STUDIES ON RATIONAL DESIGN PRINCIPLE FOR FLUORESCENCE ON/OFF SWITCHING

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Fluorescence imaging is the most powerful technique currently available for continuous observation of the dynamic intracellular processes of living cells. Fluorescein is widely employed as the core of various fluorescence probes used in imaging important biological effectors, such as nitric oxide and calcium. Despite the extensive use of fluorescein derivatives and the importance of the applications, the mechanism that controls the quantum yield of fluorescence has not been fully established. Without such mechanistic information there can be no rational design strategy to develop new, practically useful fluorescence probes. A plausible mechanism that might control the fluorescence quantum yields of fluorescein derivatives has been suggested to be photoinduced electron transfer from the electron donor moiety to the singlet excited state of the xanthene moiety. Since the fluorescence quantum yields in photoinduced electron transfer reactions may be quantitatively predicted based on the M … More arcus theory of electron transfer, confirmation of the above mechanism would provide a quantitative basis for rational design of fluorescence probes. Further, the stability of fluorescence probes could be increased by accelerating the back electron transfer in the radical ion pair generated by photoinduced electron transfer. However, the occurrence of photoinduced electron transfer in fluorescein-based probes has yet to be established.We obtained the first definitive evidence for the occurrence of photoinduced electron transfer in fluorescein-based probes in which the electron donor moiety is directly linked with the xanthene moiety. Formation of the radical ion pair upon photoirradiation of the fluorescein-based probes was detected by means of laser flash photolysis experiments, which afforded transient absorption spectra showing bands due to the radical cation of the electron donor moiety and the xanthene radical anion. The rates of photoinduced electron transfer and the back electron transfer were determined and analyzed in terms of the Marcus theory of electron transfer. The results provide for the first time a quantitative basis for rational design of fluorescein-based probes with high efficiency in fluorescence ON/OFF switching, as well as the high stability to repeated photoirradiation. We have developed the novel chemical probes for bioimaging of nitric oxide, highly reacrive oxygen species, and zinc ion. Less