title: Synthesis and Electron Spin Relaxation of Tetracarboxylate Pyrroline Nitroxides.
abstract: We report the design, synthesis, and electron spin relaxation properties of hydrophilic tetracarboxylate ester pyrroline nitroxides 1 and 2, which serve as models in the search for new spin labels for DEER distance measurement at room temperature. The nitroxides are designed to have the methyl groups further away from the N-O spin site to decrease the inequivalent couplings of the unpaired electron to the methyl protons that shorten Tmat T > 70 K in currently used labels. The key step in the synthesis of 1 and 2 is the reaction of the dianion of pyrrole-1,2,5-tricarboxylic acid tert-butyl ester dimethyl ester with electrophiles such as methyl chloroformate and methyl bromoacetate. Structures of 1 and 2 are confirmed by X-ray crystallography. Studies of electron spin relaxation rates in rigid trehalose/sucrose matrices reveal approximately temperature independent values of 1/Tmfor 1 and 2 up to about 160 K and modest temperature dependence up to 295 K, demonstrating that increasing the distance between the nitroxide moiety and methyl groups is effective in lengthening Tmat T > 70 K.
authors.affiliation: Department of Chemistry, University of Nebraska , Lincoln, Nebraska 68588-0304, United States.
authors.forename: Shengdian
authors.initials: S
authors.lastname: Huang
authors.affiliation: Department of Chemistry, University of Nebraska , Lincoln, Nebraska 68588-0304, United States.
authors.forename: Joseph T
authors.initials: JT
authors.lastname: Paletta
authors.affiliation: Department of Chemistry and Biochemistry, University of Denver , Denver, Colorado 80208-2436, United States.
authors.forename: Hanan
authors.initials: H
authors.lastname: Elajaili
authors.affiliation: Department of Chemistry and Biochemistry, University of Denver , Denver, Colorado 80208-2436, United States.
authors.forename: Kirby
authors.initials: K
authors.lastname: Huber
authors.affiliation: IUMSC, Department of Chemistry, Indiana University , Bloomington, Indiana 47405-7102, United States.
authors.forename: Maren
authors.initials: M
authors.lastname: Pink
authors.affiliation: Department of Chemistry, University of Nebraska , Lincoln, Nebraska 68588-0304, United States.
authors.forename: Suchada
authors.initials: S
authors.lastname: Rajca
authors.affiliation: Department of Chemistry and Biochemistry, University of Denver , Denver, Colorado 80208-2436, United States.
authors.forename: Gareth R
authors.initials: GR
authors.lastname: Eaton
authors.affiliation: Department of Chemistry and Biochemistry, University of Denver , Denver, Colorado 80208-2436, United States.
authors.forename: Sandra S
authors.initials: SS
authors.lastname: Eaton
authors.affiliation: Department of Chemistry, University of Nebraska , Lincoln, Nebraska 68588-0304, United States.
authors.forename: Andrzej
authors.identifier: 0000-0002-8856-1536
authors.initials: A
authors.lastname: Rajca
mesh_terms: D002264:Carboxylic Acids / Q000138:chemical synthesis / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004578:Electron Spin Resonance Spectroscopy; D008958:Models, Molecular; D015394:Molecular Structure; D009589:Nitrogen Oxides / Q000138:chemical synthesis / Q000737:chemistry*; D011758:Pyrroles / Q000138:chemical synthesis / Q000737:chemistry*
chemical_list: D002264:Carboxylic Acids; D009589:Nitrogen Oxides; D011758:Pyrroles; C013231:pyrroline