Micellar catalysis of nitric oxide dissociation from diazeniumdiolates

The effect of surfactant micelles on the acid-catalyzed dissociation of NO from diazeniumdiolate ions of structure (RRN)-R-1-N-2[N(O)NO](-) has been examined in phosphate- buffered solutions at 37 degreesC. The reaction behavior of zwitterionic substrates [2, R-1 = R-2 = H2N(CH2)(2); 3, R-1 = R-2 = H2N(CH2)(3); 4, R-1 = n-Pr, R-2 = H2N(CH2)(3); 5, R-1 = H2N(CH2)(3), R-2 = H2N(CH2)(3)NH(CH2)(4)] and anionic substrates [1, R-1 = R-2 = Et; 6, R-1 = R-2 = n-Pr] has been compared. All but DEANO (1) are catalyzed by anionic micelles of sodium dodecyl sulfate (SDS) but are unaffected by the presence of cationic cetyltrimethylammonium bromide or the zwitterionic surfactant 3-(N-dodecyl-N,N-dimethylammonio)-1- propanesulfonate (lauryl sulfobetaine). Catalysis by sodium decylphosphonate micelles has also been demonstrated for 2 (DETANO). The surfactant-mediated catalysis is discussed in terms of a distribution model with simultaneous reaction in the water and micellar pseudophases. Binding constants (K-s) for diazeniumdiolate association with the surfactant ;micelles have been obtained, and a comparison of second-order rate constants, k(2m) and k(2w), for their acid-catalyzed dissociation in the micellar and aqueous phases, respectively, has been made. For the zwitterionic polyamine diazeniumdiolates 2-5, the K-s values show good correlation with the number of positively charged nitrogen centers in the substrates, consistent with micellar association between protonated nitrogens in the zwitterions and the anionic headgroups of the micelle. The Coulombic interaction of zwitterionic substrates with SDS micelles is compared with the weak hydrophobic association which was found with the anionic diazeniumdiolate 6.

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