Electrochemistry

Since many metalloenzymes catalyze transformations that involve oxidation/reduction reactions, it is important to be able to measure the reduction potentials of the metal sites. The MRIL operates an excellent electrochemical setup in the Adams laboratory [Mike Adams, UGa].

Acronyms, synonyms

• Cyclic Voltammetry (the most common electrochemical method)
• (There are many other electrochemical methods)

Measured physical quantities

• Current as a function of potential applied to an electrode
• Current as a function of time at an applied potential

Information available

• Reduction potentials and electron transfer rate constants
• Diffusion coefficient and concentration of electroactive species
• Electrochemical reversibility of an electrode reaction
• Chemical reversibility of an electrode reaction
• Electrode reaction mechanism

Information NOT available, limitations

• The location of the electroactive species at the time of electron transfer
• Identity of the electroactive species, except from the formal reduction potential
• Absolute delineation of electrode reaction mechanism involving coupled chemical reactions in solution
• Kinetic parameters for fully reversible and for fully irreversible electrode reactions

Examples of questions that can be answered

• Strength of ligand coordination of metal site(s)
• Temperature dependencies of conformational changes that affect electron transfer properties (e.g., reaction center entropy changes)
• Kinetics of coupled chemical reactions between electrochemically generated redox states and solution-resident reaction partners
• Kinetics of redox-coupled ligand/metal association and dissociationreactions

Major advantages

• Thermodynamics and kinetics of electron transfer reactions can be measured
• Methods are easy to implement and relatively inexpensive
• Very negative redox reactions can be approached (e.g., photosynthesis)
• Minimal sample volumes

Major disadvantages

• Easy to obtain results that are ambiguous
• Easy to obtain results that are corrupted by experimental artifacts
• The sample becomes an electronic component of the instrument whose properties can affect the results obtained

Sample constraints

• Minimum sample concentration of ca. 0.05 mM and minimum convenient volume of ca. 1 mL (special experiments can use very small volumes and concentrations)
• Sample must be electroactive or react with an electroactive couple
• Sample solution should be electrically conductive for most experiments