Electrochemical Redox Patterns for Pyridinium Species: 1-Methylnicotinamide and Nicotinamide Mononucleotide

Abstract

The detailed mechanisms for the electrochemical reduction in aqueous media of pyridinium species have been elucidated on the basis of the electrochemical, spectrophotometric, and chemical properties of two such species and their reduction products: 1‐methyl‐3‐carbarnoylpyridinium ion (1‐methylnicotinamide; MCP⁺) and nicotinamide mononucleotide (NMN⁺). An initial reversible pH‐independent one‐electron (1e) addition to the pyridinium ion to produce a free radical is followed by its irreversible dimerization to an apparent 6,6' species. At more negative potential, the free radical is reduced to an apparent 1,6 dihydropyridine species (2e reduction product; NMN⁺ may form some 1,4 isomer); the dimer is stable to reduction within the available potential range. At sufficiently positive potential, both le and 2e reduction products can be oxidized back to MCP⁺ or NMN⁺. Rate constants for the free radical dimerization at and ; activation energies are . While MCP⁺ is negligibly adsorbed, its dimeric and dihydropyridine products are strongly and moderately adsorbed, respectively; NMN⁺ and its reduction products are negligibly adsorbed. Both reduction products are susceptible to acid‐catalyzed hydrolysis; the rate increases with decreasing pH; at any given pH, the dimer is less stable than the dihydropyridine species.