Subcellular distributions and excited-state processes of hypericin in neurons

The photodynamic drug, hypericin, is studied in fetal rat neurons using flu orescence microscopy. Hypericin has an extremely high affinity for the cell membrane and is found to a smaller extent in the nucleus. Fluorescent exci tation of hypericin is shown to cause irreversible damage to the cell membr anes of living neurons. Fixed cells were used to make ultrafast time-resolv ed measurements to avoid the deleterious effects of long-term exposure to i ntense light and room temperatures. To our knowledge, these are the first u ltrafast time-resolved measurements of the fluorescence lifetime of hyperic in in a subcellular environment. Nonexponential fluorescence decay is obser ved in hypericin in the neurons. This nonexponential decay is discussed in terms of other examples where nonexponential decay is induced in hypericin upon its binding to biomolecules. The nonradiative processes giving rise to the nonexponential hypericin decay are attributed to excited-state electro n transfer, excited-state proton transfer or both.