Martín Gómez

Towards a molecular-level understanding of the reactivity differences for radical anions of juglone and plumbagin: an electrochemical and spectroelectrochemical approach

An electrochemical and spectroelectrochemical strategy is presented for evaluating reactivity differences in the semiquinone anions from naturally occurring quinones juglone (5-hydroxy-1,4-naphthoquinone) and plumbagin (2-methyl-5-hydroxy-1,4-naphthoquinone). By employing cyclic voltammetry and in situ spectroelectrochemical electron spin resonance measurements, it was found that while semiquinone species generated from plumbagin are stable radical anions in DMSO solution, the species generated from juglone are more reactive. These latter species are involved in a self-protonation process involving a slow rate of protonation (1.8-2.1 mol L(-1)) due to the mild acidity of the OH group at the C-5 position. This result is important when considering observed differences in biochemical reactivity for these quinones, particularly in cases where mediated cytotoxic action is provoked by these agents, as is discussed in this work.

The association of neutral systems linked by hydrogen bond interactions: a quantitative electrochemical approach

The electrochemical characterization of the neutral–neutral association by hydrogen bonds was performed on the basis of voltammetric current measurements. The diffusion coefficient of the electroactive compound is modified by effect of association, this provoke important variations in the voltammetric current peak. As an example of the weak hydrogen bond between neutral complexes, it was determined that 1,4-benzoquinone (Q) and benzoic acid (HBz) can associate with a 1:1 stoichiometry with a conditional association constant between 10 and 15 M � 1 . The Q(HBz) complex geometry was optimized using density functional theory and Moller–Plesset perturbation theory. In both theories, the most stable geometry is flat and exhibits two hydrogen bond interactions: O–H ��� O and C–H ��� O interactions. The binding energy at our best level of theory was )7.7 kcal/mol, that supports the stability of the 1:1 Q–HBz complex and which is accord with the values of the conditional association constant obtained from the electrochemical method here described. 2002 Elsevier Science B.V. All rights reserved.

Correlation between Hydrogen Bonding Association Constants in Solution with Quantum Chemistry Indexes: The Case of Successive Association between Reduced Species of Quinones and Methanol

The functional M05-2X together with the SMD solvent model have been used to calculate hydrogen bonding association constants in dimethylsulfoxide (DMSO) solution. Data of equilibrium constants in DMSO for the case of electrochemically generated dianions interacting with methanol have been considered to test the reliability of the chemistry theoretical approach. From this approach, it was found that the successive association constants involved in the formation of the complexes depend on a linear combination of three quantum chemistry indexes which are the ionization energy, the electron affinity, and the charge on the oxygen atom receiving the methanol molecule. Under this perspective, the stoichiometry of all the dianion-methanol complexes was explained on the basis of the relative strength of the hydrogen bonding interaction compared to that of the methanol-DMSO and methanol dimer complexes. This linear combination seems to be valid regardless of the nature of the dianion structure and the number of methanol molecules in the complex, which is a relevant finding to generalize the applicability of both the functional M05-2X and the SMD solvent model, to calculate association constants for any other neutral or anionic molecules interacting by hydrogen bonding with proton donors.