Chemistry and Biochemistry
Bayer School of Natural and Environmental Sciences
Jeffrey D. Evanseck
Ralph A. Wheeler
David W. Seybert
Density Functional Calculations, MitoNEET, Rieske, Ferredoxin, Broken Symmetry, Extended Broken Symmetry, Isotopic Shift, Electron Transfer, Protonation
Iron-sulfur proteins perform a wide variety of biological functions that assist in mediating protein function via electron transfer reactions. The Rieske and mitoNEET iron-sulfur clusters have been shown to undergo proton coupled electron transfer facilitated by a histidine ligand. The protonation state of the histidine residue is key to understanding the mechanism of proton coupled electron transfer. This work reports a study of the 2Fe-2S ferredoxin, Rieske, and mitoNEET clusters using the extended broken symmetry approximation. Calculations were performed on the ferredoxin cluster to establish appropriate methodologies. Calculations were then performed for Rieske and mitoNEET clusters to suggest experiments capable of determining directly the protonation state(s) of histidine ligands to the 2Fe-2S core. The structure and vibrations have been modeled using quantum mechanics. Heisenberg coupling constants, J, and 15N isotopic vibrational frequency shifts were also calculated. It was determined that J values may not be suitable for determining the protonation state of the Rieske and mitoNEET clusters due to small differences in the J values between the protonation states of the clusters. It was determined that 15N isotopic frequency shifts may provide a method for determining if the histidine ligand(s) to the Riekse and mitoNEET clusters are protonated or deprotonated based upon a qualitative difference in the number of vibrational frequency shifts observed. The doubly-protonated Rieske clusters (both oxidized and reduced) show two vibrational frequency shifts of ~9 cm-1 upon 15N isotopic labeling. In contrast, singly-protonated Rieske clusters (both oxidized and reduced) display only one vibrational frequency shift of ~9 cm-1 upon 15N isotopic labeling. No other isotopic vibrational frequency shifts larger than 3 cm-1 were observed for either the oxidized or reduced Rieske clusters. The oxidized and reduced, protonated mitoNEET clusters show one vibrational frequency shift of 8 cm-1 upon 15N labeling and no vibrational frequency shifts greater than 2 cm-1 when the oxidized and reduced clusters are deprotonated. Thus, vibrational difference spectroscopy combined with 15N isotopic labeling may allow determination of the protonation states for the histidine ligands to the 2Fe-2S core of the Rieske and mitoNEET proteins.
Koval, A. M. (2019). Using an Extended Broken Symmetry Approximation to Characterize Structure and Spectroscopic Properties of 2Fe-2S Clusters (Doctoral dissertation, Duquesne University). Retrieved from https://dsc.duq.edu/etd/1819