Physicochemical modeling of the role of free radicals in photodynamic therapy

A new kinetic approach is suggested and experimentally supported for quantification of the spin-trapping procedure. Accordingly, the concentration of the spin adduct formed in the interaction between the spin-trap DMPO and various free radicals (cyanopropylperoxy, cumylperoxy, phenylethylperoxy, and hydroperoxy radicals) generated by the initiated oxidation of the parent molecules is followed by kinetic ESR spectrometry. The initial sections of the corresponding kinetics are linear indicating that during this period the consumption of the adduct is negligible and thus the rate of accumulation (W(A)) approximates the rate of formation (W(f)): W(A) approximately W(f) = k(ST)[Rad(*)][DMPO], supported also by the fact that the rate of initiation of oxidation equals W(A) at high [DMPO]. In addition, the circulatory experimental apparatus enables calculation of the rate of molecular decomposition of the adduct by stopping circulation (W(f) becomes negligible) and following the decrease of the ESR signal. Corresponding rate constants are summarized.