Chemistry of the diazeniumdiolates. 2. Kinetics and mechanism of dissociation to nitric oxide in aqueous solution

Diazeniumdiolate ions of structure R2N[N(O)NO](-) (1) are of pharmacologica l interest because they spontaneously generate the natural bioregulatory sp ecies, nitric oxide (NO), when dissolved in aqueous media. Here we report t he kinetic details for four representative reactivity patterns: (a) straigh tforward dissociation of the otherwise unfunctionalized diethylamine deriva tive 2 (anion 1, where R = Et) to diethylamine and NO; (b) results for the zwitterionic piperazin-1-yl analogue 4, for which the protonation state of the neighboring basic amine site is an important determinant of dissociatio n rate; (c) data for 5, a diazeniumdiolate derived from the polyamine sperm ine, whose complex rate equation can include terms for a variety of medium effects; and (d) the outcome for triamine 6 (R = CH2CH2NH3+), the most stab le structure 1 ion identified to date. All of these dissociations are acid- catalyzed, with equilibrium protonation of the substrate preceding release of NO. Specific rate constants and pK(a) values for 2-6 have been determine d from pH/rate profiles. Additionally, a hypsochromic shift (from similar t o 250 to similar to 230 nm) was observed on acidifying these ions, allowing determination of a separate pK(a) for each substrate. For 6, the pK(a) val ue obtained kinetically was 2-3 pK(a) units higher than the value obtained from the spectral shift. Comparison of the ultraviolet spectra for 6 at var ious pH values with those for O- and N-alkylated diazeniumdiolates suggests that protonation at the R2N nitrogen initiates dissociation to NO at physi ological pH, with a second protonation (at oxygen) accounting for both the spectral change and the enhanced dissociation rate at pH <4. Our results he lp to explain the previously noted variability in dissociation rate of 5, w hose half-life we found to increase by an order of magnitude when its conce ntration was raised from near-zero to 1 mM, and provide mechanistic insight into the factors that govern dissociation rates among diazeniumdiolates of importance as pharmacologic progenitors of NO.