Laser Excited Fluorescence Lifetimes of Hematoporphyrin IX as a Function of Solution pH: Implication in Photodynamic Therapy
The combination of a harmless chemical photoactivated with safe visible electromagnetic radiation in order to eradicate tumors with a photo-sensitization reaction, offers much hope for the treatment of a large variety of neoplastic diseases. The origin for the selective biodistribution of photosensitizer toward tumor tissue is not yet clearly understood. The pH of many rapidly growing tumors is often found to be significantly lower than that of normal tissue in the same individual. Recent volumetric titration of hematoporphyrin IX (HP) have yielded the distribution diagrams of five HP ionic species, having net charges of +2, +1, 0, -1 and -2. However, only three ionic species may be associated with the steady state absorbance and fluorescence spectra. Further, fluorescence decay measurements carried out in aqueous solutions at physiological pH generally show two lifetimes; a 15 nsec component normally ascribed to a monomeric species and a 3.8 nsec component usually assigned to a dimeric species . Since both monomeric and dimeric species, as well as different ionic species, may play a role in the selective biodistribution of drugs toward tumor tissue, it was deemed important to evaluate the fluorescence lifetimes of HP as a function of both pH and concentration. These measurements were carried out in aqueous solutions as well as in the presence of a surfactant, sodium dodecyl sulfate (SDS).
KeywordsIonic Species Fluorescence Lifetime Volumetric Titration Sodium Dodecyl Sulfate Concentration Monomeric Species
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