Redox properties of rubredoxin variants as a function of solvent composition and temperature: investigation of monopolar and dipolar interactions.

The role of solvent composition and temperature on equilibrium electron transfer in seven rubredoxin variants [ Clostridium pasteurianum ( Cp), V8D, V8R, V8A, V44A Cp, Pyrococcus furiosus ( Pf), and A44V Pf] were investigated to examine the role of both monopolar and dipolar interactions. The reduction potentials of all variants decreased as the polarity of the solvent decreased. The enthalpy and entropy associated with electron transfer were determined from temperature-controlled voltammetric studies. The entropic contribution [delta( Tdelta S degrees )] to the change in the reduction potential was larger for charged variants (V8D and V8R), while the enthalpic contribution [delta(-delta H degrees )] was larger for the other mutants. The large entropy change observed for monopolar variants is likely due to solvent reorganization that occurs between oxidation states. Entropic-enthalpic compensation phenomena, an observation that most proteins have an entropic term [delta( Tdelta S degrees )] and enthalpic term [delta(-delta H degrees )] with opposite signs, was observed. A correlation of the size of the amino acid side chain with delta E degrees ', delta(-delta H degrees ), and delta( Tdelta S degrees ) is also discussed.