EPR of Primitive Organic Matter: A Tool for Astrobiology

Abstract

Insoluble organic matter (IOM) conserved in ancient sedimentary rocks and in carbonaceous meteorites can reveal valuable information about the origin of Life on Earth and on the birth of the Solar System, respectively. These IOMs are also reference materials for the search for possible organic traces of extinct life on Mars. The combination of continuous-wave and pulsed EPR of the radicals in IOM provided several markers distinguishing these materials and related to their histories. For terrestrial IOM, the EPR linewidth of the radicals is mostly determined by unresolved 1H hyperfine interactions for IOM younger than 2500 million years (H-rich), and by dipolar interactions for older (H-depleted) IOM. The age of very primitive IOM could be estimated through the lineshape, which continuously evolves from Lorentzian to stretched Lorentzian upon ageing due to a change in the dimensionality of the radical spatial distribution. Nuclear spins within or near the radicals and the hyperfine interactions probed by pulsed EPR (through ESEEM and HYSCORE sequences) clearly distinguish meteoritic from terrestrial IOM. Radicals in meteorites are massively enriched in deuterium compared to terrestrial radicals, as a result of specific deuterium enrichment processes in the outer early Solar System. Meteoritic and terrestrial IOMs are also distinguished by the isotropic vs dipolar relative contributions in the 1H hyperfine interactions and by the 13C/1H HYSCORE signal ratio. Strong 31P and 14N HYSCORE signals were detected in terrestrial IOM, which point to possible P and N rich biological precursors. The spin states of the radicals could also be determined either indirectly from the temperature dependence of the EPR intensity or directly by transient nutation spectroscopy. Meteoritic IOM, in addition to S = 1/2 radicals, specifically contains species with either triplet ground state or thermally excited triplet states, which are lacking in terrestrial IOM.