Physicochemical modeling of the role of free radicals in photodynamic therapy V. Quantification of spin-trapping kinetics

A new kinetic approach is suggested and experimentally supported for quanti fication of the spin-trapping procedure. Accordingly, the concentration of the spin adduct formed in the interaction between the spin-trap DMPO and va rious free radicals (cyanopropylperoxy, cumylperoxy, phenylethylperoxy, and hydroperoxy radicals) generated by the initiated oxidation of the parent m olecules is followed by kinetic ESR spectrometry, The initial sections of t he 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 similar to W-f = k(ST) [Rad(.)][DMPO], supported also by the fact that the rate of initiation of o xidation equals W-A at high [DMPO]. In addition, the circulatory experiment al apparatus enables calculation of the rate of molecular decomposition of the adduct by stopping circulation (W-f becomes negligible) and following t he decrease of the ESR signal. Corresponding rate constants are summarized. (C) 2000 Academic Press.