Interfacial electrochemistry of hypophyseal polypeptide hormones

The adsorption and the electrode reaction of the cyclic polypeptide hormones oxytocin, desamino-oxytocin and vasopressin at the mercury/water interface was investigated by a.c. and d.c. polarography and by cyclic voltammetry. The saturation surface concentrations as obtained by the different methods under different potential oscillations, and at two pH values (2 and 6.6) varied between 3 × 10¹³ and 5 × 10¹³ molecules cm^-2. The kinetics of the adsorption-desorption reaction and of the electrode process was in every case the slowest for the non-charged desamino-oxytocin. Vasopressin behaved the most reversibly in a.c. polarography, while oxytocin showed the most reversible behavior in cyclic voltammetry. The difference in behavior is related to the difference in charge and hydrophobicity of the different polypeptides, affecting adsorption interactions and the catalytic reaction with the mercury surface.