Direct electron transfer to putidaredoxin (Pdx) in solution from bare gold and mercaptoethylamine-modified gold electrodes was studied by cyclic voltammetry. The current response of Pdx at a bare gold electrode was irreversible and highly unstable. The modified electrode offered improved, but still impersistent, quasi-reversible current response. Surface-enhanced Raman spectroscopy revealed that mercaptoethylamine adsorbed on gold was in mixed trans and gauche conformations at 0.0 V (Ag/AgCl) initially. As the potential was changed anodically or cathodically over the range of +0.4 to -0.8 V, the adsorbate population became largely gauche in conformation. A link between the conformation change and deterioration of Pdx current response at the electrode was suggested. Simulations of the electrochemical system yielded an estimated Pdx diffusion coefficient D = 1.4 x 10(-7) cm(2)/s and the apparent heterogeneous electron transfer rate constant k degrees = 1.0 x 10(-4) cm/s.