Electron tunneling in Ru-modified His46Asp azurin. Coupling through the Cu ligands

Abstract

We compare electron transfer (ET) rates in two chemically modified Pseudomonas aeruginosa azurins, to examine the role of the Cu ligands in the ET coupling. In both cases, ET proceeded from Cu^+ to a Ru(bpy)_2(im)^(3+) attached to the native His83. In this otherwise native system, the Cu^+ Ru^(3+) → Cu^(2+)Ru^(2+) rate is 1.2(1) × 10^6 s^(−1) (−ΔG° = 0.76 eV) (J.J. Regan, A.J. Di Bilio, R. Langen, L.K. Skov, J.R. Winkler, H.B. Gray and J.N. Onuchic, Chem. Biol., 2 (1995) 489 [1]; A.J. Di Bilio, M.G. Hill, N. Bonander, B.G. Karlsson, R.M. Villahermosa, B.G. Malmström, J.R. Winkler and H.B. Gray, J. Am. Chem. Soc., 119 (1997) 9921 [2]). In the His46Asp mutant (46 being one of the Cu ligands), the rate drops to 3.2(4) × 10^4 s^(−1) (−ΔG° ∼ 0.78 eV). Analysis suggests that this drop is primarily due to a disruption of electronic tube coupling, rather than to just a change in the driving force and/or reorganization energy of the reaction.

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