John Eiler, Ph.D.

SCOL Project: Molecular Isotopic Structure as a Window on Prebiotic Chemistry and Its Comparison with Biochemistry

The study of prebiotic chemistry aims to define the substrates, conditions and mechanisms of chemical pathways that could have generated the compounds, such as amino acids and nucleobases, that are thought to have served as the precursors for the first biochemistry. The findings of this field inform our thinking regarding the origin of life on Earth and also serve as a reference frame for the design and interpretation of measurements made on extraterrestrial materials to search for life elsewhere.

One fundamental challenge of this field is that it is difficult to uniquely map laboratory and theoretical studies of prebiotic chemistry to specific samples from natural environments. Many biologically relevant compounds can be formed by a diversity of abiological mechanisms at a range of conditions, and, of course, they are synthesized by terrestrial life now. This means that when an amino acid or other biorelevant compound is detected in the environment, we are left with a range of possible interpretations as to its origins. We may narrow the range of interpretations using a variety of contextual clues (independent constraints on the environment, ensembles of co-occurring compounds, etc.). Nevertheless, it remains true that most compounds of interest to this field are somewhat “anonymous” when they are found in a natural setting.

The abundances of naturally occurring isotopes (¹³C, ¹⁵N, D, etc.) can provide additional constraints on origins of biorelevant compounds, though common measurements of molecule-average abundances of these nuclides often don’t lead to unique interpretations because many factors drive rare-isotope enrichment or depletion. Molecular isotopic structure — the collection of all possible forms and combinations of rare isotope substitution in a molecule — offers a more diagnostic set of constraints. The combination of site-specific and multiply-substituted isotopic substitutions leads to an enormous diversity of isotopologues of even relatively simple organic molecules, and the proportions of these many isotopic forms can serve as a high-dimensionality fingerprint for substrates, conditions and formation mechanisms.

This study will apply the emerging technologies of high-mass-resolution isotope ratio mass spectrometry, particularly Fourier-transform-based approaches, to document the molecular isotopic structures of key compounds in prebiotic chemistry, including amino acids, sugars and nucleobases, with the aim of creating a library of recognizable and interpretable isotopic structures arising from understood synthesis pathways. A recent pilot study applying this approach to alanine from the Murchison meteorite documents the potential of such measurements to narrow the range of permitted hypotheses for the origins of a compound of interest and to concretely connect multiple co-occurring compounds into detailed reaction networks.

Bio:
John Eiler is the Robert P. Sharp Professor of Geology and Geochemistry in the Division of Geological and Planetary Sciences at the California Institute of Technology. Eiler received his Ph.D. in geology from the University of Wisconsin in 1994 and worked as a postdoctoral scholar and research scientist at Caltech for four years before beginning his faculty appointment there. He directs a laboratory for stable isotope geochemistry that pursues a wide range of research in the Earth, environmental and planetary sciences, and also serves as director of the Caltech Microanalysis Center. His research interests include the development and application of “clumped” isotope geochemistry, uses of stable isotopes to study organic and biogeochemistry, the history of climate and surface elevation, igneous petrogenesis and mantle differentiation on the Earth, moon and Mars, the thermal and fluid evolution of metamorphic rocks and primitive meteorites, and atmospheric and environmental chemistry. Eiler is a fellow of the Geochemical Society of America, the American Geophysical Union and the Mineralogical Society of America, recipient of the MSA award, the Macelwane medal of the AGU, the Epstein medal of the European Association of Geochemistry, and the Day medal of the GSA, and a member of the National Academy of Sciences.