EPR, ENDOR, and HYSCORE Study of the Structure and the Stability of Vanadyl−Porphyrin Complexes Encapsulated in Silica: Potential Paramagnetic Biomarkers for the Origin of Life

Abstract

The possibility of using vanadyl ions as paramagnetic biomarkers for the identification of traces of primitive life fossilized in silica rocks is studied by cw-EPR, ENDOR, HYSCORE, and DFT calculations. It is well-known that porphyrins, which are common to all living organisms, form vanadyl-porphyrin complexes in sediments deposited in oceans. However, the stability of these complexes over a very long time (more than 3 billion years) is not known. By encapsulating vanadyl-porphyrin complexes in silica synthesized by a sol-gel method to mimic SiO(2) sediments, we studied the structure and stability of these complexes upon step heating treatments by monitoring the evolution of the g factor and of the hyperfine interactions with (51)V, (1)H, (14)N, (13)C, and (29)Si nuclei. It is found that vanadyl-porphyrin complexes are progressively transformed into oxygenated vanadyl complexes by transfer of the VO(2+) ion from the porphyrin ring to the mineral matrix. The organic component is transformed into carbonaceous matter which contains paramagnetic centers (IOM(*) centers). To test the validity of this approach, we studied by EPR a 3490 million years old chert (polycrystalline SiO(2) rock) containing some of the oldest putative traces of life. This rock contains oxygenated vanadyl complexes and IOM(*) centers very similar to those found in the synthetic analogues.

In Situ Identification of Various Structural Features of Vanadyl Porphyrins in Crude Oil by High-Field (3.4 T) Electron–Nuclear Double Resonance Spectroscopy Combined with Density Functional Theory Calculations

“…Figure 2) and the spectroscopic parameters listed in Table 2. The obtained experimental and calculated data are in a good correspondence with those for different vanadyls model systems [24,28,34]. The second signal with g-factor of 2.004 is attributed to stable organic "free" radical(s) [15,35].…”

Nuclear Magnetic Biosignatures in the Carbonaceous Matter of Ancient Cherts: Comparison with Carbonaceous Meteorites

“…), cw-EPR parameters (g-factor, width, lineshape, magnetism), and pulsed-EPR parameters (intensity and shape of 1 H, 13 C, 31 P resonances) would constitute fingerprints of the origin, age, and evolution of primitive CM. In addition to CM, it is worth noting that EPR can also be used to detect inorganic biosignatures, as for example biogenic vanadyl VO 2 + ions (Gourier et al, 2010) and biogenic MnO 2 mineralizations (Kim et al, 2011).…”

“…In addition to a weak Fe 3 + signal at g & 4.2, we always observed E¢-center of quartz (an electron trapped at a dangling sp 3 orbital of SiO 3 À 3 ), and the [AlO 4 /h] 0 center, which is an O -ion adjacent to a substitutional Al 3 + in Si site. In addition to these three paramagnetic defects, the 3.49-billion-year-old Warrawoona chert shows a very weak signal of vanadyl VO 2 + ions, which is an inorganic biosignature (Gourier et al, 2010). Also, the 45-million-year-old Clarno chert exhibits four weak, very narrow lines that are unambiguously attributed to isolated CH 3°r adicals .…”

EPR of Primitive Organic Matter: A Tool for Astrobiology

“…In all cases, the EPR signal is a more or less symmetrical line at nearly free electron spin g-value. In this field range the signal is often superimposed to signals of defects and impurities of the mineral matrix, such as Fe III , VO II , E′ centres and Al Si° centres in SiO 2 (in Kröger-Vink notation) [22,26]. At room temperature and at X-band, the EPR signal of radicals in IOM is superimposed with the g // component of E′ centre of the chert.…”

“…It is important to note that EPR in such Astrobiological context is only emerging, so that considerable work remains to be done before reaching an EPR's maturity equivalent to that of Raman spectroscopy. It is worth noticing that EPR of transition metal ions can also be used to search for biosignatures of the origin of life [22,23], however this aspect of EPR is not treated in the present chapter.…”