Diffusionless electron transfer of microperoxidase-11 on gold electrodes

Microperoxidase-11, MP-11, is made by proteolytic digestion of cytochrome c , cyt. c. It consists of a polypeptide of 11 amino residues attached covale ntly to the heme. Given that MP-11 has a more exposed heme than the complet e protein, it would seem that electron transfer, ET, between immobilized MP -11 and electrodes would be at least as fast as for intact cyt. c. However, while the maximal heterogeneous ET rate for immobilized cyt. c is around 1 000 s(-1), that reported previously for immobilized MP-11 does not exceed 2 0 s(-1). This work attempts to understand this difference in measured ET ra tes. The MP-11 was immobilized on gold electrodes using several protocols: (electrode A) the immobilization was done following a previously published carbodiimide based recipe yielding ET rates of the order of 20 s(-1); (B) M P-11 was bound to gold electrodes by Lomant's reagent and gave an ET rate c lose to 4000 s(-1); (C) physisorbed MP-11 on gold electrodes with a self as sembled monolayer, SAM, of alkane thiols gave an ET rate approaching 2000 s (-1) for the shortest length alkane thiol. Inspection of the immobilization chemistries suggests that the procedure employed in producing electrodes B and C are likely to lead to a monolayer or less of immobilized MP-11 while the procedure employed for electrode A may lead to a film comprised of a m ultilayer of MP-11. The presence of such a film on electrode A complicates the ET process since the MP-11 in the layer adjacent to the electrode could have fast ET rates while the MP-11 in the outer layers may have significan tly slower ET rates. The net result would be an apparent ET rate constant w hich is much smaller than the value for the first layer. The measurements a nd calculations are presented in support of such an interpretation. (C) 199 9 Elsevier Science S.A. All rights reserved.