Two main high-density lipoprotein subfractions, HDL2 and HDL3, were spin labelled with TEMPO which partitions both in aqueous and lipid phase. The dynamics of the lipid phase was monitored ina the reduction of incorporated TEMPO with ascorbic acid. The reduction of the paramagnetic nitroxide into the nonparamagnetic hydroxilamine form decreases the ESR signal with time. The reduction curves show complex behaviour while the partition coefficients of TEMPO remain unchanged during the reaction. The reduction process in the samples containing HDL particles proceeds faster than in the aqueous solution of pure reactants, e.g. spin label and ascorbic acid. In order to explain experimental data the model for reduction of TEMPO by ascorbic acid is proposed. It assumes that the processes taking place in these heterogeneous systems are determined by the overall reaction rate, which depends on the local concentrations of the reactants, as well as on their transport properties in the particular phases
