Distance dependence of photoinduced long-range electron transfer in zinc/ruthenium-modified myoglobins

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) (a_5Ru;a = NH_3) derivatives of sperm whale myoglobin (Mb): a_5Ru(His-48)Mb, a_5Ru(His-12)Mb, a_5Ru(His-116)Mb, a_5Ru(His-81)Mb. Electron transfer from the ZnP triplet excited state (^3ZnP*) to Ru^3+, ^3ZnP*-Ru^3+ → ZnP^+-Ru^2+ (ΔE° ~ 0.8V) was measured by time-resolved transient absorption spectroscopy: rate constants (k_f) are 7.0 × 10^4 (His-48), 1.0 × 10^2 (His-12), 8.9 × 10^1 (His-116), and 8.5 × 10^1 (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 a_5Ru(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 k_f = 7.8 × 10^8 exp[-0.9l(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)