EPR and ENDOR Characterization of Fe- and S-Ethylated [4Fe-4S]1+ Clusters, and a Cascade of H-Atom Abstractions Following Photolysis of the Fe–C Bond

Abstract

Initiation of radical reactions by radical S-adenosyl-l-methionine (SAM) enzymes involves a central organometallic intermediate denoted Ω, with an Fe–C bond between the unique iron of a [4Fe-4S]3+ cluster and 5′C of 5′-dAdo. In some radical SAM (RS) enzyme processes, the 5′-dAdo moiety has been proposed to migrate from the unique iron to a neighboring sulfide, a process occurring reversibly in synthetic [4Fe-4S] clusters. Sulfide-alkylated [4Fe-4S] clusters are further proposed as functional intermediates involving auxiliary (non-SAM-binding) clusters in some RS enzymes, especially those involved in formation of Fe–S or C–S bonds. Here we use electron paramagnetic resonance EPR and 13C/1,2H electron nuclear double resonance ENDOR spectroscopies to characterize a synthetic organometallic [4Fe-4S]1+ cluster with an Fe-ethyl bond, Fe-Et, and its sulfide-ethylated counterpart, [S-Et]2+. Notably, Fe-Et shows large 13C and 1H dipolar-couplings, but unusually low isotropic 13C hyperfine coupling to the Fe-bound CH2. [S-Et]2+ shows even smaller 13C and 1H couplings, consistent with minimal spin delocalization. From the iron ions onto the cluster sulfides. Inspired by photoinduced electron transfer from enzymatic SAM-bound [4Fe-4S]1+ clusters and by adenosylcobalamin Co–C bond photocleavage, we performed 405 nm photolysis of both complexes at 12 K. While we have found that Fe-alkylated [4Fe-4S]3+ clusters, and now [S-Et]2+, are not photoactive, the Fe-alkylated [4Fe-4S]1+ cluster Fe-Et undergoes photocleavage of the Fe–C bond to [4Fe-4S]1+. This initiates a cascade of H-atom transfers paralleling that occurring during catalysis by radical SAM glycyl-radical-enzyme activating-enzymes. This observation broadens our understanding both of photochemistry of [4Fe-4S] clusters and of active-site chaperoning of radicals during catalysis by RS enzymes.