Carbonaceous meteorites contain a wide range of extraterrestrial nucleobases
Michael P. Callahana,1, Karen E. Smithb, H. James Cleaves IIc, Josef Ruzickad, Jennifer C. Sterna, Daniel P. Glavina, Christopher H. Houseb, and Jason P. Dworkina
Author Affiliations
aNational Aeronautics and Space Administration Goddard Space Flight Center and The Goddard Center for Astrobiology, Greenbelt, MD 20771;
bDepartment of Geosciences and Penn State Astrobiology Research Center, Pennsylvania State University, 220 Deike Building, University Park, PA 16802;
cGeophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015; and
dScientific Instruments Division, Thermo Fisher Scientific, Somerset, NJ 08873
Edited by Mark H. Thiemens, University of California San Diego, La Jolla, CA, and approved July 12, 2011 (received for review April 25, 2011)
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.
Footnotes
1To whom correspondence should be addressed.
E-mail: michael.p.callahan@nasa.gov.
Author contributions: M.P.C., H.J.C., J.C.S., D.P.G., and J.P.D. designed research; M.P.C., K.E.S., H.J.C., and J.R. performed research; H.J.C., J.R., and J.P.D. contributed new reagents/analytic tools; M.P.C. and K.E.S. analyzed data; and M.P.C., K.E.S., H.J.C., D.P.G., C.H.H., and J.P.D. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at
Freely available online through the PNAS open access option.
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