Highly sensitive fluorescence probes for nitric oxide based on boron dipyrromethene chromophore-rational design of potentially useful bioimaging fluorescence probe

Boron dipyrromethene (BODIPY) is known to have a high quantum yield (phi) of fluorescence in aqueous solution but has not been utilized much for biological applications, compared to fluorescein. We developed 8-(3,4-diaminophenyl)-2,6-bis(2-carboxyethyl)-4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene (DAMBO-P(H)), based on the BODIPY chromophore, as a highly sensitive fluorescence probe for nitric oxide (NO). DAMBO-P(H) had a low phi value of 0.002, whereas its triazole derivative (DAMBO-P(H)-T), the product of the reaction of DAMBO-P(H) with NO, fluoresced strongly (phi = 0.74). The change of the fluorescence intensity was found to be controlled by an intramolecular photoinduced electron transfer (PeT) mechanism. The strategy for development of DAMBO-P(H) was as follows: (1) in order to design a highly sensitive probe of NO, the reactivity of o-phenylenediamine derivatives as NO-reactive moieties was examined using 4,5-diaminofluorescein (DAF-2, a widely used NO fluorescence probe), (2) in order to avoid pH-dependency of the fluorescence intensity, the PeT process was controlled by modulating the spectroscopic and electrochemical properties of BODIPY chromophores according to the Rehm-Weller equation based on measurement of excitation energies of chromophores, ground-state reduction potentials of PeT acceptors (BODIPYs), and calculation of the HOMO energy level of the PeT donor (o-phenylenediamine moiety) at the B3LYP/6-31G level, (3) in order to avoid quenching of fluorescence by stacking of the probes and to obtain probes suitable for biological applications, hydrophilic functional groups were introduced. This strategy should be applicable for the rational design of other novel and potentially useful bioimaging fluorescence probes.