Carbonaceous meteorites contain a wide range of extraterrestrial nucleobases

Abstract

All terrestrial organisms depend on nucleic acids (RNA and DNA), which use pyrimidine and purine nucleobases to encode genetic information. Carbon-rich meteorites may have been important sources of organic compounds required for the emergence of life on the early Earth; however, the origin and formation of nucleobases in meteorites has been debated for over 50 y. So far, the few nucleobases reported in meteorites are biologically common and lacked the structural diversity typical of other indigenous meteoritic organics. Here, we investigated the abundance and distribution of nucleobases and nucleobase analogs in formic acid extracts of 12 different meteorites by liquid chromatography-mass spectrometry. The Murchison and Lonewolf Nunataks 94102 meteorites contained a diverse suite of nucleobases, which included three unusual and terrestrially rare nucleobase analogs: purine, 2,6-diaminopurine, and 6,8-diaminopurine. In a parallel experiment, we found an identical suite of nucleobases and nucleobase analogs generated in reactions of ammonium cyanide. Additionally, these nucleobase analogs were not detected above our parts-per-billion detection limits in any of the procedural blanks, control samples, a terrestrial soil sample, and an Antarctic ice sample. Our results demonstrate that the purines detected in meteorites are consistent with products of ammonium cyanide chemistry, which provides a plausible mechanism for their synthesis in the asteroid parent bodies, and strongly supports an extraterrestrial origin. The discovery of new nucleobase analogs in meteorites also expands the prebiotic molecular inventory available for constructing the first genetic molecules.

Fig. 1.

Distribution of guanine, hypoxanthine, xanthine, adenine, purine, and 2,6-diaminopurine in 11 carbonaceous chondrites and one ureilite. The three CM2 carbonaceous chondrites in this study (Murchison, LEW 90500, and LON 94102) contained significantly higher (approximately 4× to 12×) abundances of purine nucleobases as well as greater structurally diversity. The * represents a tentative assignment. The meteorites are roughly ordered by increasing aqueous alteration (Right to Left) as determined using mineralogical and isotopic evidence (–41). The relative degree of aqueous alteration among carbonaceous chondrites within the same group and of the same petrologic type is less certain, although some ordering can be made.

Fig. 2.

Mass-selected fragmentation spectra of reference standards (left spectra) and compounds found in the meteorite LON 94102 (right spectra) measured on an LTQ Orbitrap XL hybrid mass spectrometer using an HCD (higher energy collision dissociation) setting of 90 to 100%. Purine, adenine, 2,6-diaminopurine, and 6,8-diaminopurine were identified using accurate mass measurements on the parent mass and multiple fragment masses and chromatographic retention time. Mass accuracy of less than 5 ppm allows for the unambiguous assignment of elemental formulae. The * represents inferences in the fragmentation spectra that are present in both the meteorite and reference standard spectra.