Probing Excited State Dynamics in Metalloporphyrins and Hemoproteins with X-Ray Transient Absorption Spectroscopy

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Metalloporphyrins fulfill incredibly diverse chemical roles in biology and photocatalysis, where they act as photosensitizers, redox sites, substrate binding sites, and facilitators of long range electron transfer. Metalloporphyrin chemistry is uniquely tuneable through conformation and functionalization of the porphyrin ring, choice of metal, and interaction with the environment as these impact the interaction between ligand orbitals and metal d orbitals. Many biological or photocatalytic processes in these systems can be optically triggered. To understand how excited state chemistry can be controlled, either by the rational design of photocatalysts or by metalloproteins to regulate their function, it is key to ask how porphyrin structure and electronic structure evolve during these dynamic processes. X-ray Transient Absorption (XTA) spectroscopy is a pump-probe technique that utilizes the metal site electronic structure and local coordination geometry sensitivity of core-level X-ray absorption to follow excited state dynamics. This work uses XTA to investigate the excited state dynamics of three metalloporphyrin systems: Nickel tetramesitylporphyrin (NiTMP), Carbmonoxymyoglobin (MbCO), and Zinc porphyrins, including zinc-protoporphyrin IX (ZnPPIX) substituted hemoproteins Cytochrome C and Myoglobin. NiTMP and MbCO are both open shell metalloporphyrins where excited states rapidly relax through vacant d-orbitals. Our ultrafast XTA studies are among those shaping a new field using the ultrafast pulses of X-ray Free Electron Lasers (XFELs) to perform X-ray spectroscopy, in this case to measure sub-ps dynamics in these systems. In both cases, changes in the porphyrin structure occur in response to the excited state rearrangement of d electrons within a picosecond. In the case of NiTMP, an intermediate species not resolved by other ultrafast techniques was observed. ZnPPIX is a closed shell metalloporphyrin, and thus has very long lived porphyrin-centered excited states that are Jahn Teller unstable. Here we use this instability as a probe of the degree of porphyrin interaction with the protein in which it is bound.

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  • 02/19/2018
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