Development of enzyme-responsive ferrocene bioconjugates as electrochemical biosensors

Abstract

Bioconjugates, molecules designed to elicit a measurable response when interacting with a target compound, represent a field of growing interest in recent years as advancements in both medical technology and knowledge have allowed researchers to understand the molecular basis of disease better than ever before. Research into enzyme-responsive biosensors, in particular, has gained significant traction as many diseases have been linked to dysfunction of enzymes or enzyme regulation mechanisms in the body. Many enzyme-responsive biosensors developed to date, however, utilize a turn-off mechanism to indicate substrate presence, which is not ideal when dealing with low concentrations of and/or subtle changes in substrate concentration. The bioconjugates developed in this work, in contrast, utilize a change in redox potential to detect the presence of a target enzyme in solution. The basic setup of these biosensor molecules involves an enzyme-specific substrate attached to a redox active molecule (ferrocene). The enzymatic reaction is expected to cleave the substrate from the parent molecule, thus creating a new molecule with a new redox potential. Several bioconjugates, compounds 1-10, were developed to be sensitive for the enzyme caspase-3. Of these, 1-4 provided the most complete characterization and thus were chosen as subjects of further study. Compounds 1 and 2 were designed primarily to evaluate the ability to detect caspase-3 in solution while compounds 3 and 4 were designed to be immobilized on a gold electrode and detect caspase-3. Cyclic voltammetry experiments performed on each bioconjugate individually in bulk aqueous solution revealed a ~100 mV shift in both Epa and Epc between 1 and 2 and a ~300 mV shift in both Epa and Epc between 3 and 4, indicating that significant shift in potential could be observed upon cleavage of either 1 to 2 or 3 to 4 by caspase action.Reactivity studies with caspase-3 revealed 1 was able to undergo near complete cleavage by enzyme action within one hour while virtually no activity between caspase-3 and 3 was observed during the four hour experiment. The reactivity studies involving 3 were performed multiple times, with two experiments performed with the bioconjugate immobilized on a gold ball electrode and another performed with the bioconjugate in solution. All experiments provided similar results. It is hypothesized that the differences in reactivity observed are likely due to a difference in linker length between the ferrocene core and the DEVD (enzyme-binding) substrate in 1 and 3, but further studies are needed to explore this topic further. Nevertheless, our studies show promising work in the development of the first small molecule, enzyme-responsive biosensors that operate through signal (potential) shifts versus traditional amperometric or turn-off mechanisms.