www.elsevier.nl/locate/jelechem Journal of Electroanalytical Chemistry 469 (1999) 123 – 131 Diffusionless electron transfer of microperoxidase-11 on gold electrodes Tautgirdas Ruzgas a, *, Adolfas Gaigalas b , Lo Gorton a a Department of Analytical Chemistry, Lund Uniersity, PO Box 124, S -22100 Lund, Sweden b Biotechnology Diision, NIST, Gaithersburg, MD 20899 USA Received 13 January 1999; received in revised form 12 April 1999; accepted 23 April 1999 Abstract Microperoxidase-11, MP-11, is made by proteolytic digestion of cytochrome c, cyt. c. It consists of a polypeptide of 11 amino residues attached covalently to the heme. Given that MP-11 has a more exposed heme than the complete 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 1000 s -1 , that reported previously for immobilized MP-11 does not exceed 20 s -1 . This work attempts to understand this difference in measured ET rates. 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) MP-11 was bound to gold electrodes by Lomant’s reagent and gave an ET rate close to 4000 s -1 ;(C) physisorbed MP-11 on gold electrodes with a self assembled 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 multilayer 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 significantly slower ET rates. The net result would be an apparent ET rate constant which is much smaller than the value for the first layer. The measurements and calculations are presented in support of such an interpretation. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Hemepeptide; Microperoxidase-11; Electrocatalysis; Peptide-modified electrodes; Interfacial electron-transfer 1. Introduction Porphyrins have considerable potential in medicine for clinical application [1,2], in material science for analytical application [3,4] and for construction of pho- toelectronic devices [5,6]. The poor aqueous solubility of porphyrins, their high tendency to aggregate and chemical instability are serious constraints [7,8]. Cova- lent coupling of porphyrins to short polypeptides or artificial proteins increases the robustness of porphyrin- based systems [9–11]. In this context heme peptides produced by proteolytic digestion of cyt. c exhibit superior characteristics too [12]. The structure of the heme peptide frequently also called microperoxidase-11, MP-11, due to its peroxidase activity, is presented in Fig. 1. Heme peptides having polypeptide of 6, 8, or 9 amino acids can also be obtained by proteolytic diges- tion of cyt. c [12]. Heme peptide molecules have several superficial and very attractive properties compared with those of por- phyrin. These are: aqueous solubility, a weaker ten- dency to aggregation, the availability of a few chemical functionalities for covalent coupling or modification, as well as relative simplicity of the structure exhibiting enzymatic activity (peroxidase) which is beneficial for a simplified theoretical description of different phenom- ena, e.g., structure-function relationship [12,13], or modeling of a biomolecule behavior at interfaces, i.e. adsorption on solid surfaces [14,15], electrochemi- * Corresponding author. Tel.: +46-46-2228191; fax: +46-46- 2224544. E-mail address: tautgirdas.ruzgas@analykem.lu.se (T. Ruzgas) 0022-0728/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII:S0022-0728(99)00194-1