TEMPERATURE-INDEPENDENT ELECTRON-TRANSFER IN MIXED-METAL HEMOGLOBIN HYBRIDS

Long-range electron transfer (ET) in mixed-metal hemoglobin hybrids [(MP),FE(CN-)P] (M = Mg, Zn) is measured as a function of temperature from ambient to 100 K. Triplet-state quenching of 3(MP) is found to be inadequate to determine ET rate constants when the quenching rate is low, as it is in these hybrids at low temperature. Direct observation of the ET intermediate, [(MP)+,Fe2+(CN-)P], has allowed us to determine the temperature response of the ET rate constants k(t) and k(b) for the 3(MP) --> Fe3+(CN-)P and Fe2+(CN-)P --> (MP)+ electron-transfer steps, respectively. The ET process is not affected by the freezing of the cryosolvent, which may indicate that coupling of ET to low-frequency solvent modes may be minimal. For both M, but especially for M = Mg, k(b) is nearly temperature independent. Calculation of the electron coupling matrix element gives H(AB) almost-equal-to 10(-3) cm-1. Comparison with results for ruthenated myoglobin indicates that the Hb interface exhibits efficient electronic coupling even across the noncovalent interface between the subunits.