With the global energy crisis, there are growing interests in making redox reagents for applications ranging from fuel cells to solar energy transfer. A critical barrier to the success is the design of such reagents with tunable redox potentials (E°) and reorganization energies (λ) for efficient long-range electron transfer (ET). A long-standing issue is how the E° can be fine-tuned over a broad range with little change to the metal-binding site or other ET properties. To meet the challenge, we have demonstrated tuning E° of a single cupredoxin, azurin, across a 700 mV range, which is broader than that of native cupredoxins (~500 mV), through manipulation of secondary coordination sphere interactions such as hydrophobicity, hydrogen-bonding and peptide bond oxygen interactions.1 We have also shown that these features are additive, making redox potential tuning of azurin predictable. Furthermore, lower λ in the ET process generally increase ET rate constants and efficiency. However, rational design of ET centers to lower the λ has rarely been demonstrated. Such a task is particularly challenging for ET proteins like the azurin, which have been shown to possess very low λ in comparison to the majority of other proteins. We have reported an analysis of the intramolecular ET from pulse radiolytically produced disulfide radicals to Cu(II) in the above rationally designed azurin mutants, and showed that the λ of ET is indeed smaller than that of native azurin, interpreted as being due to an increased flexibility of the copper site.2 Finally, we have also provided experimental support for the Marcus “inverted region” behavior in a protein only system and found it to be related to the lower λ. It has been hypothesized that the dramatically different forward and backward ET rates in the photosynthetic center is due to the lower λ and large activation barrier for the reverse processes that result from the inverted region. Therefore, our report of successful lowering of λ, achieving the Marcus inverted region and establishing the relationship between these two features furthers the understanding of ET reactivity in proteins and may facilitate de novo design of ET centers for applications such as advanced energy conversions. 1. N. M. Marshall et al. Nature 462, 113-116 (2009). 2. O. Farver et al. Proc. Natl. Acad. Sci. USA 110, 10536-10540 (2013).

Designing metalloproteins with tunable redox potentials and reorganization energies for efficient long-range electron transfer

249th National Meeting (2015)

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