Thiol activation of a model O-aryl diazeniumdiolate prodrug in phospholipid vesicle media

Thiolysis of the model diazeniumdiolate prodrug, O²-(2,4-dinitrophenyl) 1-(N,N-diethylamino)diazen-1-ium-1,2-diolate (DNP-DEA/NO, 1), by glutathione (GSH), cysteine (CYSH) and 1-heptanethiol (heptylmercaptan, HM) has been examined in anionic (DOPG), neutral (DPPC, DOPE) and cationic (DOTAP) vesicle media and in glycine buffered aqueous solutions. DOTAP vesicles accelerate the bimolecular reaction with glutathione, cysteine and 1-heptanethiol by factors of 81, 8.2 and 4630, respectively, while reaction is inhibited 5- to 10-fold in the presence of neutral and anionic vesicles. The intrinsic nucleophilicity of the thiols has been compared through the second-order rate constants, 22.9, 5.24 and 43.1 M⁻¹ s⁻¹, for nucleophilic attack on 1 by GS⁻, CYS⁻ and M⁻, respectively, obtained in buffered aqueous media. Analysis of the catalysis by DOTAP vesicles, using pseudophase ion-exchange formalism, suggests that the rate increase is due to reactant concentration in the bilayer and interfacial region coupled with enhanced dissociation of the thiol at the vesicle surface. Some contribution from enhanced nucleophilic reactivity at the vesicle interface may also contribute to the greater catalysis by HM. Inhibition of the thiolysis reaction by phospholipid liposomes is attributed to repulsion of the thiolate anions by the negatively charged acyl phosphate of the lipid head group. DOPG = 1,2-dioleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)], DPPC = 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, DOPE = 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine, DOTAP = 1,2-dioleoyl-3-trimethylammonium-propane.