Benzophenoxazine-based fluorescent dyes for labeling biomolecules
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Some desirable attributes of Medola's Blue 1, Nile Red 2, and Nile Blue 3, contained as fluorescent probes, are analyzed. Phenoxazines without strongly electron withdrawing or donating substituents have unexceptional absorbance characteristics. Meloda's Blue has one dimethylamino substituent and the heterocyclic core is oxidized. Nile Red has a neutral oxidized phenoxazine system such as phenoxazinone. The 9-diethylamino substituent is able to donate electron density into the carbonyl group across the ring. The water-solubility of Nile-Red is extremely poor, but in other solvents its fluorescence maxima and intensity are good indicators of the dye's environment-polarity. The first synthesis of Nile red was a condensation reaction of a nitrosophenol. The original consequence of the difference in charges for Nile Red 2 and Blue 3 is that water-solubility of Nile Blue is significantly better.
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