title: Decomposition of amino diazeniumdiolates (NONOates): molecular mechanisms.
abstract: Although diazeniumdiolates (X[N(O)NO](-)) are extensively used in biochemical, physiological, and pharmacological studies due to their ability to release NO and/or its congeneric nitroxyl, the mechanisms of these processes remain obscure. In this work, we used a combination of spectroscopic, kinetic, and computational techniques to arrive at a quantitatively consistent molecular mechanism for decomposition of amino diazeniumdiolates (amino NONOates: R2N[N(O)NO](-), where R=N(C2H5)2 (1), N(C3H4NH2)2 (2), or N(C2H4NH2)2 (3)). Decomposition of these NONOates is triggered by protonation of their [NN(O)NO](-) group with the apparent pKa and decomposition rate constants of 4.6 and 1 s(-1) for 1; 3.5 and 0.083 s(-1) for 2; and 3.8 and 0.0033 s(-1) for 3. Although protonation occurs mainly on the O atoms of the functional group, only the minor R2N(H)N(O)NO tautomer (population ~10(-7), for 1) undergoes the NN heterolytic bond cleavage (kd~10(7) s(-1) for 1) leading to amine and NO. Decompositions of protonated amino NONOates are strongly temperature-dependent; activation enthalpies are 20.4 and 19.4 kcal/mol for 1 and 2, respectively, which includes contributions from both the tautomerization and bond cleavage. The bond cleavage rates exhibit exceptional sensitivity to the nature of R substituents which strongly modulate activation entropy. At pH<2, decompositions of all three NONOates that have been investigated are subject to additional acid catalysis that occurs through di-protonation of the [NN(O)NO](-) group.
authors.affiliation: Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973, United States.
authors.forename: Nizamuddin
authors.initials: N
authors.lastname: Shaikh
authors.affiliation: William R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, United States.
authors.forename: Marat
authors.initials: M
authors.lastname: Valiev
authors.affiliation: Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973, United States. Electronic address: Lymar@bnl.gov.
authors.forename: Sergei V
authors.initials: SV
authors.lastname: Lymar
mesh_terms: D001391:Azo Compounds / Q000737:chemistry*; D002384:Catalysis; D007700:Kinetics; D009569:Nitric Oxide / Q000737:chemistry*; D020030:Nitric Oxide Donors / Q000737:chemistry*; D009589:Nitrogen Oxides / Q000737:chemistry*; D011522:Protons; D012996:Solutions; D013237:Stereoisomerism; D013696:Temperature; D013816:Thermodynamics
chemical_list: D001391:Azo Compounds; D020030:Nitric Oxide Donors; D009589:Nitrogen Oxides; D011522:Protons; D012996:Solutions; D009569:Nitric Oxide; C039900:nitroxyl
keywords: Ab initio calculations; Diazeniumdiolates; Mechanism; NO donor; NONOates; Tautomerization