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 Didier Gourier,* ,† Olivier Delpoux, † Audrey Bonduelle, † Laurent Binet, † Ilaria Ciofini, ‡ and Herve ´ Vezin § Laboratoire de Chimie de la Matie `re Condense ´e de Paris, Ecole Nationale Supe ´rieure de Chimie de Paris (Chimie ParisTech) and UniVersite ´ Pierre et Marie Curie, UMR-CNRS 7574, 11 rue Pierre et Marie Curie, 75231 Paris cedex 05, France, Laboratoire d’Electrochimie, Chimie des Interfaces et Mode ´lisation pour l’Energie, Ecole Nationale Supe ´rieure de Chimie de Paris (Chimie ParisTech), UMR-CNRS 7575, 11 rue Pierre et Marie Curie, 75231 Paris cedex 05, France, and Laboratoire de Spectrochimie Infrarouge et Raman, UniVersite ´ des Sciences et Technologes de Lille, UMR-CNRS 8516, 59655 VilleneuVe d’Ascq, France ReceiVed: December 11, 2009; ReVised Manuscript ReceiVed: February 8, 2010 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. Introduction The identification of physicochemical signatures of the oldest fossilized life on Earth or on Mars is a challenging issue to determine with regards to when, where, and how the first microorganisms appeared. 1 The oldest (∼3500 million years (Myr)) and best preserved putative traces of life are found in the form of carbonaceous microstructures embedded in cherts, which are polycrystalline SiO 2 rocks originating from silica sediments deposited on the floor of primitive seas. Their attribution to fossilized bacteria was initially proposed on the basis of morphological observations 2 and of laser Raman microspectroscopy for the identification of amorphous carbon material. 3 However, Raman spectroscopy alone cannot assess the biologic origin of these microstructures, 4 and abiotic processes such as Fischer-Tropsch reactions or thermal de- composition of carbonates are able to give the same types of carbonaceous material with the same Raman signature. 5–7 Also, carbon isotope fractionation has often been considered as a criterion for the biologic origin of the carbonaceous matter in the oldest rocks. 8,9 Unfortunately, abiotic processes can also lead to the same carbon isotope composition. 10–12 Thus, there is still a lack of consensus in the scientific community on which observables could be considered as reliable biosignatures, and at present, there are no physicochemical markers which can help to determine the origin of ancient carbonaceous matter. There- fore, the determination of stable and reliable biosignatures is a fundamental issue in the search for traces of primitive life on Earth, and also on Mars, which offers conditions of habitability similar to that of early Earth at the same epoch. 1 Even when the biogenic origin of a fossilized carbonaceous matter is ascertained, it must be checked that it is indigenous to, and synchronous with, the host rock and not the result of a recent biological contamination. As an emblematic example, the polycyclic aromatic compounds and the related organic matter found in the famous Martian meteorite ALH84001 13 were found to be recent and of terrestrial origin. 14,15 An undisputable molecular biomarker of primitive life should exhibit several characteristics: (i) it must be common to all known living organisms, especially prokaryotes (bacteria, archea) and cannot be synthesized by abiotic processes existing in primitive geological environments; (ii) it must be demonstrated that it is synchronous with the host rock and does not originate from more recent biological contamination (water circulation, endo- lithic bacteria, anthropic contamination from chemical opera- tions, etc.); (iii) it must be detectable at very small concentration, preferably without chemical sample preparation, to avoid anthropic contamination and to be compatible with Mars sample studies. * To whom correspondence should be addressed. E-mail: didier-gourier@chimie-paristech.fr. † Laboratoire de Chimie de la Matie `re Condense ´e de Paris, Ecole Nationale Supe ´rieure de Chimie de Paris (Chimie ParisTech) and Universite ´ Pierre et Marie Curie. ‡ Laboratoire d’Electrochimie, Chimie des Interfaces et Mode ´lisation pour l’Energie, Ecole Nationale Supe ´rieure de Chimie de Paris (Chimie ParisTech). § Universite ´ des Sciences et Technologes de Lille. J. Phys. Chem. B 2010, 114, 3714–3725 3714 10.1021/jp911728e  2010 American Chemical Society Published on Web 02/22/2010