Ultrafast Electronic and Vibrational Relaxation Dynamics in Iron-Sulfur Protein

Iron-sulfur (FeS) clusters are ubiquitous in nature and play a wide range of important roles such as electron transfer, FeS cluster biogenesis, regulation of DNA repair, small molecule sensing, and the catalysis of chemical reactions, etc. They are also involved in many essential biological processes including photosynthesis and cellular respiration. Their electronic and vibrational dynamics are important to the understanding of their rich chemistry, but difficult to characterize because of their structural complexity and the fact that a large number of states exist in close proximity. Photo-induced chemical reactions involving FeS clusters have also attracted much attention recently. However, despite many studies on the light-induced dynamics of charge insertion in FeS complexes, the directly excited photodynamics is poorly known. This knowledge is important because it helps to unravel their photochemical properties as well as to learn how to better use them in light-induced charge transfer reactions.;In this dissertation, the directly photo-excited dynamics in a series of FeS clusters, including the 1Fe-4S cluster in Pyrococcus furiosus rubredoxin (PfRd), the 2Fe-2S clusters in Rhodobacter capsulatus ferredoxin VI (Rc6) and Pseudomonas putida (Pdx), the 4Fe-4S cluster in nitrogenase iron protein, as well as the 8Fe-7S P-cluster and the 7Fe-9S-1Mo FeMo cofactor in nitrogenase MoFe protein, are characterized using ultrafast laser pump probe spectroscopy.;Specifically, Chapter 1 of the thesis gives an overview of FeS clusters and the significance of studying their directly-excited photo-dynamics. Chapter 2 of the thesis introduces the ultrafast laser pump-probe spectroscopic techniques that are used for the study of these FeS complexes. Chapter 3 gives a brief overview of the global analysis methodology adopted for the analysis of the ultrafast transient absorption (TA) spectroscopic data.;Chapter 4 reports a study on the ultrafast electronic relaxation dynamics in the 2Fe-2S cluster of Rc6 characterized using ultrafast TA spectroscopy. Multiple ligand-to-metal charge-transfer populations were found to be induced by laser excitation that evolve to low-lying states. Two long-lived states were identified. The longer one was attributed to a potential long-range electron-transfer pathway. Chapter 5 presents the impulsive coherent vibrational spectroscopic (ICVS) study on Rc6's vibrational relaxation dynamics. Two ICVS bands were identified, with the 484 cm-1 band attributed to excited electronic state vibration. Its time-dependent shift in frequency is also consistent with the excited state evolution characterized in Chapter 4.;Chapter 6 extends the study of charge-transfer dynamics in Rc6 to a series of FeS proteins that contain 1-Fe, 2-Fe, 4-Fe, 7-Fe and 8-Fe clusters: the 1Fe-4S cluster from PfRd, the 2Fe-2S cluster from Pdx, the 4Fe-4S cluster from nitrogenase iron protein, and the 8Fe-7S P-cluster and the 7Fe-9S-1Mo FeMo cofactor from nitrogenase MoFe protein. We aim to characterize and ultimately direct critical charge-transfer dynamics in these systems, as well as to study the cluster dependence of their electronic relaxation dynamics. A competition between the cluster dependence of reorganization energies and density of states was proposed to mediate the electronic relaxation lifetimes.;Chapter 7 presents the early-stage development of a novel single-shot time-resolved infrared spectroscopic system that, once functional, can be used in the study of nitrogenase reaction intermediates that are too transient for conventional vibrational spectroscopic techniques to capture. Future directions for the improvement of this system are also discussed.;In summary, the transient absorption spectroscopic studies on these important FeS clusters have contributed more insights to the directly photo-induced dynamics in these clusters. Understanding these dynamics holds potential to enable the utilization of these clusters in photo-activated chemical reactions such as solar fuel production.