時間：2026-08-10(一) 10:10 am

講題：Exploring Pigment-Derived Molecules Preserved in Ancient Rocks: From Strong Chemical Oxidation to Potential-Controlled Selective Oxidation

講者 :  Prof. Ryosuke Saito

服務單位：Department of Earth Science, Graduate School of Sciences and Technology for Innovation, Yamaguchi University  

地點：4樓創意講堂

主持人：林家裕教授

摘要：

Understanding perturbations of the ancient carbon cycle requires information about the photosynthetic ecosystems. Photosynthetic pigments, including chlorophylls, potentially preserve valuable molecular information about primary producers and their light environments. However, pigment-derived molecules are rarely recovered by conventional solvent extraction from ancient rocks, particularly from thermally mature samples. This absence does not necessarily mean that pigment-derived structures have been destroyed. Instead, some tetrapyrrole structures may have been incorporated into the macromolecular structure of kerogen and thus become inaccessible to conventional extraction methods.

In our previous studies, chromic acid oxidation of kerogens isolated from approximately 200-million-year-old and 2.1-billion-year-old rocks yielded maleimides interpreted as degradation products of tetrapyrrole structures. These results suggest that tetrapyrrole-derived structures, including those potentially related to biological pigments, can remain preserved even in thermally mature ancient rocks. However, chromic acid oxidation causes extensive degradation of kerogen and removes much of the structural information associated with the original tetrapyrroles, including their macrocyclic structures, peripheral substituents, and central metal ions. Consequently, the resulting maleimides cannot be used to clearly distinguish structures derived from photosynthetic pigments, such as chlorophylls, from those derived from non-photosynthetic tetrapyrroles, such as heme.

To overcome this limitation, we aim to develop a new extraction and identification approach based on potential-controlled electrochemical oxidation. Our central hypothesis is that tetrapyrrole structures with different central metal ions and peripheral molecular structures can be degraded stepwise and selectively by controlling the applied potential. Porphyrin standards containing different central metal ions and peripheral substituents will be used to compare their oxidation onset potentials, reaction behavior, and product distributions. These experiments will examine whether individual porphyrin structures exhibit characteristic electrochemical fingerprints. Electrode materials and reaction conditions will also be optimized while the applied potential is increased stepwise from lower to higher values. This approach will allow us to evaluate the extent to which information on the origin and molecular structure of tetrapyrroles, which is lost during conventional strong oxidation, can be retained or reconstructed.

In this seminar, I will present our previous findings on kerogen-bound tetrapyrrole structures in ancient rocks and introduce the concept and experimental strategy of potential-controlled selective oxidation. Establishing this method may enable the selective recovery of molecular information derived from photosynthetic pigments preserved in ancient rocks. Ultimately, this approach could provide a new means of reconstructing the relationships between the evolution of photosynthetic ecosystems and global carbon-cycle perturbations throughout Earth history.