N-Diazeniumdiolate (NONOate) als Stickstoffmonoxid (NO)-Donatoren
In the past two decades, the chemistry of nitric oxide (NO) has become an area of high interest in biomedical research. For living organisms, nitric oxide has been shown to mediate a variety of physiological and pathophysiological processes. Diazeniumdiolates (NONOates) are known as a class of compounds that can release NO spontaneously under physiological conditions. NONOates derived from aliphatic amines such as diethylamine (DEA/NO), spermine (SPER/NO), N,N’-dimethyl-1,6-hexanediamine (MAHMA/NO) and diethylenetriamine (DETA/NO) have extensively been described and their NO-releasing properties have been studied in detail. In aqueous solution, the decomposition of NONOates follows first-order kinetics, yielding nitric oxide and the parent amine. Depending on pH, temperature and structure, the reported half-life times range from a few seconds to several hours. Figure 1. Structures of four different diazeniumdiolates. Half-life times t1/2 at pH 7.4 and 37 °C. In view of modern applications of NO-donating compounds as drugs, the release of nitric oxide should not occur spontaneously. Thus, there has been a growing interest in the synthesis of novel diazeniumdiolates with the aim to deliver NO to a specific organ or cell type. With the view to guide diazeniumdiolates to specific organs, we were interested in the synthesis of new branched polyamines. As compared to the well studied linear donors NONOates derived from branched polyamines are believed to be more stable. Due to the number of amino groups the release of NO should take place only by interaction with a natural receptor. Glycosaminoglycans (GAGs), a component of the extracellular matrix, are useful targets for polyamine-based diazeniumdiolates because the high number of sulphate and uronic acid groups in the carbohydrate polymers leads to a significant negative charge density under physiological conditions. Thus, the possible interactions of the NO donors with GAGs include electrostatic interactions such as hydrogen bonding and ion-ion interactions. Polyamine 1 was synthesized in a multistep reaction by using Michael addition and reduction methods. Compound 2 was prepared by alkylation of tris(2-aminoethyl)amine with ethelenedibromide. Starting from α-methylenglutaronitrile and ethylenediamine the tetranitrile 3 was synthesized by Michael addition reaction. Figure 2. Sructures of the branched polyamines (1, 2) and polynitrile (3) synthesized by using alkylation, reduction and Michael addition reactions. Plasma cell membranes are useful targets for hydrophobic diazeniumdiolates. However, these NO donors are not only promising NO-releasing agents, but are also useful compounds for the molecular characterization of micellar systems. Ionic micelles have long been considered as an excellent model system for studying aqueous interfaces and interfacial phenomena. In order to estimate the pH value in the Stern region of SDS micelles, we focused on the preparation of novel, surfactant-like diazeniumdiolates with a predominant hydrophobic structure. Zwitterionic diazeniumdiolates of the form RN[N(O)NO]─(CH2)2NH2+R, where R = CH3 (4), R = (CH2)3CH3 (5), R = (CH2)5CH3 (6) and R = (CH2)7CH3 (7) were synthesized by reaction of the corresponding diamine with nitric oxide. It was shown that the release of NO from the diazeniumdiolates is markedly catalyzed by anionic micelles of SDS. First–order rate constants for the decay of 4 – 7 were determined in phosphate buffer (pH 7.4) at 22 °C as function of the SDS concentration. Micellar binding constants, KSM, for the association of diazeniumdiolates with the SDS micelles were also obtained for 4 ‑ 6. The KSM values show a significant increase with increasing length of the alkyl side chains. The release of NO from 4 – 6 was further studied in presence of dioxane, cationic and nonionic surfactants. No special effect has been observed. Finally, all kinetic data were compared and discussed in terms of the particular structure of the negatively charged micelles. The utility of 5 and 6 as probes for the determination of the pH value in the Stern region of SDS micelles was demonstrated. For the zwitterionic donors 5 and 6 the dissociation of NO was studied in various buffered solutions between pH 5 and 8 at 22°C. From the pH dependence of the first–order rate constants and the rate data obtained for the surfactant–mediated decay, the pH value in the Stern region was evaluated. For a bulk phase of pH 7.4 in the Stern region a pH value of 5.9 was determined, consistent with the distribution of H+ in the vicinity of negatively charged micelles. Figure 3. Structures of the zwitterionic diazeniumdiolates 4 – 7 prepared for the study of the Stern region of sodium dodecyl sulfate (SDS) micelles. From simple kinetic studies, we could get more insight into the molecular structure of the micellar aggregates. The novel method described represents a simple and fast route for advanced studies of interfacial phenomena in micellar systems. Due to their hydrophobic structure, diazeniumdiolates such as 5 – 7 may also be useful for the preparation of novel NO-releasing polymers.