tradiol in 50% v/v methanol/water 0.1 M buffer. However, when the percentage of methanol was reduced to 5% v/v, an adsorptive couple was observed even at "pH" 4.0.Cyclic voltammograms of -estradiol (Figure 16B), the stereoisomer of ß-estradiol in the 17-OH position, under identical conditions to the /3-estradiol experiments, showed significant differences in position and shape of the adsorptive responses relative to that of ß-estradiol (Figure 16A). Oestrone (a phenolic steroid not possessing a 17-OH group) did not show any peaks at all in CV. Obviously the addition of the 17-OH group played an important role in the manifestation of the capacitative phenomena observed in CV as did the 3-OH group of the "A" ring phenol.
ConclusionsThe electron-transfer reactions of acetylenic steroid hormones at the mercury electrode are strongly influenced by interaction with the electrode surface. The surface interactions in turn are dependent upon the type and orientation of the substituents.The dichotomy in behavior between the oxidation process for the "phenolic" acetylenic steroid ethynylestradiol and the non-phenolic acetylene steroids can be rationalized in terms of their adsorptive properties at the mercury electrode. Phenolic steroids in general have specific reversible interactions with the mercury surface at high "pH". These interactions are presumably a result of specific orientations on the electrode surface and depend upon the surface charge, stereoisomerism of 17-OH group, phenol/phenolate A ring/"pH", and methanol content. Such physical and chemical considerations have been expressed for specific orientations of other aromatic compounds at the platinum electrode.21"23 Furthermore it is likely that these specific orientations for phenolic steroids facilitate the electrode process for the oxidation of mercury in the presence of ethynylestradiol, resulting in a comparatively well-defined process for the latter.However, at pH >10, the non-phenolic acetylenic steroids do not exhibit specific electrode adsorption behavior and are much more strongly adsorbed than the corresponding phenolic steroids. The preferred orientation and strong adsorption of this class of steroid is believed to induce the observed irreversible electrode reaction behavior. The orientation of adsorbed intermediates is known to exert profound effects on electrochemical processes.24
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