No abstract

The electrochemical behavior over the available pH range of the fundamental sequence of adenine (20) P. J. Elving, Pure Appl. Chem., 7, 423 (1963). ( 21) Inflections or breaks on £>/2-pH plots between pH 2.0 and 2.5 (Table I) are probably due to the change from chloride to Mcllvaine buffer rather than to pH.(22) C. A.

No abstract

The electrochemical reduction of 6‐substituted purines has been examined over the available pH range by direct‐ and alternating‐current polarography, and the effects of substitution evaluated in respect to significant electrochemical characteristics. Where the 1,6 normaland 3,2N=C bonds are available in the pyrimidine moiety, a four‐electron polarographic wave due to hydrogenation of these bonds is observed; purine itself and 6‐methylpurine give two twoelectron waves. The energy‐controlling step involves reduction of the protonated 1,6normalthinsp;N=C bond. The current is essentially diffusion controlled at low pH where the wave is relatively constant in height and kinetically controlled at higher pH where it begins to disappear. Correlation of the ease of reduction, as measured by E½ , with total polar substituent constant false(σpfalse) and polar substituent constant (σ*) indicates either an absence of mesomeric and steric effects due to the substituents or their transmittal by an inductive mechanism and, further, that none of the substituents exerts any specific effect different from those exerted by the other substituents. Substitution in the 6‐position influences mostly the Nfalse(1false)‒Cfalse(6false) bond order and the electronic charge distributions on N(1) and N(3), which, in turn, largely determine the reducibility; E½ correlates linearly with the quantum mechanically calculated magnitudes of these quantities, but not with other bond orders and charges. E½ also correlates linearly with electron‐acceptor properties as represented by the calculated energy of the lowest empty molecular orbital ( ki coefficients) and with pKa for proton acquisition (the protonation, which occurs prior to electron transfer, seems to have no significant role in the potential‐determining step except in the region where the wave has nearly disappeared). Isoguanine generally deviates from the various correlations. Adsorption involving an uncharged site in the molecule is markedly greater with derivatives having bulky substituents.

No abstract