The photodynamic modification of DNA by hematoporphyrin.

Illumination (white light: 300-750 nm) of DNA in the presence of hematoporphyrin (less than or equal to 5-10 (-4) M) results in selective degradation of the guanine moiety. DNA so illuminated exhibits physical chemical properties (lowered sedimentation coefficients, lower temperatures of helix-coil transitions, increased buoyant density values) consistent with single-chain scissions (and the generation of single-stranded regions) which presumably are secondary to the photodegradation of the guanine residue. Illumination of DNA in the presence of low levels of hematoporphyrin (greater than or equal to 2.5 - 10 (-4) M) results in a biopolymer exhibiting all of the physical properties described above with the exception of a lowered sedimentation coefficient; on the contrary such DNA is aggregated. Of the four usual deoxynucleosides irradiated in the presence of hematoporphyrin, only deoxyguanosine is destroyed.