In electrochemistry, protein film voltammetry (or protein film electrochemistry, or direct electrochemistry of proteins) is a technique for examining the behavior of proteins immobilized (either adsorbed or covalently attached) on an electrode. The technique is applicable to proteins and enzymes that engage in electron transfer reactions and it is part of the methods available to study enzyme kinetics. Provided that it makes suitable contact with the electrode surface (electron transfer between the electrode and the protein is direct) and provided that it is not denatured, the protein can be fruitfully interrogated by monitoring current as a function of electrode potential and other experimental parameters. Various electrode materials can be used. Special electrode designs are required to address membrane-bound proteins.
Experiments with redox proteins Small redox proteins such as cytochromes and ferredoxins can be investigated on condition that their electroactive coverage (the amount of protein undergoing direct electron transfer) is large enough (in practice, greater than a fraction of pmol/cm2). Electrochemical data obtained with small proteins can be used to measure the redox potentials of the protein's redox sites, the rate of electron transfer between the protein and the electrode, or the rates of chemical reactions (such as protonations) that are coupled to electron transfer.
Interpretation of the peak current and peak area
In a cyclic voltammetry experiment carried out with an adsorbed redox protein, the oxidation and reduction of each redox site shows as a pair of positive and negative peaks. Since all the sample is oxidised or reduced during the potential sweep, the peak current and peak area should be proportional to scan rate (observing that the peak current is proportional to scan rate proves that the redox species that gives the peak is actually immobilised). The same is true for experiments performed with non-biological redox molecules adsorbed onto electrodes. The theory was mainly developed by the French electrochemist Etienne Laviron in the 1980s,,. Since both this faradaic current (which results from the oxidation/reduction of the adsorbed molecule) and the capacitive current (which results from electrode charging) increase in proportion to scan rate, the peaks should remain visible when the scan rate is increased. In contrast, when the redox analyte is in solution and diffuses to/from the electrode, the peak current is proportional to the square root of the scan rate (see: Randles–Sevcik equation).
Peak area Irrespective of scan rate, the area under the peak (in units of AV) is equal to n F A Γ ν {\displaystyle nFA\Gamma \nu } , where n {\displaystyle n} is the number of electrons exchanged in the oxidation/reduction of the center, A {\displaystyle A} is the electrode surface and Γ {\displaystyle \Gamma } is the electroactive coverage (in units of mol/cm2). The latter can therefore be deduced from the area under the peak after subtraction of the capacitive current.
Peak shape
Slow scan rate At slow scan rates there should be no separation between the oxidative and reductive peaks.
A one-electron site (e.g. a heme or FeS cluster) gives a broad peak (fig 1A). The equation that gives the shape and intensity of the peak is:
i F 2 ν A Γ / R T = ± exp ( F R T ( E − E 0 ) ) ( 1 + exp ( F R T ( E − E 0 ) ) 2 {\displaystyle {\frac {i}{F^{2}\nu A\Gamma /RT}}=\pm {\frac {\exp \left({\frac {F}{RT}}(E-E^{0})\right)}{(1+\exp \left({\frac {F}{RT}}(E-E^{0})\right)^{2}}}}
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