Polarography and Voltammetry of Nucleosides and Nucleotides and Their Parent Bases as an Analytical and Investigative Tool

Elving, Philip J.; O'Reilly, James E.; Schmakel, Conrad O.

doi:10.1002/9780470110416.ch6

Cited by 12 publications

(2 citation statements)

References 266 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…6), peak potential Ic does not become less pH-dependent as the pH is lowered. The essential reactions in the wave I process are, consequently, as follows, where R represents nicotinamide R d-H + ~ RH + [4] RH + -{-e ~:~ RH [5] 2RH-~ (RH)2 [6] The assumption that protonation is essentially complete before electron transfer is supported by the relatively slight pH-dependence of E1/2 and by the closeness of Ell2 for nicotinamide (--1.11V) and N'-methylnicotinamide (--1.fl8V) to that for 1-methyl-3-carbamoylpyridium ion (--1.03V) (23), where the nitrogen lone pair is not available and the electroactive species exists as a cation.…”

Section: Discussionmentioning

confidence: 99%

Nicotinamide and N'-Methylnicotinamide: Electrochemical Redox Pattern

Schmakel¹,

Santhanam²,

Elving³

1974

J. Electrochem. Soc.

View full text Add to dashboard Cite

The mechanisms for the electrochemical reduction of nicotinamide (3-carbamoylpyridine) and N'-methylnicotins_mide [3-(N-methylcarbamoyl)-pyridine] in aqueous media, as well as those for reduction and oxidation of intermediate and final products, have been investigated. Both compounds involve the same redox pattern of two successive one-electron additions. The initial step involves simultaneous addition of an electron and a proton to form a free radical (only an electron in the pH range where the compound is protonated), which rapidly dimerizes to an apparent 6,6' species; oxidation of the free radical can be seen at pH 9. At more negative potential, the nicotinamide is reduced to an apparent 1,6-dihydroDyridine species (the dimer is stable to reduction in the available potential range); the two polarographie waves begin to merge above pH 8 and are completely merged by pH 12. El~2 for both electron-transfer processes becomes more negative with increasing pH between pH 4 and 9. The rate constants for dimerization of the initially produced free radicals are 1.8 X l0 s and 4.9 X 106 liter mol-1 sec-Z for nicotinamide and N'-methylnicotinamide, respectively, at 30 ~ and pH 9. Adsorption of both comuounds and their reduction products at the mercury-solution interface is negligible. Both dimer and dihydrouyridine species are oxidized to the original nicotinamide but at considerably more positive potentials than those necessary for their formation. Nicotinamide(3-carbamoylpyridine) ( Fig. 1) is widely distributed in plant and animal tissues and is an essential dietary factor for nearly all mammalian systems. Its absence, along with that of nicotinic acid and other members of the B complex, results in pellegra. The essential role of nicotinamide undoubtedly stems from its importance in the production of certain types of pyridine nucleotides,3 e.g., NAD+ and NMN +, under physiological conditions. The function of such nucleotides in biological systems, e.g., the role of NAD+ as a coenzyme, generally involves oxidationreduction of the pyridine ring. For this reason, redox processes involving nicotinamide and its derivatives are of obvious interest.Nicotinamide itself differs from the pyridine nucleotides in having a ring nitrogen, which is basic due to the presence of the lone electron pair. Consequently, its redox behavior would be expected to show a pHdependence differing from those of the pyridine nucleotides except in the pH region where the ring nitrogen is protonated.The polarography of nicotinamide has been previously studied (1-4). Essentially, nicotinamide is reported to yield in acidic media two rather ill-defined polarographic waves having half-wave potentials, E1/2, close to one another, which have been ascribed (5, 6) to the protonated and neutraI species, presumably in an acid-base equilibrium of a type frequently seen in polarography. Others (7) observed nearly the same behavior except that the double wave was not noted. Above pH 8, a single well-defined wave was reported (5-7), whose EI/2 became 40 mV more ne...

show abstract