The electrochemical behavior over the available pH range of the fundamental sequence of adenine nucleosides and nucleotides at mercury electrodes is similar, in principle, to that of the parent adenine; up to pH 4-5, all exhibit a diffusion-controlled 4e wave due to reduction of the 1,6 and 3,2 N=C bonds in the protonated species; the wave is kinetically controlled at higher pH where it begins to disappear. El/, becomes more negative with increasing pH. Attachment of a ribose or ribosophosphate moiety decreases the ease of reducibility; electro- static effects, association, and adsorption overcome the electron-withdrawing effect of the ribose. The compounds are strongly adsorbed at ca. -0.6 V, which involves an uncharged portion of the molecule; at more negative po- tential, the species gradually desorb and may be readsorbed via the protonated portion of the molecule. The marked decrease of the experimental diffusion coefficients with concentration, and their being 3-4 times greater than those calculated on the basis of the Stokes-Einstein relation, are interpreted in terms of association, preferential orienta- tion, and conformation of the species diffusing to the electrode. The latter involves planar arrangement of the rings and vertical stacking, which results in the effective barrier to diffusional transport being essentially the minimal cross-sectional area of the planar purine moiety; these factors also influence the ease of reduction. The variation in diffusion coefficient mentioned may also provide a test for association at the millimolar concentration level.
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