Free radical reactions of peroxidizing lipids with amino acids and proteins: an ESR study.

Free radical transfer from oxidizing methyl linoleate to amino acids and proteins was studied in dry model systems incubated for periods up to 20 days. Electron spin resonance was used to study free radical production. Free radicals were detectable in the amino acids lysine, arginine, histidine, tryptophan, and cysteine. Reduced glutathione and, to a limited extent, cystine also gave free radical signals. Free radicals produced in proteins primarily showed central singlet lines, attributable to carbon-centered radicals, with g= 2.004+/- 0.001. Sulfhydryl proteins also exhibited downfield shoulders at g approximately equal to 2.015 and 2.023 that were essentially identical to peaks observed in cysteine and reduced gluathione. The field positions of sulfur resonace in cysteine and proteins suggested a sulfur-oxygen complex rather than thiyl radicals.