| Abstract
| - Fluorescein is one of the best available fluorophores for biological applications, but the factors thatcontrol its fluorescence properties are not fully established. Thus, we initiated a study aimed at providing astrategy for rational design of functional fluorescence probes bearing fluorescein structure. We have synthesizedvarious kinds of fluorescein derivatives and examined the relationship between their fluorescence propertiesand the highest occupied molecular orbital (HOMO) levels of their benzoic acid moieties obtained bysemiempirical PM3 calculations. It was concluded that the fluorescence properties of fluorescein derivativesare controlled by a photoinduced electron transfer (PET) process from the benzoic acid moiety to the xanthenering and that the threshold of fluorescence OFF/ON switching lies around −8.9 eV for the HOMO level of thebenzoic acid moiety. This information provides the basis for a practical strategy for rational design of functionalfluorescence probes to detect certain biomolecules. We used this approach to design and synthesize 9-[2-(3-carboxy-9,10-dimethyl)anthryl]-6-hydroxy-3H-xanthen-3-one (DMAX) as a singlet oxygen probe and confirmedthat it is the most sensitive probe currently known for 1O2. This novel fluorescence probe has a 9,10-dimethylanthracene moiety as an extremely fast chemical trap of 1O2. As was expected from PM3 calculations,DMAX scarcely fluoresces, while DMAX endoperoxide (DMAX-EP) is strongly fluorescent. Further, DMAXreacts with 1O2 more rapidly, and its sensitivity is 53-fold higher than that of 9-[2-(3-carboxy-9,10-diphenyl)anthryl]-6-hydroxy-3H-xanthen-3-ones (DPAXs), which are a series of fluorescence probes for singlet oxygenthat we recently developed. DMAX should be useful as a fluorescence probe for detecting 1O2 in a variety ofbiological systems.
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