Photochemical consequences of porphyrin and phthalocyanine aggregation on nucleoprotein histone

Anionic photosensitizers, meso-tetrakis(4-sulfonatophenyl)porphyrin (TPPS) and chloroaluminium phthalocyanine tetrasulfonate (AlPCS), are bound to his tone by Coulombic attraction as has been evidenced by absorption and fluore scence spectra. At variance to serum albumins, histone promotes aggregation of the bound monomeric sensitizer, the apparent dimerization constants bei ng K-D = 4.2 x 10(5) M-1 (+/-0.4 x 10(5)) and 3.3 x 10(5) M-1 (+/-0.7 x 10( 5)) for TPPS and AlPCS, respectively. Hydrophobic environment and the shiel ding effect of positively charged histone act as self-aggregating driving f orces for the formation of stacked aggregates. Sensitizer-histone ground st ate interactions and their influence on photophysical properties have been analyzed. Reduction of the triplet quantum yields Phi(T) induced by aggrega tion on histone considerably decreases the sensitizing ability. The bimolec ular rate constant for quenching of histone bound triplet (TPPS)-T-3 by oxy gen is (3.9 +/- 0.7) x 10(8) M-1 s(-1), which is one order of magnitude low er than that far the free sensitizer. (C) 1998 Elsevier Science S.A. All ri ghts reserved.