Electron transfer processes driving free energy

Straus et al. [223] noticed that the quantization of the librational and vibrational modes of water (by means of the Feynman path integral formalism [224]) can significantly influence the solvent free energy barrier and the thermodynamic driving force of the heterogeneous electron transfer process. For more information the reader is referred to the cited publications and the references therein. 

The free energy change, AG , of an electron transfer process is the driving force of the process. The free energy of activation, AG, is needed to reach the transition state,. It is related to the reorganizational energy, "k of the system. From the geometry of the parabolas ... 

FIGURE 10. Possible origin of dispersive kinetics of primary electron transfer. Forward electron transfer is indicated by the solid arrows, thermal repopulation of the P state (a minor process) by the dotted arrows. (A) Static heterogeneityoelectron transfer takes place from the P state to P Ha states with distribution of free energies The reaction therefore occurs with a distribution of driving forces, and hence a distribution of rates. Most thermal repopulation of the P state occurs from the P Ha" states that are highest in energy. A similar model can be constructed based upon a P —> P a" with two or more values for the... 

The rate constants and the associated free-energy snrfaces available to the peroxide and native intermediates deserve comment since they differ overall by nearly 10 (or ca. Vkcalmol in absolnte valne). Given the relatively electronentral nature of electron transfers between the copper sites (the E° values for the three sites differ overall by only 60 mV), the differences in rate in the first instance reflect the difference in the E° value for le versus 2e reduction of dioxygen (leading to the peroxy intermediate) and peroxide (leading to the native intermediate). Second, the differences reflect the work available from the favorable 4e reduction that drives the turnover from native intermediate to fully reduced enzyme primed, now, to react with O2. This latter process, k 100 s (compare to k = 0.34s for decay of the native intermediate to fully oxidized enzyme), is functionally equivalent to the reductive release of Fe + from Fe +-transferrin catalyzed by the membrane metalloreductase, Dcytb in both cases, the lower valent metal species is more loosely coordinating. Whereas Fe + dissociates in the latter case, in MCO turnover the bound water(s) dissociate. 

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