine and cytosine, together with the purines adenine and guanine are, of course, the building blocks of terrestrial genetic inheritance. Therefore their presence or absence in meteorites is also of considerable interest to theories of chemical evolution and the origin of life. We report here the positive identification of uracil in water and formic acid extracts of all three meteorites.