Nuclear-magnetic-resonance determination of the electron self-exchange rate constant of Clostridium pasteurianum rubredoxin.

The iron ion of rubredoxins efficiently exchanges one electron between the Fe(II) and Fe(III) oxidation states in mixtures of oxidized and reduced protein. The conditions under which the relaxation properties of the NMR signals can provide information about this exchange process have been worked out. The rate constant for the rubredoxin electron self-exchange ranges between 1.5 x 10(5) M-1 s-1 at 12 degrees C and 3 x 10(5) M-1 s-1 at 30 degrees C with an activation energy of the order of 24-30 kJ mol-1 in 50 mM potassium phosphate, pH 7. The increase of the electron self-exchange rate constant with ionic strength suggests that neutralizing electrostatic repulsion between the active sites of two molecules further accelerates the already fast electron exchange.