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Cluster-Dependent Charge-Transfer Dynamics in Iron–Sulfur Proteins

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journal contribution
posted on 05.01.2018, 00:00 by Ziliang Mao, Shu-Hao Liou, Nimesh Khadka, Francis E. Jenney, David B. Goodin, Lance C. Seefeldt, Michael W. W. Adams, Stephen P. Cramer, Delmar S. Larsen
Photoinduced charge-transfer dynamics and the influence of cluster size on the dynamics were investigated using five iron–sulfur clusters: the 1Fe–4S cluster in Pyrococcus furiosus rubredoxin, the 2Fe–2S cluster in Pseudomonas putida putidaredoxin, the 4Fe–4S cluster in nitrogenase iron protein, and the 8Fe–7S P-cluster and the 7Fe–9S–1Mo FeMo cofactor in nitrogenase MoFe protein. Laser excitation promotes the iron–sulfur clusters to excited electronic states that relax to lower states. The electronic relaxation lifetimes of the 1Fe–4S, 8Fe–7S, and 7Fe–9S–1Mo clusters are on the picosecond time scale, although the dynamics of the MoFe protein is a mixture of the dynamics of the latter two clusters. The lifetimes of the 2Fe–2S and 4Fe–4S clusters, however, extend to several nanoseconds. A competition between reorganization energies and the density of electronic states (thus electronic coupling between states) mediates the charge-transfer lifetimes, with the 2Fe–2S cluster of Pdx and the 4Fe–4S cluster of Fe protein lying at the optimum leading to them having significantly longer lifetimes. Their long lifetimes make them the optimal candidates for long-range electron transfer and as external photosensitizers for other photoactivated chemical reactions like solar hydrogen production. Potential electron-transfer and hole-transfer pathways that possibly facilitate these charge transfers are proposed.