Facilitated protamine transfer at polarized water/1,2-dichloroethane interfaces studied by cyclic voltammetry and chronoamperometry at micropipet electrodes

Cyclic voltammetry and chronoamperometry at micropipet electrodes were applied to study the phase transfer of polypeptide protamine facilitated by complexation with charged ionophore dinonylnaphthalenesulfonate (DNNS) at polarized water/1,2-dichloroethane (DCE) interfaces, i.e., sDNNS(-) (DCE) + protamine(n+) (aq) reversible arrow protamine-DNNS complex (DCE). Well-defined current responses based on the selective protamine transfer were obtained reproducibly even in the presence of 0.12 M NaCl. The selective and reproducible responses make this voltammetric/amperometric approach an attractive alternative to the traditional potentiometric counterpart based on mixed potential responses, for which both protamine and Na+ need to be transferred simultaneously. Using both organic- and water-filled micropipet electrodes, the reaction mechanism was studied under different mass-transfer conditions controlled by diffusion of protamine, DNNS, and the complex in the outer solution of the pipets. Both charge number of transferred protamine, n, and complexation stoichiometry, s, were determined to be similar to20 by chronoamperometry. With these parameters, the electrochemically irreversible voltammograms were analyzed by assuming a one-step transfer model to obtain experimental transfer coefficients, which represent apparent dependence of the transfer rate on the interfacial potential. The analysis showed that the transfer coefficients are much larger or smaller than a normal value of similar to0.5 and strongly depend on the diffusion-limiting species, i.e., 0.088 +/- 0.005, 0.89 +/- 0.01, and 0.065 +/- 0.008 for protamine, DNNS, and the complex, respectively. The apparently anomalous transfer coefficients were explained consistently by a phenomenological model based on adsorption and transfer processes.