Layered, electrochemically active films of bacterial iron-sulfur protein putidaredoxin (Pdx) and poly(dimethyldiallyammonium) (PDDA) polycations were constructed on gold electrodes coated with mercaptopropane sulfonate (MPS) and on quartz slides. Second-derivative UV-vis spectra suggested similar structures of Pdx in films and solutions at pH 7. Direct electrochemistry was achieved between Pdx and gold electrodes in these films, with significantly better electrochemical reversibility than in cast Nafion-lipid-Pdx films. A formal potential dispersion model gave a good fit to square wave voltammograms by regression analysis and was used to estimate an average apparent rate constant of 4.5 s(-1). Reduced Pdx in the polyion films did not react with its natural redox partner cytochrome P450(cam) because of unfavorable thermodynamics in the film environment.