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EPR Study of Nitroxides Formed from the Reaction of Nitric Oxide with Photolyzed Amides

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EPR Study of Nitroxides Formed from the Reaction of Nitric Oxide with Photolyzed Amides __________________________________________________________________________www.paper.edu.cn MAGNETIC RESONANCE IN CHEMISTRY Magn. Reson. Chem. 2003; 41: 647–659 Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/mrc.1229 EPR study of nitroxides formed from the reaction of nitric oxide with photolyzed amides Fan Wang, Jing Jin and Longmin Wu∗ State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, China Received 17 March 2003; Revised 12 May 2003; Accepted 13 May 2003 Free radicals generated from UV irradiation of simple aliphatic amides in anaerobic and nitric oxide (NO)-saturated liquid mixtures or solutions gave EPR spectra of nitroxides. The application of isotopic effects to EPR spectra and the generation of radicals by transient radical attack on substrate molecules or by photolysing amine or acetoin were used to help identify photochemically produced radicals from the amides. The aliphatic amides used were formamide, acetamide and their N-methyl- or deuterium- substituted derivatives. Transient radicals used to attack the amides via hydrogen-atom abstraction were generated from the initiator AIBN or AAPH. The observation of various nitroxides indicates the reactivity of NO for trapping acyl, carbamoyl and other carbon-centered radicals. Possibly mechanistic pathways diagnosed with this trap are proposed. Copyright 2003 John Wiley & Sons, Ltd. KEYWORDS: EPR; nitric oxide; nitroxide; amides; acyl; carbamoyl INTRODUCTION ‘Non-’ acyl radicals can be generated by different pro- cesses and trapped by nitroso compounds, yielding acylalkyl It has been well established that nitric oxide (NO) is a highly nitroxides.9–11 The EPR spectroscopic features of acylalkyl stable free radical under chemical conditions. NO shows no nitroxides are characterized by a significantly smaller nitro- tendency to dimerize or disproportionate. It does not abstract gen hyperfine splitting constant (HFSC), generally in the a hydrogen atom nor add itself to an inactivated double bond. range 0.7–0.8 mT, and by a larger g-value than those of nor- However, increasing experimental facts have indicated that mal dialkyl nitroxides.10 Therefore, they will be important NO can be used as a long-lived paramagnetic free radical criteria determining whether an acyl group bears directly scavenger in certain cases.1–5 The range of reactions of NO the nitroxide function or not. Generally, the EPR data with alkyl radicals is complicated. The (RNOR) dimer or 2 bank, especially for acylalkyl nitroxides, seems limited. It R NOR was found to be its major product.6,7 This suggested 2 is well known that the UV photolysis of aliphatic amides can that an oxyaminyl radical or a nitroxide appeared, although generate aliphatic acyl radicals.12–14 Other radicals derived the reaction yielding oxyaminyl-type radicals was ruled from amides, carbamoyl radicals in particular, could be out in some cases.8 There were also indications of aminyl formed by various methods, such as sonolysis,15 radiolysis,16 formation.5,9 In previous studies,4,5 we found that NO tended radicals or excited triplet organic molecule attack17 and to couple with carbon (C)-centered and less stereo-hindered photo-oxidation,18 etc. The radicals so formed were directly alkyl radicals to give nitroso compounds. The compounds , observed in the matrix at a low temperature,18 19 by the spin formed could trap other C-centered radicals, sulfinyl radicals trapping technique9,15,17e or by the flow technique.17a,d or thiyl radicals to yield nitroxides (i.e. aminoxyls) or Amides have long been of practical importance and of oxyaminyls, but they did not seem to trap alkyloxy or phenyl fundamental interest. The amide group is a ubiquitous moi- 4,5 radicals. The long-lived nitroxides thus formed were ety in biologically important macromolecules. Amides can observed by the EPR spectroscopic technique. Furthermore, serve as a linkage in proteins and act as a building block we were encouraged to investigate the availability of NO for many polymers. For instance, the simplest formamide for trapping other kinds of transient radicals, particularly is the smallest model molecule of the peptide prototype 9 the ‘non- ’ aliphatic acyl radicals, although Forrester et al. NH—C O linkage. It has been reported that some dipep- 9a suggested that nitrosoacyls, which stemmed from acyloxy tide amides showed selective inhibition effects on nitric oxide amidyl radicals generated by hydrogen-atom abstraction synthases (NOS) and exhibited therapeutic potential in the from N-acyloxyamides, might be useful traps, especially for treatment of some diseases resulting from NO overproduc- nucleophilic radicals. tion, such as septic shock and inflammation.20 In particular, organic radicals derived from DMF played a significant ŁCorrespondence to: Longmin Wu, State Key Laboratory of role in cell killing, with possible implications for cancer Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, treatment.21 This is another reason why we are interested in China. E-mail: [email protected] Contract/grant sponsor: Natural Science Foundation of China; spin trapping reactions of NO with acyl and other radicals Contract/grant number: 20072013. generated or derived from aliphatic amides. Copyright 2003 John Wiley & Sons, Ltd. 中国科技论文在线_________________________________________________________________________www.paper.edu.cn 648 F.Wang,J.JinandL.Wu Table 1. EPR parameters of nitroxides R1R2N(Ož) No. R1 R2 Solvent HFSC (mT)a g-Valueb 1 O O Benzene 1N: 0.962 2.0062 H C C H 2 O O Benzene 1N: 0.983 2.0060 D C C D Aqueous 1N: 1.028 3 (CH3)2N N(CH3)2 Aqueous 1N: 1.678 2.0053 2N: 1.447 O 4 C(CH3)2CN Benzene 1N: 1.080 2.0061 H C O 5 C(CH3)2CN Benzene 1N: 1.116 2.0060 D C 6 O O Acetoin 1N: 0.614 2.0064 H3C C CCH3 OH O 7 Acetoin 1N: 0.614 2.0068 H C C CCH 3 3 1H: 1.064 H O 8 H C(CH3)2CN Benzene 1N: 1.010 2.0062 N C H 1H: 0.098 1H: 0.047 O 9 D C(CH3)2C(ND2 NH D2O 1N: 0.962 2.0060 N C D O 10 H C(CH3)2C(NH2 NH Aqueous 1N: 0.970 2.0061 N C H 1H: 0.105 O 11 C(CH3)2C(NH2 NH Aqueous 1N: 0.956 2.0060 HN C CH3 O 12 C(CH3)2C(ND2 NH D2O 1N: 1.035 2.0060 DN C CH3 O 13 C(CH3)2CN Benzene 1N: 1.001 2.0060 HN C CH3 O H 14 C(CH3)2CN Benzene 1N: 1.438 2.0061 H C N C 1N: 0.219 H C H 3 1H: 1.713 1H: unresolved O H 15 C(CH3)2C(NH2 NH Aqueous 1N: 1.476 2.0059 H C N C 1N: 0.218 H3C H 1H: 1.821 1H: unresolved O D 16 C(CH3)2CN Benzene 1N: 1.550 2.0061 D C N C 1N: 0.250 D C D 3 1D: 0.132 O D 17 C(CH3)2C(NX2 NH Aqueous or D2O 1N: 1.479 2.0060 D C N C (X D HorD) 1N: 0.220 D C D 3 1D: 0.137 1D: 0.127 O H 18 C(CH3)2C(NH2 NH Aqueous 1N: 1.400 2.0059 H C N C 1N: 0.229 H H 1H: 1.324 1H: unresolved Copyright 2003 John Wiley & Sons, Ltd. Magn. Reson. Chem. 2003; 41: 647–659 中国科技论文在线_________________________________________________________________________www.paper.edu.cn EPR study of nitroxides from NO and photolyzed amides 649 Table 1. (Continued) No. R1 R2 Solvent HFSC (mT)a g-Valueb O 19 C(CH3)2CN Benzene 1N: 1.475 2.0059 H C C N CH 3 2 1N: 0.271 CH 3 2H: 0.685 O H 20 C(CH3)2CN Benzene 1N: 1.502 2.0059 H C C N C 3 1N: 0.132 H C H 3 1H: 2.196 1H: unresolved O 21 C(CH3)2C(NH2 NH Aqueous 1N: 1.462 2.0060 H C C N CHC 3 2 1N: 0.184 CH 3 2H: 1.027 O H 22 C(CH3)2C(NH2 NH Aqueous 1N: 1.651 2.0060 H C C N C 3 1N: 0.159 H C H 3 1H: 2.044 1H: unresolved O 23 C(CH3)2C(NX2 NH Aqueous or D2O 1N: 1.410 2.0056 X N C CH 2 2 (X D HorD) 2H: 0.796 (X=H or D) O 24 C(CH3)2C(NX2 NH Aqueous or D2O 1N: 1.502 2.0059 H C C N CH 3 2 (X D HorD) 1N: 0.219 X (X=H or D) 2H: 0.915 O H 25 C(CH3)2C(NX2 NH Aqueous or D2O 1N: 1.461 2.0059 H C C N C 3 (X D HorD) 1N: 0.229 X H (X=H or D) 1H: 1.416 1H: unresolved a Absolute accuracy š0.003 mT. b Absolute accuracy š0.0001. EXPERIMENTAL described previously.4,5 All the EPR determinations were carried out at ambient temperature. EPR parameters such as N,N-Dimethylformamide (DMF) (Xian Chemicals, AP) was HFSCs and g-values are given in Table 1. dried over magnesium sulfate for 24 h and then treated with potassium hydroxide to remove water and formic acid. It was distilled and the middle fraction was col- RESULTS AND DISCUSSION lected for use.22 N,N-Dimethylacetamide (DMA) (Tianjin Nitroxides generated from reaction of NO with Chemicals, AP) was distilled at reduced pressure from bar- photo-produced acyl radicals 22 ium oxide. Formamide (Xian Chemicals, AP) was treated The EPR spectrum shown in Fig. 1(a) was recorded during 22 according to the literature. Acetamide (Xian Chemicals, the UV photolysis of NO-saturated DMF mixed with benzene 22 AP) was recrystallized from acetone.
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