Kinetics and mechanism of photoinduced electron-transfer reaction between cytochrome c and zinc myoglobin modified with diethylenetriaminepentaacetate ions

The photoinduced electron-transfer (ET) reaction between the oxidized form of cytochrome c (cyt C(III)) and zinc-substituted myoglobin, whose lysine r esidue(s) was modified with diethylenetriaminepentaacetate ions (DTPA), was reexamined in an aqueous degassed solution at 25.0 degreesC, pH = 7.0 (0.0 10 M phosphate buffer) and ionic strength from 0.020 to 0.10 M (NaCl). The first-order rate constant for the quenching of the excited triplet state of ZnMb(DTPA)(n) (n = 2, 4, and 5) was saturated with increasing concentratio ns of cyt c(III), suggesting that the ET quenching occurs within a diprotei n complex. The formation constant for the diprotein complex increased with increasing the number of DTPA ions on the lysine residues of myoglobin and decreased with increasing ionic strength, indicating that cyt c(III) intera cts with the DTPA anionic patch. The intramolecular ET rate constants in th e diprotein complex increased with increasing the number of DTPA ions and d ecreased with increasing ionic strength. It is suggested that a gating occu rs by fluctuation in the configuration of a diprotein complex.