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Project Progress

Our project to identify the underlying mechanisms of stable isotope fractionation in biological systems is moving forward. We have recently collected evidence that there is a charge-transfer contribution to kinetic stable isotope fractionation that should have significance for the biological fractionation of ⁵⁶Fe, ⁵⁷ Fe, and ⁵⁴ Fe and other transition metal isotopes. We performed a series of electroplating experiments that demonstrated a voltage-dependent isotope fractionation during reduction of Fe ⁺² to Fe metal, with magnitudes covering the range observed in natural biotic and abiotic systems. We have developed a theory, based on the fundamental theory for electron transfer developed previously by Marcus, that accounts for the observed voltage dependent fractionation. The theory is general and makes a broad-based series of predictions concerning stable isotope fractionation in a wide variety of charge-transfer reactions. Our results demonstrate a specific mechanistic origin of stable isotope fractionations in Fe during redox phenomena.

Armed with this new tool, we are now in position to use observed stable isotope fractionations as markers for the specific driving forces in charge-transfer reactions, including biologically mediated electron exchange reactions. In particular, the theory makes testable predictions about the isotope fractionation to be observed based on the ligands that surround a given transition metal during the electron-transfer processes. We are presently expanding our experiments to test these predictions.