Thermal generation of stable spin trap adducts with super-hyperfine structure in their EPR spectra: An alternative EPR spin trapping assay for radical scavenging capacity determination in dimethylsulphoxide

Abstract

Thermal decomposition of potassium persulphate (K₂S₂O₈) was studied in detail by the EPR spin trapping technique in dimethylsulphoxide (DMSO), employing 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), 5-ethoxycarbonyl-5-methyl-1-pyrroline-N-oxide (EMPO) and 5-(diisopropoxyphosphoryl)-5-methyl-1-pyrroline-N-oxide (DIPPMPO) as spin traps. DMPO and/or its derivatives exclusively trapped the primary formed SO₄^{• −} radical anions producing stable spin adducts with half-lives exceeding 2 h at room temperature. High-resolution EPR spectra of these adducts showed unusually rich hyperfine structure due to the interaction of the unpaired electron with all magnetically active nuclei of the spin trap moiety. In contrast to aprotic DMSO solvent, ^•DMPO-OH adducts dominated in mixed DMSO/water solutions with water content higher than 50%. The thermal decomposition of K₂S₂O₈ in DMSO represents an effective source of free radicals for the radical scavenging capacity (RSC) determination assay, applicable to hydrophilic as well as hydrophobic antioxidants. Efficiency of the assay is demonstrated with a series of cereal samples.

• EPR spectroscopy
• spin trapping
• DMSO
• sulphate radical anion
• antioxidant

Abbreviations
ACN	=	acetonitrile
BHT	=	2,6-di-tert-butyl-4-methyl-phenol
DEPMPO	=	5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide
DIPPMPO	=	5-(diisopropoxyphosphoryl)-5-methyl-1-pyrroline-N-oxide
DMF	=	N,N-dimethylformamide
DMPO	=	5,5-dimethyl-1-pyrroline-N-oxide
DMSO	=	dimethylsulphoxide
EMPO	=	5-ethoxycarbonyl-5-methyl-1-pyrroline-N-oxide
hfcs	=	hyperfine coupling constants
r	=	correlation coefficient
RSC	=	radical scavenging capacity
TEAC	=	Trolox equivalent antioxidant capacity
Trolox	=	6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid

Abbreviations
ACN	=	acetonitrile
BHT	=	2,6-di-tert-butyl-4-methyl-phenol
DEPMPO	=	5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide
DIPPMPO	=	5-(diisopropoxyphosphoryl)-5-methyl-1-pyrroline-N-oxide
DMF	=	N,N-dimethylformamide
DMPO	=	5,5-dimethyl-1-pyrroline-N-oxide
DMSO	=	dimethylsulphoxide
EMPO	=	5-ethoxycarbonyl-5-methyl-1-pyrroline-N-oxide
hfcs	=	hyperfine coupling constants
r	=	correlation coefficient
RSC	=	radical scavenging capacity
TEAC	=	Trolox equivalent antioxidant capacity
Trolox	=	6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid

EPR spectra (black lines) with the corresponding simulation (red lines) detected 0, 2, 4 and 13 min after mixing 200 µL of water with 20 µL of reaction mixture containing 1 mM K₂S₂O₈ and 20 mM DMPO in DMSO, that was incubated for 30 min at 333 K before mixing: (•) ^•DMPO-SO₄⁻, (▪) ^•DMPO-OH; microwave power, 10 mW; modulation amplitude, 0.1 mT).