Comprehensive study of the possible electrotransformations of the sodium salt of 3-nitro-4-hydroxy-7-methylthio-4H-[1,2,4]triazolo[5,1-c][1,2,4]triazinide monohydrate as potential antiviral drug

The electrochemical transformation of nitroheterocyclic compounds plays a critical role in understanding their biological activity. Despite structural similarities among nitrotriazolotriazines, their electroreduction mechanisms remain incompletely elucidated. This study focuses on the sodium salt of 3-nitro-4-hydroxy-7-methylthio-4H-[1,2,4]triazolo[5,1-c][1,2,4]triazinide monohydrate (TZV-OH), a structural analogue of the antiviral drug Triazavirin® (Riamilovir) – sodium salt of 3-nitro-4-oxo-7-methylthio-4H-[1,2,4]triazolo[5,1-c][1,2,4]triazinide dihydrate (TZV), and aims to investigate how structural differences affect the redox behavior of nitrotriazolotriazines. Using a combined experimental and computational approach, including cyclic voltammetry, chronoamperometry, electron paramagnetic resonance (EPR) spectroscopy, quantum chemical calculations, and preparative electrolysis with LC-HRMS analysis, we examined the electroreduction mechanism in both aqueous and aprotic media. Our findings reveal that TZV-OH undergoes a single-wave, diffusion-controlled, four-electron irreversible reduction of the nitro group, whereas TZV is reduced in two steps, complicated by a preceding chemical reaction. EPR spectroscopy confirms a higher concentration of radical intermediates for TZV-OH, supported by experiments with the DMPO spin trap and quantum chemical modeling. Mass spectrometry identified hydroxylamines, amines, and dimeric products as the primary products of the reduction process of TZV-OH. Quantum chemical calculations indicate that the formation of radical intermediates is thermodynamically more favorable for TZV-OH than for TZV, facilitating radical generation. These structural and mechanistic insights highlight the potential of TZV-OH as a superior antiviral agent. The integrated methodology outlined here serves as a predictive tool for assessing the pharmacological potential of nitrotriazolotriazines.