Spin-Lattice Relaxation of Coupled Metal-Radical Spin-Dimers in Proteins: Application to Fe2+-Cofactor (QA−, QB−,ϕ− ) Dimers in Reaction Centers from Photosynthetic Bacteria

AbstractThe spin-lattice relaxation times (T1) for the reduced quinone acceptors QA− and QB−, and the intermediate pheophytin acceptor ϕ−, were measured in native photosynthetic reaction centers (RC) containing a high spin Fe2+ (S=2) and in RCs in which Fe2+ was replaced by diamagnetic Zn2+. From these data, the contribution of the Fe2+ to the spin-lattice relaxation of the cofactors was determined. To relate the spin-lattice relaxation rate to the spin-spin interaction between the Fe2+ and the cofactors, we developed a spin-dimer model that takes into account the zero field splitting and the rhombicity of the Fe2+ ion. The relaxation mechanism of the spin-dimer involves a two-phonon process that couples the fast relaxing Fe2+ spin to the cofactor spin. The process is analogous to the one proposed by R. Orbach (Proc. R. Soc. A. (Lond.). 264:458–484) for rare earth ions. The spin-spin interactions are, in general, composed of exchange and dipolar contributions. For the spin dimers studied in this work the exchange interaction, Jo, is predominant. The values of Jo for QA−Fe2+, QB−Fe2+, and ϕ−Fe2+ were determined to be (in kelvin) −0.58, −0.92, and −1.3×10−3, respectively. The |Jo| of the various cofactors (obtained in this work and those of others) could be fitted with the relation exp(−βJd), where d is the distance between cofactor spins and βJ had a value of (0.66-0.86) Å−1. The relation between Jo and the matrix element |Vij|2 involved in electron transfer rates is discussed

Engineering the Redox Potential over a Wide Range within a New Class of FeS Proteins

Abstract: MitoNEET is a newly discovered mitochondrial protein and a target of the TZD class of antidiabetes drugs. MitoNEET is homodimeric with each protomer binding a [2Fe-2S] center through a rare 3-Cys and 1-His coordination geometry. Both the fold and the coordination of the [2Fe-2S] centers suggest that it could have novel properties compared to other known [2Fe-2S] proteins. We tested the robustness of mitoNEET to mutation and the range over which the redox potential (EM) could be tuned. We found that the protein could tolerate an array of mutations that modified the EM of the [2Fe-2S] center over a range of ∼700 mV, which is the largest EM range engineered in an FeS protein and, importantly, spans the cellular redox range (+200 to-300 mV). These properties make mitoNEET potentially useful for both physiological studies and industrial applications as a stable, water-soluble, redox agent