Diazeniumdiolate reactivity in model membrane systems

The effect of small unilamellar phospholipid vesicles on the acid-catalyzed dissociation of nitric oxide from diazeniumdiolate ions, R1R2N[N(O)NO]−, [1: R1 = H2N(CH2)3-, R2 = H2N(CH2)3NH(CH2)4-; 2: R1 = R2 = H2N(CH2)3-; 3: R1 = n-butyl-, R2 = n-butyl-NH2+(CH2)6-; 4: R1 = R2 = nPr-] has been examined at pH 7.4 and 37 °C. NO release was catalyzed by anionic liposomes (DPPG, DOPG, DMPS, POPS and DOPA) and by mixed phosphatidylglycerol/phosphatidylcholine (DPPG/DPPC and DOPG/DPPC) covesicles, while cationic liposomes derived from 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) and the zwitterionic liposome DMPC did not significantly affect the dissociation rates of the substrates examined. Enhancement of the dissociation rate constant in DPPG liposome media (0.010 M phosphate buffer, pH 7.4, 37 °C) at 10 mM phosphoglycerol levels, ranged from 37 for 1 to 1.2 for the anionic diazeniumdiolate 4, while DOPA effected the greatest rate enhancement, achieving 49-fold rate increases with 1 under similar conditions. The observed catalysis decreases with increase in the bulk concentration of electrolytes in the reaction media. Quantitative analysis of catalytic effects has been obtained through the application of pseudo-phase kinetic models and equilibrium binding constants at different liposome interfaces are compared. The stoichiometry of nitric oxide release from 1 and 2 in DPPG/DPPC liposome media has been obtained through oxyhemoglobin assay. DPPG = 1,2-dipalmitoyl-sn-glycero-3-[phospho-rac-(1-glycerol)], DOPG = 1,2-dioleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)], DMPS = 1,2-dimyristoyl-sn-glycero-3-[phospho-l-serine], POPS = 1-palmitoyl-2-oleoyl-sn-glycero-3-[phospho-l-serine], DOPA = 1,2-dioleoyl-sn-glycero-3-phosphate; DPPC = 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, DMPC = 1,2-dimyristoyl-sn-glycero-3-phosphocholine, DOTAP = 1,2-dioleoyl-3-trimethylammonium-propane.