This report presents the first full temperature response of long-range electron transfer within a protein electron-transfer complex of known architecture, the [Zn,Fem]hemoglobin hybrids. For both hybrids kx exhibits a thermally activated regime in which nonadiabatic electron transfer is coupled to thermal vibrations and/or fluctuations within the ,-ß2 electron-transfer complex and its environment. Below ca. 160 K, kt is independent of temperature and the transfer process occurs by nonadiabatic electron tunneling in which the accompanying nuclear rearrangement proceeds by nuclear tunneling. The transition between these two regimes differs in the two hybrids because of different responses of the ferriheme (FemP) coordination state to cooling. Variable-temperature optical measurements and EPR spectroscopy show that the H20 heme ligand of the [cr(FemH20),/3(Zn)] species is replaced by the imidazole of the distal histidine upon cooling, but the heme ligation of the [a(Zn),/3(FenlH20)] hybrid remains invariant. Analysis of the data in terms of the quantum-mechanical theory of vibronically coupled electron tunneling permits us to make comparisons with results for electron transfer in ruthenium-modified proteins, as well as with electron transfer from the cytochrome-to-bacteriochlorophyll special pair in the photosynthetic reaction center of C. vinosum.
Measurements characterizing electron transfer from a photoexcited zinc protoporphyrin triplet (3ZnP) to a ferriheme electron acceptor within the [alpha 1,beta 2] electron-transfer complex of [FeIII,Zn] hybrid hemoglobins are reported. Analytical results demonstrate that the hybrids studied are pure, homogeneous proteins with 1:1 ZnP:FeP content. Within the T quaternary structure adopted by these hybrids, the optical spectrum of a FeIIIP is perturbed by the protein environment. Room temperature kinetic studies of the rate of 3ZnP decay as a function of the heme oxidation and ligation state demonstrate that quenching of 3ZnP by FeIII(H2O)P occurs by long-range intramolecular electron transfer with rate constant kt = 100 (+/- 10) s-1 and is not complicated by spin-quenching or energy-transfer processes; results are the same for alpha(Zn) and beta(Zn) hybrids. Replacement of H2O as a ligand to the ferriheme changes the 3ZnP----FeIIIP electron-transfer rate constant, kt, which demonstrates that electron transfer, not conformational conversion, is rate limiting. However, the trend is not readily explained by simple considerations of spin-state and bonding geometry: kt decreases in the order imidazole greater than H2O greater than F- approximately CN- approximately N3-. The reverse electron-transfer process FeIIP----ZnP+ has not been observed directly but has been shown to be much more rapid, with rate constant kb greater than 10(3) s-1, consistent with the possible importance of "hole" superexchange in electron tunneling within protein complexes.
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