Abstract: Soybean proteins were prepared from the model systems consisting of lipid-reduced soybean proteins, linoleic acid and lipoxygenase, and the EPR spectra of the proteins were investigated and compared with at different microwaves. The effects of preparation methods on the free radical concentration and species were examined. Four different type free radicals were identified as the damage of protein side chains or free radical chain reaction. The carbon radicals (g value range 2.0041 to 2.0054) formed at either on the backbone of the α-carbon or on the side chains of the proteins, while the free radical concentration or central singlet signal decreased with increased microwaves. Sulphur radicals (g value range 2.019 to 2.028) attributed to sulfur- containing side-chain of methionine or cysteine in the proteins were all detected at different microwaves, and were stable even at room temperature as observed in two samples. The radical ·OH was originated from free radical chain reaction and water molecules, and related to dityrosine formation from oxidative modification of soybean proteins examined in the previous study. Aldehyde radical must be originated from the enzymatic reactions which occurred between soybean proteins and the degradation product of linoleic acid oxidation catalyzed by lipoxygenase I-B (LOX). It was contained in the central carbon signal, but shifted in some sort to high magnetic field.

Key words: soybean proteins, lipoxygenase, linoleic acid, free radicals, EPR