Electrochemical determination of structure and interactions in spread lipid monolayers

The structure of spread monolayers of phosphatidyl choline (egg)-PC and of phosphatidyl serine (bovine spinal cord) PS in the pure state and when interacting with polypeptides were investigated by a.c. polarography and by cyclic voltammetry with the aid of D.M.E. contacting the monolayer from the gaseous (N₂) phase. The pure lipid monolayers produced around the electrocapillary maximum (ecm) a flat low-capa-citance region ((≈1.6 μF/cm²) until high adsorption-desorption capacitance peaks were obtained at high positive or negative polarization.. The relaxation time for the formation of the monolayer from the disrupted state, obtained by the cyclic voltammetry at different sweep rates and by measuring the decay of current after a potential step, varied between 0.5 and 1 ms. The access of polar groups to the mercury electrode surface was investigated by introducing the electroactive dinitrophenyl label into a fraction (≈5%) of the polar groups. From the shift in the reduction potential towards negative potentials when the monolayer was compressed, we estimated the frequency of fluctuations allowing access of polar groups to the electrode surface through the hydrocarbon region. On this basis of energetic considerations, we suggested fluctuating indentation in the planar layer. Synthetic basic polypeptides and the oligopeptide hormones, oxytocin, vasopressin and desamino-oxytocin, shift back the reduction potentials of the probe in compressed PS monolayers, indicating enhancement of the access of the polar groups to the electrode surface by the.