| Summary: | An experimental investigation of the distance dependence of long-range electron transfer in zinc/ruthenium-modified myoglobins has been performed. The modified proteins were prepared by substitution of zinc mesoporphyrin IX diacid (ZnP) for the heme in each of four previously characterized pentaammineruthenium(III) (a5Ru; a = NH3) derivatives of sperm whale myoglobin (Mb): a5Ru(His-48)Mb, a5Ru(His-12)Mb, a5Ru(His-116)Mb, a5Ru(His-81)Mb. Electron transfer from the ZnP triplet excited state (3ZnP*) to Ru3+, 3ZnP*-Ru3+ → ZnP+-Ru2+ (ΔE° ∼ 0.8 V) was measured by time-resolved transient absorption spectroscopy: rate constants (kf) are 7.0 × 104 (His-48), 1.0 × 102 (His-12), 8.9 × 101 (His-116), and 8.5 × 101 (His-81) s-1 at 25°C. Activation enthalpies calculated from the temperature dependences of the electron-transfer rates over the range 5-40°C are 1.7 ± 1.6 (His-48), 4.7 ± 0.9 (His-12), 5.4 ± 0.4 (His-116), and 5.6 ± 2.5 (His-81) kcal mol-1. Electron-transfer distances (d = closest ZnP edge to ȧ5Ru(His) edge; angstroms) were calculated to fall in the following ranges: His-48, 11.8-16.6; His-12, 21.5-22.3; His-116, 19.8-20.4; His-81, 18.8-19.3. The rate-distance equation is kf = 7.8 × 108 exp[-0.91(d- 3)] s-1. The data indicate that the 3ZnP*-Ru(His-12)3+ electronic coupling may be enhanced by an intervening tryptophan (Trp-14).
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