Polarography of biological purines and pyrimidines.

Since electron-transfer reactions are so important in biological systems, information which can be obtained in respect to the energy levels (electrical potentials) and reaction pathways involved should be helpful in the understanding, control and utilization of such reactions. Polarographically determinded half-wave potentials ( and electrode reaction paths or mechanisms readily provide such data under conditions generally similar to those encountered in biological systems, e.g., electrolytic reductions and oxidations often occur under conditions resembling those of enzymatic transformations: electron-transfer at a heterogeneous interface, previous occurrence of adsorption and/or adduct formation, dilute aqueous solution, pH between 2 and 9, and moderate temperature. Postulation of the extent of correlation that may exist between electrolytic and biological redox processes must at present be largely speculation. Nevertheless, to the extent that the two kinds of processes occur under similar conditions, information obtained from the examination by electrochemical techniques of the redox behavior of compounds of biological interest may help to clarify and explain biological processes with respect to the nature of the factors, such as pH, controlling the electron-transfer process. The value of electrical measurement in elucidating redox behavior of biologically important organic compounds has long been recognized. However, due to the fact that few organic redox systems behave reversibly, valid potentiometric data have been obtained for only a limited number of compounds. Where such data have been systematically gathered and analyzed, the results have been fruitful, e.g., the work of Michaelis, Clark and their couaborators during the 1920's and 1930's. Since then, most studies of the redox behavior of organic compounds have been based on polarography, generally at the dropping mercury electrode (D.M.E.) . Frequently, Elj2 data on organic compounds so determined are the only energetic data readily obtainable by electrochemical measurement. The present paper summarizes the results of polarographic examination of purines and pyrimidines in aqueous solution with emphasis on (a) the reaction path involved in electrochemical reduction and oxidation, (b) the effect of substituents and structure in altering the ease of reduction and the reaction path, and (c ) the correlation of E,,? data with electronic and other indices. The stress on reduction is simply due to the fact that, until relatively recently, systematic study of the electrochemical behavior of organic compounds was largely restricted to their reduction at mercury electrodes due to the lack of an electrode suitable for studying oxidation. In addition to classical direct-current constant-potential polarography at the D.M.E., derived techniques-controlled electrode potential electrolysis and coulometry, cyclic voltammetry, chronopotentiometry and alternating current polarography at mercury and graphite electrodes-have been used to obtain the data to be discussed. The polarography of purines and pyrimidines was reviewed several years