title: Kinetic analysis of nitroxide radical formation under oxygenated photolysis: toward quantitative singlet oxygen topology.
abstract: Reaction kinetics for two sterically hindered secondary amines with singlet oxygen have been studied in detail. A water soluble porphyrin sensitizer, 5,10,15,20-tetrakis-(4-sulfunatophenyl)-21,23H-porphyrin (TPPS), was irradiated in oxygenated aqueous solutions containing either 2,2,6,6-tetramethylpiperidin-4-one (TMPD) or 4-[N,N,N-trimethyl-ammonium]-2,2,6,6-tetramethylpiperidinyl chloride (N-TMPCl). The resulting sensitization reaction produced singlet oxygen in high yield, ultimately leading to the formation of the corresponding nitroxide free radicals (R2NO) which were detected using steady-state electron paramagnetic resonance (EPR) spectroscopy. Careful actinometry and EPR calibration curves, coupled with a detailed kinetic analysis, led to a simple and compact expression relating the nitroxide quantum yield ΦR2NO (from the doubly-integrated EPR signal intensity) to the initial amine concentration [R2NH]i. With all other parameters held constant, a plot of ΦR2NOvs. [R2NH]i gave a straight line with a slope proportional to the rate constant for nitroxide formation, kR2NO. This establishment of a rigorous quantitative relationship between the EPR signal and the rate constant provides a mechanism for quantifying singlet oxygen production as a function of its topology in heterogeneous media. Implications for in vivo assessment of singlet oxygen topology are briefly discussed.
authors.affiliation: Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA. mdef@unc.edu.
authors.forename: David F
authors.initials: DF
authors.lastname: Zigler
authors.forename: Eva Chuheng
authors.initials: EC
authors.lastname: Ding
authors.forename: Lauren E
authors.initials: LE
authors.lastname: Jarocha
authors.forename: Renat R
authors.initials: RR
authors.lastname: Khatmullin
authors.forename: Vanessa M
authors.initials: VM
authors.lastname: DiPasquale
authors.forename: R Brendan
authors.initials: RB
authors.lastname: Sykes
authors.forename: Valery F
authors.initials: VF
authors.lastname: Tarasov
authors.forename: Malcolm D E
authors.initials: MD
authors.lastname: Forbes
mesh_terms: D000588:Amines / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy; D004735:Energy Transfer; D007700:Kinetics; D015394:Molecular Structure; D009589:Nitrogen Oxides / Q000737:chemistry; D010782:Photolysis; D011166:Porphyrins / Q000737:chemistry; D026082:Singlet Oxygen / Q000737:chemistry*; D012996:Solutions; D014867:Water / Q000737:chemistry
chemical_list: D000588:Amines; D009589:Nitrogen Oxides; D011166:Porphyrins; D012996:Solutions; D014867:Water; D026082:Singlet Oxygen; C016136:tetraphenylporphine sulfonate; C039900:nitroxyl