RADIATION-INDUCED FORMATION OF HEMATOPORPHYRIN-TRANSIENTS IN AQUEOUS-SOLUTION - A PULSE-RADIOLYSIS STUDY

The radiation-induced formation of some haematoporphyrin (HP) transien ts in aqueous solution was studied by pulse radiolysis. The OH radical s attack HP (rate constant k = (1.3 +/- 0.2) x 10(10) dm(3) mol(-1) s( -1)) and the resulting transients decay with 2k = (2.5 +/- 0.2) x 10(8 ) dm(3) mol(-1) s(-1). The spectrum of the HP-OH species shows several maxima in the range of 450-850 nm (epsilon(590) = 3880, epsilon(640) = 3850, epsilon(800) = 3670 dm(3) mol(-1) cm(-1)). k for one electron reduction of HP (radcial anion, HP.-) was determined by a direct e(aq) (-) attack on HP (at pH 9.0, k = (1.6 +/- 0.2) x 10(10) dm(3) mol(-1) s(-1)) as well as by electron transfer from hydroxyisopropyl radicals to the substrate (at pH 8.6, k = (3.2 +/- 0.1) x 10(8), at pH 3.8, k = (4.0 +/- 0.2) x 10(8), and at pH 2.0, k = (2.1 +/- 0.1) x 10(9) dm(3) mol(-1) s(-1)). In slightly alkaline media (pH 8.6-9.5) the HP.- tran sients have main absorption bands at lambda < 450 and 640 nm (epsilon( 660) = 7500 dm(3) mol(-1) cm(-1)), whereas at pH 2.0 they are at lambd a < 430 and 640 nm (epsilon(640) = 8170 dm(3) mol(-1) cm(-1)) and 710 nm (epsilon(710) = 8500 dm(3) mol(-1) cm(-1)). The one-electron oxidat ion of HP (formation of HP.+, radical cation) was studied by means of N-3(.) or SO4.- species as electron acceptors. Using N-3(.) at pH 9.5 k (HP + N-3(.)) = (3.2 +/- 0.2) x 10(9) and for the decay 2k = (3.5 +/ - 0.3) x 10(8) dm(3) mol(-1) s(-1) were determined; at 3.6, k = (HP N-3(.)) = (7.1 +/- 0.3) x 10(8) and 2k = (2.6 +/- 0.3) x dm(3) mol(-1) s(-1) were found. In the case of SO4.- at pH9.8 a buildup (5.6 +/- 0. 3) x 10.7 dm(3) mol(-1) s(-1)) was obtained. The spectrum of the HP.transients at pH 9.6 possesses several mardma with epsilon(345) = 38 8 00, epsilon(450) = 2400, epsilon(595) = 2880, and epsilon(660) = 3300 dm(3) mol(-1) cm(-1). The corresponding values at pH 3.8 are: epsilon( 640) = 2450, epsilon(710) = 2600, and epsilon(780) = 1580 dm(3) mol(-1 ) cm(-1). It is conceivable that the reported HP transients might be f ormed also in tumour cells during the radiation or photodynamic treatm ent of patients when HP is used as sensitizer.