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Molecular Taphonomy of Heme: Chemical Degradation of Hemin under Presumed Fossilization Conditions

ORCID
0000-0001-8787-0399
Affiliation
PharmaCenter Bonn & Pharmaceutical Institute, Department of Pharmaceutical & Medicinal Chemistry, University of Bonn, An der Immenburg 4, 53121 Bonn, Germany;(M.T.);(L.S.)
Tahoun, Mariam;
ORCID
0000-0001-6987-4126
Affiliation
Kekulé Institute for Organic Chemistry and Biochemistry, University of Bonn, 53121 Bonn, Germany
Engeser, Marianne;
ORCID
0009-0007-7472-7412
Affiliation
PharmaCenter Bonn & Pharmaceutical Institute, Department of Pharmaceutical & Medicinal Chemistry, University of Bonn, An der Immenburg 4, 53121 Bonn, Germany;(M.T.);(L.S.)
Svolacchia, Luca;
ORCID
0000-0003-4981-4307
Affiliation
Section Paleontology, Institute of Geosciences, University of Bonn, 53115 Bonn, Germany;
Sander, Paul Martin;
ORCID
0000-0002-0013-6624
Affiliation
PharmaCenter Bonn & Pharmaceutical Institute, Department of Pharmaceutical & Medicinal Chemistry, University of Bonn, An der Immenburg 4, 53121 Bonn, Germany;(M.T.);(L.S.)
Müller, Christa E.

The metalloporphyrin heme acts as the oxygen-complexing prosthetic group of hemoglobin in blood. Heme has been noted to survive for many millions of years in fossils. Here, we investigate its stability and degradation under various conditions expected to occur during fossilization. Oxidative, reductive, aerobic, and anaerobic conditions were studied at neutral and alkaline pH values. Elevated temperatures were applied to accelerate degradation. High-performance liquid chromatography coupled to tandem mass spectrometry (HPLC-MS/MS) identified four main degradation products. The vinyl residues are oxidized to formyl and further to carboxylate groups. In the presence of air or H 2 O 2 , cleavage of the tetrapyrrole ring occurs, and hematinic acid is formed. The highest stability of heme was observed under anaerobic reductive conditions (half-life 9.5 days), while the lowest stability was found in the presence of H 2 O 2 (half-life 1 min). We confirmed that the iron cation plays a crucial role in degradation, since protoporphyrin IX, lacking iron, remained significantly more stable. Under anaerobic, reductive conditions, the above-mentioned degradation products were not observed, suggesting a different degradation pathway. To our knowledge, this is the first molecular taphonomy study on heme, which will be useful for understanding its fate during fossilization.

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