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An electron spin resonance (ESR) method combined with a spin trapping technique was applied to trap and characterize unstable radicals which were generated by ultrasonic irradiation of the dimethylsulfoxide (DMSO) solution of a softwood, Yezo Spruce (Picea jezoensis carr.) lignin. It was found that an unstable secondary carbon radical,~CH • in the solution was trapped as the stable nitroxide spin adduct when the DMSO solution was subjected to ultrasonic irradiation in the presence of a spin trapping reagent: 2,4,6-tri-tert-butylnitrosobenzene (BNB) at 50°C for 30 min. This means that the alkyl phenyl ether bonds,~CH-O-phenyl, known as interunitary bonds in lignins were homolytically cleaved by the ultrasonic irradiation, although the phenoxy radical Ph-O •, called guaiacoxy radical, i.e. the counter radical of the secondary carbon radical, was not trapped by the BNB spin trap. This suggests that the trapping of the guaiacoxy radical, having a methoxy group in an ortho-position, by the BNB molecule, carrying two bulky butyl groups in the ortho-positions, is sterically hindered. KeywordsYezo spruce (Picea jezoensis carr.) Milled wood lignin (MWL) Electron spin resonance (ESR) Spin trapping Phenoxy radicals Alkyl phenyl ether bonds Secondary carbon radical Ultrasonic irradiation Brought to you by | University of Queensland -UQ Library Authenticated Download Date | 6/23/15 7:21 AM

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Unstable chemical species, that is, radicals generated by the thermal treatment of a dimethyl sulfoxide (DMSO) solution of the lignin of a softwood, Yezo spruce (Picea jezoensis Carr.), were studied in detail with an electron spin resonance (ESR) method combined with a spin-trapping technique. An unstable secondary carbon radical (ϳCH ⅐ ) in the solution was trapped as a stable nitroxide spin adduct [RO(NOO ⅐ )OCHϳ (R ϭ tert-butyl benzene)] when the DMSO solution was heat-treated in the presence of a spin-trapping reagent [2,4,6-tri-tert-butylnitrosobenzene (BNB)] at about 40°C. This meant that alkyl phenyl ether bonds (ϳCHOO-phenyl), known as interunitary lignin bonds, were homolytically scissioned by the thermal treatment in the lignin solution. A detailed analysis of the ESR spectrum revealed that three kinds of radicals-primary (ϳCH 2 ⅐ ), secondary (ϳCH ⅐ ), and tertiary (ϳC ⅐ ) carbon radicals-were trapped as stable spin adducts at about 60°C, although the phenoxy radical (PhOO ⅐ ) was not trapped by the BNB spin trap as the counter radical of the secondary carbon radical. This suggested that a fairly large steric hindrance existed between the so-called guaiacoxy radical with a methoxy group in the ortho position and the BNB molecule bearing two butyl groups as bulky moieties in the ortho positions. However, the phenoxy radicals in the lignin solution were stable up to about 60°C.