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Cytometry A. 2013 May;83(5):441-51. doi: 10.1002/cyto.a.22260. Epub 2013 Feb 15.

UV-activated conversion of Hoechst 33258, DAPI, and Vybrant DyeCycle fluorescent dyes into blue-excited, green-emitting protonated forms.

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Division of Cell Biophysics, Faculty of Biochemistry, Biophysics, and Biotechnology, Jagiellonian University, Kraków, Poland.


Hoechst 33258, DAPI and Vybrant DyeCycle are commonly known DNA fluorescent dyes that are excited by UV and emit in the blue region of the spectrum of visible light. Conveniently, they leave the reminder of the spectrum for microscopy detection of other cellular targets labeled with probes emitting in green, yellow or red. However, an exposure of these dyes to UV induces their photoconversion and results in production of the forms of these dyes that are excited by blue light and show fluoresce maxima in green and a detectable fluorescence in yellow and orange regions of the spectrum. Photoconversion of Hoechst 33258 and DAPI is reversible and independent of the dye concentration or the presence of DNA. Spectrofluorimetry and mass spectrometry analyses indicate that exposure to UV induces protonation of Hoechst 33258 and DAPI.

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