Redox Cycling within Nanoparticle-Nucleated Protein Superstructures: Electron Transfer between Nanoparticulate Gold, Molecular Reductant, and Cytochrome c

Tripeptide Self-Assembled Monolayers as Biocompatible Surfaces for Cytochrome c Electrochemistry

Biocompatible tripeptide self-assembled monolayers (SAMs) are designed with a carboxylate group on the terminal amino acid (glutamate, aspartate, or amino adipate) to electrostatically attract the lysine groups around the heme crevice in horse heart cytochrome c (cyt c), creating an electroactive protein/tripeptide/Au interfacial structure. Exposing the peptide/Au electrode to cyt c resulted in an 11 ± 3 pmol/cm² electroactive protein surface coverage. Topographical images of the interfacial structure are obtained down to single-protein resolution by atomic force microscopy. Uniform protein monolayer assemblies are formed on the Au electrode with no major surface roughness changes. The cyt c/peptide/Au electrode systems were examined electrochemically to probe surface charge effects on the redox thermodynamics and kinetics of cyt c. Neutralization of protein surface charge due to adsorption on anionic COOH-terminated SAMs was found to change the formal potential, as determined by cyclic voltammetry. The cyt c/peptide/Au electrodes exhibit formal potentials shifted to more positive values, have a surface carboxylic acid pK_a of 6 or higher, and produce effective cyt c surface charges (Z_ox) of -6 to -14. The Marcus theory is utilized to determine the protein electron transfer rates, which are ∼5 times faster for cyt c/tripeptide/Au compared to cyt c/11-mercaptoundecanoic acid SAMs of similar chain lengths.

Competition between Photoinduced Electron Transfer and Resonance Energy Transfer in an Example of Substituted Cytochrome c-Quantum Dot Systems

Colloidal quantum dots (QDs) are nanoparticles that are able to photoreduce redox proteins by electron transfer (ET). QDs are also able to transfer energy by resonance energy transfer (RET). Here, we address the question of the competition between these two routes of QDs’ excitation quenching, using cadmium telluride QDs and cytochrome c (CytC) or its metal-substituted derivatives. We used both oxidized and reduced versions of native CytC, as well as fluorescent, nonreducible Zn(II)CytC, Sn(II)CytC, and metal-free porphyrin CytC. We found that all of the CytC versions quench QD fluorescence, although the interaction may be described differently in terms of static and dynamic quenching. QDs may be quenchers of fluorescent CytC derivatives, with significant differences in effectiveness depending on QD size. SnCytC and porphyrin CytC increased the rate of Fe(III)CytC photoreduction, and Fe(II)CytC slightly decreased the rate and ZnCytC presence significantly decreased the rate and final level of reduced FeCytC. These might be partially explained by the tendency to form a stable complex between protein and QDs, which promoted RET and collisional quenching. Our findings show that there is a net preference for photoinduced ET over other ways of energy transfer, at least partially, due to a lack of donors, regenerating a hole at QDs and leading to irreversibility of ET events. There may also be a common part of pathways leading to photoinduced ET and RET. The nature of synergistic action observed in some cases allows the hypothesis that RET may be an additional way to power up the ET.