Binding of a porphyrin conjugate of Hoechst 33258 to DNA. I. UV-visible and melting studies detect multiple binding modes to a 12-mer nonself-complementary duplex

Relative to ligand-free duplex DNA, the melting temperature of the 1:1 complex of the duplex d(CGAATTGTATGC):d(GCATACAATTCG) with the conjugate of Hoechst 33258 with a des-metalloporphyrin, increased from 42 to 60.5 degrees C indicating strong ligand binding. UV-vis spectrophotometric titration detected more than one class of binding site (apparent dissociation constants approximately 0.2 microM for simple noncooperative binding and 1 microM for the simultaneous cooperative mode with Hill coefficient approximately 2).