Inner reorganization during the radical–biradical transition in a nor-β-lapachone derivative possessing two redox centers

Abstract

In this work, the electrochemical behaviour of an antitumoral nitro o-quinone derivative obtained from 3-bromo-nor-beta-lapachone was studied. Cyclic voltammetric experiments, in acetonitrile solution, revealed that both quinone and nitro functions are reduced independently as quasi-reversible one-electron transfer processes in this order. Depending on the reduction potential, a radical anion or a biradical dianion is obtained. The formation of these paramagnetic species was confirmed by performing in situ Electrochemical-Electron Spin Resonance (E-ESR) experiments. Analysis of the kinetics of electron transfer associated to those electron uptake processes, in terms of the Marcus-Hush-Levich model, revealed differences in the reorganization energy (lambda((k))) for both steps (lambda((I)): 1.07-1.11 eV; lambda((II)): 1.21-1.30 eV). By evaluating the conformations of the radical and biradical systems by calculations at the BLYP//TZVP level of theory, it was found that the inner component, for the second reduction process (lambda((II))) was approximately 72% of lambda((II)), reflecting modifications in the molecular structure during the radical anion-biradical dianion transition. This change is also reflected in the differences presented by line widths of the ESR signals of both electrogenerated radical and diradical species.

Revisiting the electrochemical formation, stability and structure of radical and biradical anionic structures in dinitrobenzenes

“…This behavior is consistent with the presented mechanism of reduction of this compound. Both radical species behave independently in the same structure, as already described for the reduction of independent redox centers that also generate stable biradical dianionic species [47]. This independent behavior of both radical species indicates that inductive effects which allow the transport of electrons from the reduced sites do not allow a proper coupling of the electron spins being added upon reduction of the respective sites in the molecule (at -NO 2 groups at positions C-3 and C-5 and the COOCH 3 function at C-1), which would not be the case if resonant effects were present (as in ortho or para substituted compounds).…”

Electrochemical study, on mercury, of a Meta-nitroarylamine derivative of nor-β-lapachone, an antitumor and trypanocidal compound

“…According to the previous paper for the reduction of the same compound in acetonitrile, on glassy carbon electrode, 13 the second cathodic peak appearing at a more negative potential would correspond to the further one-electron reduction of the electrogenerated radical anion in order to obtain a quinone-nitro biradical dianion, according to Equation 5:…”

“…13 Therein, it was shown that both quinone and nitro functions are reduced independently as quasi-reversible one-electron transfer processes and afford different radical ion species. Depending on the reduction potential, the presence of a radical anion or a biradical dianion was confirmed by performing in-situ Electrochemical-Electron Spin Resonance (E-ESR) experiments.…”

“…Depending on the reduction potential, the presence of a radical anion or a biradical dianion was confirmed by performing in-situ Electrochemical-Electron Spin Resonance (E-ESR) experiments. 13 In this paper, we carried out a comprehensive cyclic voltammetric study on a mercury electrode in order to study the reduction of [Q]-PhNO 2 and the consequent formation of radical anions. The focus of this work is the quantitative measurement of the stability constants applying the Olmstead's methodology.…”

Oncocalyxone A: electrochemical, spectroscopic investigation and studies of its interaction with DNA, nucleobases and N-acetylcysteine

“…electron spin resonance, ESR), would give important structural information. 14 In protic medium, electrochemical studies on DNA and also in the presence of biologically important thiols like N-acetylcysteine (NAC) can be performed 15 to test the Michael acceptor nature of this quinone, 10,11 which would favour its pro-oxidant nature, with relevance in antitumor activity. 12,16,17 More specifically, the use of NAC is relevant once it has been used as tools for investigating the role of ROS (reactive oxygen species) in numerous biological and pathological processes.…”