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 embedded in clear poly(vinyl alcohol) (PVA) films, they exhibit inter-subunit electron transfer (ET) electron-nuclear tunneling down to cryogenic temperature (5 K), making them the first protein system other than photosynthetic system to exhibit such behavior. The rate constant for the (Fe²⁺Porphyrin) → (MPorphyrin)⁺ inter-subunit ET reaction shows a roughly temperature- invariant, quantum-tunneling regime from cryogenic temperatures (5 K) up to ca. 200 K. Some of the hybrids (depending On M and the Fe ligand) begin to show a strong increase in this ET rate constant at higher temperatures. This behavior is discussed here in terms of a recent heuristic description of ET in a glassy environment that accounts for the fact that slow solvent relaxation at low temperatures, and in particular upon cooling through a glassing transition, causes the reaction pathway to deviate from the path through the equilibrium transition state, and leads to the formation of nonequilibrium ET product states represented by points on the product surface other than that of the equilibrium product state. The analysis suggests that in regard to the dynamical modes of motion that control ET, the protein 'medium' acts substantially like a frozen glass, even at room temperature. It further suggests that, although the protein acts largely as its own heat bath, the thermal characteristics of that heat can be modified by the external environment.