This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Molecular high-valent metal-oxo species are implicated in a wide variety of important biological, synthetic, and industrial applications. Both heme and non-heme high-valent metal-oxo complexes have been cited as possible crucial intermediates in the mechanisms of many metalloenzymes (e.g. Cytochrome P450, peroxidases, lipoxygenase, methane monooxygenase, photosynthetic reaction center), and related synthetic systems (e.g. metalloporphyrin oxidations, salen-based asymmetric epoxidations). Although widely invoked, direct evidence for the participation of a high-valent M=O complex is often lacking, and these species have been inferred as key intermediates from product analyses, kinetic data, or other indirect methods. We have developed a continuous flow x-ray fluorescence spectrometer that can be used to record time-resolved x-ray absorption spectra with msec resolution. As a first test, we have used this to investigate the intermediate formed during Mn catalyzed olefin epoxidation (an important step in a wide range of pharmaceutical syntheses). Our results suggest that the standard mechanisms for explaining a large class of Mn-catalyzed oxo transfer reactions must be reevaluated in light of the absence of any detectable Mn=O intermediates.
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