CRYOGENIC ELECTRON-TUNNELING WITHIN MIXED-METAL HEMOGLOBIN HYBRIDS - PROTEIN GLASSING AND ELECTRON-TRANSFER ENERGETICS

We report that when mixed-metal, [M, Fe] hemoglobin (Hb) hybrids, with Fe in one type of subunit and M = Zn or Mg in the other type, are emb edded in clear poly(vinyl alcohol) (PVA) films, they exhibit inter-sub unit electron transfer (ET) electron-nuclear tunneling down to cryogen ic temperatures (5 K), making them the first protein system other than photosynthetic systems to exhibit such behavior. The rate constant fo r the (Fe(2+)Porphyrin) --> (MPorphyrin)(+) inter-subunit ET reaction shows a roughly temperature-invariant, quantum-tunneling regime from c ryogenic temperatures (5 K) up to ca. 200 K. Some of the hybrids (depe nding on M and the Fe ligand) begin to show a strong increase in this ET rate constant at higher temperatures. This behavior is discussed he re in terms of a recent heuristic description of ET in a glassy enviro nment that accounts for the fact that slow solvent relaxation at low t emperatures, and in particular upon cooling through a glassing transit ion, causes the reaction pathway to deviate from the path through the equilibrium transition state, and leads to the formation of nonequilib rium ET product states represented by points on the product surface ot her than that of the equilibrium product state. The analysis suggests that in regard to the dynamical modes of motion that control ET, the p rotein ''medium'' acts substantially like a frozen glass, even at room temperature. It further suggests that, although the protein acts larg ely as its own heat bath, the thermal characteristics of that heat bat h can be modified by the external environment.