David L. McAllister

Electrochemical reduction of 6- and 7-hydroxypteridine: Involvement of adsorption processes in the electrochemistry of 6-hydroxypteridine

Summary Electrochemical reduction of 6-hydroxypteridine over the pH range 7–12 at mercury and pyrolytic graphite electrodes involves a 2 e − /2H + reaction to 7,8-dihydro-6-hydroxypteridine. A.c. and d.c. polarography indicate that 7,8-dihydro-6,7-dihydroxypteridine (the monohydrated form of 6-hydroxypteridine which predominates below pH 7) and 7,8-dihydro-6-hydroxypteridine are strongly adsorbed at the mercury electrode, and under proper conditions of pH and concentration, apparently form hydrogen-bonded polymeric films which cover the electrode surface. Over the pH range 0–12, 7-hydroxypteridine is reduced at mercury and pyrolytic graphite in a 2 e − /2H + reaction to 5,6-dihydro-7-hydroxypteridine. Apparently, 7-hydroxypteridine and its reduction product are not strongly adsorbed.

Electrochemistry of pteridine

Summary Over the pH range 1–12, pteridine is reduced at the dropping mercury electrode (DME) and pyrolytic graphite electrode (PGE) by way of three polarographic or voltammetric processes. The first, least negative process, is a reversible 2 e -2H + reduction of pteridine to 5,8-dihydropteridine. The latter species reacts with pteridine in a base-catalyzed Michael reaction producing a dihydro dimer, probably 7,7′,8,8′-tetrahydro-7,7′-dipteridyl. The second process involves reduction of the monohydrated form of pteridine (3,4-dihydro-4-hydroxypteridine) which exists in major amounts at low pH which is also reduced in a 2 e -2H + reaction to 5,8-dihydropteridine but in an irreversible process. Again the latter compound reacts with pteridine to give a dimer. The third process is reduction of the anion of 3,4-dihydro-4-hydroxypteridine in the same overall process as is observed for the two previous reactions forming, ultimately, the same dihydropteridine dimer. The latter dimer is also reducible electrochemically in a 2 e -2H + irreversible process to give 7,8-dihydropteridine. Rate constants have been obtained for the reversible electron transfer and for the dimerization reaction of 5,8-dihydropteridine with pteridine.

A direct method for phase-angle recording in A.C. polarography with a commercial lock-in amplifier: Analytical utility of phase-angle measurements

Summary A commercially available lock-in amplifier (PAR Model 121) is utilized in the construction of an instrument suitable for fundamental harmonic a.c. polarography, phase-selective a.c. polarography, anddirect recording of total phase angle. Equations have been derived for calculation of the total phase angle from the magnitudes and phase angles of the capacitive andfaradaic currents. The analytical utility of phase-angle measurements was investigated for the Cd2+/Cd(Hg) system. Phase-angle recording appears to have no analytical advantages over phase-selective a.c. polarography.

Electrochemical oxidation of 6- and 7-hydroxypteridine and 6,7-dihydroxypteridine at the pyrolytic graphite electrode

Summary Over the pH range 2.3–10, 6-hydroxypteridine is oxidized electrochemically at the PGE by way of two voltammetric peaks. The first, least positive, peak is due to a 2 e –2H + oxidation to 6,7-dihydroxypteridine. In very acidic solution (2 M H 2 SO 4 ), 7-hydroxypteridine also gives two voltammetric peaks. Again the first, least positive, peak is a 2 e –2H + oxidation to 6,7-dihydroxypteridine. The second, more positive, peaks of both compounds are due to electrochemical oxidation of 6,7-dihydroxypteridine which occurs over the pH range of 2.3–9. Under voltammetric conditions this peak is due to a 2 e –2H + oxidation to a di-imine of 6,7-dihydroxypteridine that very rapidly hydrates to a diol. Under conditions of controlled potential electrolysis, however, the oxidation involves transfer of approximately 6 e . Under the latter conditions the initial electrode product, the bridgehead diol of 6,7-dihydroxypteridine under goes rearrangement, further oxidation and hydrolysis yielding as final products, tetraketopiperazine, oxamide, urea, oxamic acid, ammonia, formaldehyde, formic acid and CO 2 . The electrochemical and related chemical reactions were studied by d.c. polarography, cyclic and linear sweep voltammetry, coulometry, and macroscale electrolysis followed by product isolation and identification, and radioisotope tracer techniques.

Electrochemical reduction of purine-2,6-disulfonic acid: Polarography and controlled potential electrolysis

Summary Between pH 0 and 13 purine-2,6-disulfonic acid (P-2,6-DSo) shows a total of 8 polarographic waves at the DME. The nature of these waves and the products were investigated by polarography, cyclic voltammetry at the pyrolytic graphite electrode, coulometry, macroscale electrolysis, and u.v. absorption spectra. At low pH (0–3.5), P-2,6-DSo is reduced polarographically in 3 steps: the first, a 2e process (wave Ia) to give 1,6-dihydropurine-2,6-disulfonic acid; the second, a 4e reduction (wave III) of the wave Ia product to 1,6-dihydropurine; and the third, a 2e reduction (wave IVa) of 1,6-dihydropurine to 1,2,3,6-tetrahydropurine, which hydrolyzes to a 4-aminoimidazole. A small kinetic wave (wave II), corresponding to the reduction of purine -2-sulfonic acid which is produced via slow chemical and electrochemical reactions of the wave Ia product, is also observed at low pH. Under coulometric conditions at low pH (0–3.7), P-2,6-DSo is reduced initially in a 5e process (wave Ia) involving loss of H2S and sulfurous acid to give purine-2-sulfonic acid which is reduced in a 7e step (wave II), again with loss of H2S and sulfurous acid, to 1,6-dihydropurine, which is reduced as before (wave IVa, 2e) to 1,2,3,6-tetrahydropurine. Between pH 3.8 and 4.8, waves II and III merge to give wave V. Coulometric reduction of P-2,6-DSo on the crest of wave V requires 11e and the product is 1,6-dihydropurine, which can be further reduced in a 2e process as before. At intermediate pH (5.1–6.6) only one large (12e coulometric, 8e polarographic) wave (wave VI), involving the reduction of P-2,6-DSo to a 4-aminoimidazole, is observed. Between pH 6.8 and 13, identical 2-step reductions occur under both polarographic and coulometric conditions: the first, a 2e reduction (wave Ib) to purine-2-sulfonic acid with loss of sulfurous acid; the second, a 6e reduction (wave IVb) of purine-2-sulfonic acid to the 4-aminoimidazole.