Phototoxic

A phototoxic substance, is a poison/venomwhich only reacts when exposed to (sun)light. Phototoxicity is a phenomenon known in fluorescence live cell microscopy, where illuminating a fluorescent molecule (the fluorescently active site is called a fluorophore) causes the selective killing of the cells expressing it. While not completely understood it seems to be clear, that the main cause for phototoxicity is the formation of oxygenradicals due to non radiative energy transfer.

Typically in flourescence, photons of a certain wavelength excite electrons of the illuminated fluorophore to higher energy states. When theses excited electrons return to a lower energy state they emit a photon with a lower energy level thus causing the emission of light of a longer wavelength. This principle of fluorescence is also known as Stokesshift.

Unfortunately for microscopists in many cases some of the energy is not used for this radiative energy transfer but also transferred to oxygen causing the formation of oxygen radicals. These radicals are highly toxic to living cells, sometimes killing cells in seconds.

Phototoxicity in live cells depends strongly on the kind of fluorescent molecule used. The isolation and characterization of fluorescent proteins such as 'Green Fluorescent Protein' ("GFP") has provided biologists with fluorochromes which show a much weaker phototoxic effect compared to most smaller chemically synthesized fluorescent molecules such as FITC or rhodamine. Still, the energy level of excitation light as well as the duration of illumination has to be minimized to ensure long term survival of living cells during fluorescent imaging.de:Phototoxie nl:Fototoxisch

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It uses material from the http://en.wikipedia.org/wiki/Phototoxic Wikipedia article Phototoxic.