Probing the structure of HPD by fluorescence spectroscopy.

Fluorescence emission spectra indicate that oligomers containing both hematoporphyrin and its dehydration products (vinyl porphyrins) comprise the tumor-localizing fraction of HPD. In the relatively polar solvent methanol, the vinyl porphyrins exhibit reduced fluorescence yields while the hematoporphyrin residues are relatively resistant to fluorescence quenching by Fe+3. In the less polar solvent tetrahydrofuran, fluorescence from oligomeric vinyl porphyrins was enhanced, and Fe+3-induced quenching of oligomeric hematoporphyrin promoted. These, together with other studies in biological systems, suggest a substantial degree of interaction among the porphyrin units contained in these oligomers, as a function of the polarity of the environment.