Fluorescence spectra of hematoporphyrin and hematoporphyrin-diacetate aggregates in buffer solution

Different fluorescence spectra are observed on preferential excitation of monomer and aggregate species of hematoporphyrin and hematoporphyrin-diacetate, respectively. From these measurements we obtain the fluorescence spectrum of aggregate species in buffer solution, which shows two maxima, at 628 nm and around 700 nm. The fluorescence intensity in the region between these maxima is very sensitive to environmental changes. It is suggested that the changes in the fluorescence of hematoporphyrin in vivo as compared with that in buffer solution is associated with both hematoporphyrin-substrate interactions and an enhanced contribution from aggregate species.