Radiation-induced formation of haematoporphyrin-transients in aqueous solution. A pulse radiolysis study

The radiation-induced formation of some haematoporphyrin (HP) transients in aqueous solution was studied by pulse radiolysis. The OH radicals attack HP (rate constant k = (1.3 +/- 0.2) x 10(10) dm3 mol-1 s-1) and the resulting transients decay with 2k = (2.5 +/- 0.2) x 10(8) dm3 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 dm3 mol-1 cm-1). k for one electron reduction of HP (radical anion, HP.-) was determined by a direct eaq- attack on HP (at pH 9.0, k = (1.6 +/- 0.2) x 10(10) dm3 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) dm3 mol-1 s-1). In slightly alkaline media (pH 8.6 - 9.5) the HP.- transients have main absorption bands at lambda < 450 and 640 nm (epsilon 660 = 7500 dm3 mol-1 cm-1), whereas at pH 2.0 they are at lambda < 430 and 640 nm (epsilon 640 = 8170 dm3 mol-1 cm-1) and 710 nm (epsilon 710 = 8500 dm3 mol-1 cm-1). The one-electron oxidation of HP (formation of HP.+, radical cation) was studied by means of N3. or SO4.- species as electron acceptors. Using N3. at pH 9.5 k (HP + N3.) = (3.2 +/- 0.2) x 10(9) and for the decay 2k = (3.5 x 0.3) x 10(8) dm3 mol-1 s-1 were determined; at pH 3.6, k = (HP + N3.) = (7.1 +/- 0.3) x 10(8) and 2k = (2.6 +/- 0.3) x 10(7) dm3 mol-1 s-1 were found. In the case of SO4.- at pH9.8 a buildup (5.6 +/- 0.3) x 10.7 dm3 mol-1 s-1) was obtained. The spectrum of the HP.+ transients at pH 9.6 possesses several maxima with epsilon 345 = 38,800, epsilon 450 = 2400, epsilon 595 = 2880, and epsilon 660 = 3300 dm3 mol-1 cm-1. The corresponding values at pH 3.8 are epsilon 640 = 2450, epsilon 710 = 2600, and epsilon 780 = 1580 dm3 mol-1 cm-1.(ABSTRACT TRUNCATED AT 400 WORDS)