Dimerization of HNO in aqueous solution: an interplay of solvation effects, fast acid-base equilibria, and intramolecular hydrogen bonding?

The recent unraveling of the rather complex acid-base equilibrium of nitroxyl (HNO) has stimulated a renewed interest in the significance of HNO for biology and pharmacy. HNO plays an important role in enzymatic mechanisms and is discussed as a potential therapeutic agent against heart failure. A cumbersome property for studying HNO reactions, its fast dimerization leading to the rapid formation of N(2)O, is surprisingly far from being well understood. It prevents isolation and limits intermediate concentrations of nitroxyl in solution. In this study, a new mechanism for the HNO dimerization reaction in aqueous solution has been theoretically derived on the basis of DFT calculations. Detailed analysis of the initial reaction step suggests a reversal of the cis-trans isomer preference in solution compared to the corresponding gas phase reaction. In contrast to a gas phase derived model based on intramolecular rearrangement steps, an acid-base equilibrium model is in agreement with previous experimental findings and, moreover, explains the fundamental differences between the well studied gas phase reaction and the solvent reaction in terms of polarity, cis-trans isomerizations, and acidities of the intermediates. In the case of cis-hyponitrous acid, the calculated pK(a) values of the acid-base equilibria were found to be significantly different from the corresponding experimental value of the stable trans isomer. Under physiological conditions, N(2)O formation is dominated by the decomposition of the unstable monoanion cis-N(2)O(2)H(-) rather than that of the commonly stated cis-HONNOH.