NMR 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 r educed protein. The conditions under which the relaxation properties o f the NMR signals can provide information about this exchange process have been worked out. The rate constant for the rubredoxin electron se lf-exchange ranges between 1.5x10(5)M(-1) s(-1) at 12 degrees C and 3x 10(5) M(-1) s(-1) at 30 degrees C with an activation energy of the ord er of 24-30 kJ mol(-1) in 50 mM potassium phosphate, pH 7. The increas e of the electron self-exchange rate constant with ionic strength sugg ests that neutralizing electrostatic repulsion between the active site s of two molecules further accelerates the already fast electron excha nge.