Recent applications of fluorescence decay kinetics and time-gated (nanosecond) laser microscopy in medical diagnosis and environmental research are summarized. Various fluorescent coenzymes (free and protein-bound NADH and flavin molecules) as well as chlorophyll molecules of the two photosystems were differentiated on the basis of their spectral and temporal behaviour. This allowed for measurements of (i) the function of the mitochondrial respiratory chain, (ii) tissue diagnostics and (iii) light-induced studies of plants and micro-organisms (ciliate) in different physiological states. Defects to the photosynthetic apparatus of spruce needles were correlated with high environmental light doses, which (in combination with drought and increased ozone doses) can possibly account for the widespread phenomenon of “forest decline”. In addition, the intracellular location and light-induced reactions of photosensitizing porphyrins were studied. Hydrophobic porphyrins revealed to be composed by monomers, dimers and aggregates with different patterns of distribution in the plasma membrane, nuclear membrane and adjacent cellular sites. In contrast to this, hydrophilic porphyrins like uroporphyrin III (UP III) and meso-tetra(4-sulfonatophenyl)porphyrin (TPPS4) were mainly monomeric and could be localized within the lysosomes. The composition by an unprotonated and an ionic species indicated that the pH values of both sensitizers were around 5, and even slightly lower after light-induced inactivation of the cells. Changes of lysosomal metabolism may therefore play a role in the process of photodynamic therapy (PDT).
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