Measurements characterizing electron transfer from a photoexcited zinc protoporphyrin triplet (3ZnP) to a ferriheme electron acceptor within the [α1,β2] 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-1and is not complicated by spin-quenching or energy-transfer processes; results are the same for α(Zn) and β(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: ktdecreases in the order imidazole > H20 > F- ~ CN- ~ 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> 103s-1, consistent with the possible importance of “hole” superexchange in electron tunneling within protein complexes.
ASJC Scopus subject areas