Detailed Proton Q-Band ENDOR Study of the Electron Spin Population Distribution in the Reduced [4Fe-4S]1+ State

Abstract

Proton ENDOR spectroscopy has been used in single crystals of the synthetic model compound [N(C2D5)4]2[Fe4S4(SCH2C6D5)4] in order to determine the electron spin population distribution within the [4Fe-4S]1+ paramagnetic state. This compound constitutes an excellent biomimetic model of the active sites of many [4Fe-4S] proteins, with the [4Fe-4S]1+ state corresponding to the reduced state of ferredoxin proteins and of numerous other related proteins. Because the information obtained by ENDOR and (or) by NMR from these protons constitutes the basis of the most detailed knowledge on these proteins from magnetic resonance methods, our purpose is to set a firm basis for their interpretation. Only by using Q-band ENDOR could we succeed in measuring the complete hyperfine tensors of a total of 11 protons, i.e., the eight protons of the four CH2 groups and three additional protons from nearest neighbors. By appealing to a new version of the multicentric point-dipole analysis of the dipolar parts of these tensors, we could deduce a detailed map of the electron spin population distribution. This new approach is based on the direct determination of the four spin projection coefficients centered on the four iron monomers constituting the cluster. Our findings confirm that two classes of reduced [4Fe-4S]1+ have to be distinguished, according to the magnitude of the spin coupling coefficients that best describe their fundamental magnetic spin states. Such a distinction has been observed already among oxidized [4Fe-4S]3+ centers. This opens the way for further research aiming at understanding the relations between structures, redox states, ligation, and magnetic spin states.