Mapping local electric fields in proteins at biomimetic interfaces.

We present a novel approach for determining the strength of the electric field experienced by proteins immobilised on membrane models. It is based on the vibrational Stark effect of a nitrile label introduced at different positions on engineered proteins and monitored by surface enhanced infrared absorption spectroscopy.

Surface-enhanced vibrational spectroscopy for probing transient interactions of proteins with biomimetic interfaces: electric field effects on structure, dynamics and function of cytochrome c.

Most of the biochemical and biophysical processes of proteins take place at membranes, and are thus under the influence of strong local electric fields, which are likely to affect the structure as well as the reaction mechanism and dynamics. To analyse such electric field effects, biomimetic interfaces may be employed that consist of membrane models deposited on nanostructured metal electrodes. For such devices, surface-enhanced resonance Raman and IR absorption spectroscopy are powerful techniques to disentangle the complex interfacial processes of proteins in terms of rotational diffusion, electron transfer, and protein and cofactor structural changes. The present article reviews the results obtained for the haem protein cytochrome c, which is widely used as a model protein for studying the various reaction steps of interfacial redox processes in general. In addition, it is shown that electric field effects may be functional for the natural redox processes of cytochrome c in the respiratory chain, as well as for the switch from the redox to the peroxidase function, one of the key events preceding apoptosis.

Thermal fluctuations determine the electron-transfer rates of cytochrome c in electrostatic and covalent complexes.

The heterogeneous electron-transfer (ET) reaction of cytochrome c (Cyt-c) electrostatically or covalently immobilized on electrodes coated with self-assembled monolayers (SAMs) of omega-functionalized alkanethiols is analyzed by surface-enhanced resonance Raman (SERR) spectroscopy and molecular dynamics (MD) simulations. Electrostatically bound Cyt-c on pure carboxyl-terminated and mixed carboxyl/hydroxyl-terminated SAMs reveals the same distance dependence of the rate constants, that is, electron tunneling at long distances and a regime controlled by the protein orientational distribution and dynamics that leads to a nearly distance-independent rate constant at short distances. Qualitatively, the same behavior is found for covalently bound Cyt-c, although the apparent ET rates in the plateau region are lower since protein mobility is restricted due to formation of amide bonds between the protein and the SAM. The experimental findings are consistent with the results of MD simulations indicating that thermal fluctuations of the protein and interfacial solvent molecules can effectively modulate the electron tunneling probability.

Gated electron transfer of cytochrome c6 at biomimetic interfaces: a time-resolved SERR study.

The electron shuttle heme protein Cyt-c(6) from the photosynthetic cyanobacterium Nostoc sp. PCC 7119 was immobilized on nanostructured Ag electrodes coated with SAMs that mimic different possible interactions with its natural reaction partner PSI. The structure, redox potential, and electron-transfer dynamics of the SAM-Cyt-c(6) complexes were investigated by TR-SERR spectroelectrochemistry. It is shown that the heterogeneous electron-transfer process is gated both in electrostatic and hydrophobic-hydrophilic complexes. At long tunneling distances, the reaction rate is controlled by the tunneling probability, while at shorter distances or higher driving forces, protein dynamics becomes the rate-limiting event.

Direct observation of the gating step in protein electron transfer: electric-field-controlled protein dynamics.

Heterogeneous electron transfer of proteins at biomimetic interfaces is characterized by unusual distance dependences of the electron-transfer rates, whose origin has been elusive and controversial. Using a two-color, time-resolved, surface-enhanced resonance Raman spectroelectrochemical approach, we have been able to monitor simultaneously and in real time the structure, electron-transfer kinetics, and configurational fluctuations of cytochrome c electrostatically adsorbed to electrodes coated with self-assembled monolayers. Our results show that the overall electron-transfer kinetics is determined by protein dynamics rather than by tunnelling probabilities and that the protein dynamics in turn is controlled by the interfacial electric field. Implications for interprotein electron transfer at biological membranes are discussed.