Biomaterial engineered electrodes for bioelectronics

A series of single-cysteine-containing cytochrome c, Cyt c, heme proteins i ncluding the wild-type Cyt c (from Saccharomyces cerevisiae) and the mutant s (V33C, Q21C, R18C, G1C, K9C and K4C) exhibit direct electrical contact wi th Au-electrodes upon covalent attachment to a maleimide monolayer associat ed with the electrode. With the G1C-Cyt c mutant, which includes the cystei ne residue in the polypeptide chain at position 1, the potential-induced sw itchable control of the interfacial electron transfer was observed. This he me protein includes a positively charged protein periphery that surrounds t he attachment site and faces the electrode surface. Biasing of the electrod e at a negative potential (-0.3 V vs. SCE) attracts the reduced Fe(ii)-Cyt c heme protein to the electrode surface. Upon the application of a double-p otential-step chronoamperometric signal onto the electrode, where the elect rode potential is switched to +0.3 V and back to -0.3 V, the kinetics of th e transient cathodic current, corresponding to the re-reduction of the Fe(i ii)-Cyt c, is controlled by the time interval between the oxidative and red uctive potential steps. While a short time interval results in a rapid inte rfacial electron-transfer, k(et)(1)=20 s(-1), long time intervals lead to a slow interfacial electron transfer to the Fe(iii)-Cyt c, k(et)(2)=1.5 s(-1 ). The fast interfacial electron-transfer rate-constant is attributed to th e reduction of the surface-attracted Fe(iii)-Cyt c. The slow interfacial el ectron-transfer rate constant is attributed to the electrostatic repulsion of the positively charged Cyt c from the electrode surface, resulting in lo ng-range electron transfer exhibiting a lower rate constant. At intermediat e time intervals between the oxidative and reductive steps, two populations of Cyt c, consisting of surface-attracted and surface-repelled heme protei ns, are observed. Crosslinking of a layered affinity complex between the Cy t c and cytochrome oxidase, COx, on an Au-electrode yields an electrically- contacted, integrated, electrode for the four-electron reduction of O-2 to water. Kinetic analysis reveals that the rate-limiting step in the bioelect rocatalytic reduction of O-2 by the integrated Cyt c/COx electrode is the p rimary electron transfer from the electrode support to the Cyt c units.