Radiation-induced Formation of Haematoporphyrin-transients in Aqueous Solution. A Pulse Radiolysis Study

Abstract

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) × 10¹⁰ dm³ mol⁻¹ s⁻¹) and the resulting transients decay with 2k = (2·5 ± 0·2) × 10⁸ dm³ mol⁻¹ s⁻¹. The spectrum of the HP-OH species shows several maxima in the range of 450–850 nm (ϵ₅₉₀ = 3880, ϵ₆₄₀ = 3850, ϵ₈₀₀ = 3670 dm³ mol⁻¹ cm⁻¹). k for one electron reduction of HP (radical anion, HP^.−) was determined by a direct e⁻aq attack on HP (at pH 9·0,k = (1·6 ± 0·2) × 10¹⁰ 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) × 10⁸, at pH 3·8, k = (4·0 ± 0·2) × 10⁸, and at pH 2·0, k = (2·1 ± 0·1) × 10⁹ dm³ mol⁻¹ s⁻¹). In slightly alkaline media (pH 8·6–9·5) the HP^.− transients have main absorption bands at λ < 450 and 640 nm (ϵ₆₆₀ = 7500 dm³ mol⁻¹ cm⁻¹), whereas at pH 2·0 they are at λ < 430 and 640 nm (ϵ₆₄₀ = 8170 dm³ mol⁻¹ cm⁻¹) and 710 nm (ϵ₇₁₀ = 8500 dm³ mol⁻¹ cm⁻¹). The one-electron oxidation of HP (formation of HP^.+, radical cation) was studied by means of N.3 or SO^·−4 species as electron acceptors. Using N.3 at pH 9·5 k (HP + N.3) = (3·2 ± 0·2) × 10⁹ and for the decay 2k = (3·5 ± 0·3) × 10⁸ dm³ mol⁻¹ s⁻¹ were determined; at pH 3·6, k = (HP + N.3) = (7·1 ± 0·3) × 10⁸ and 2k = (2·6 ± 0·3) × 10⁷ dm³ mol⁻¹ s⁻¹ were found. In the case of SO^·−4 at pH9·8 a buildup (5·6 ± 0·3) × 10·7 dm³ mol⁻¹ s⁻¹) was obtained. The spectrum of the HP^.+ transients at pH 9·6 possesses several maxima with ϵ₃₄₅ = 38 800, ϵ₄₅₀ = 2400, ϵ₅₉₅ = 2880, and ϵ₆₆₀ = 3300 dm³ mol⁻¹ cm⁻¹. The corresponding values at pH 3·8 are: ϵ₆₄₀ = 2450, ϵ₇₁₀ = 2600, and ϵ₇₈₀ = 1580 dm³ mol⁻¹ cm⁻¹. It is conceivable that the reported HP transients might be formed also in tumour cells during the radiation or photodynamic treatment of patients when HP is used as sensitizer.