Derivatives
There are many fluorescein derivatives. For example, fluorescein isothiocyanate 1, often abbreviated as FITC, is the original fluorescein molecule functionalized with an isothiocyanate group (-N=C=S), replacing a hydrogen atom on the bottom ring of the structure. This derivative is reactive towards primary amine groups of biologically relevant compounds including intracellular proteins to form a thiourea linkage. A succinimidyl ester functional group attached to the fluorescein core, creating NHS-fluorescein, forms another common amine-reactive derivative, yielding more stable amide adducts. Pentafluorophenyl esters (PFP) and tetrafluorophenyl esters (TFP) are other useful reagents. In oligonucleotide synthesis, several phosphoramidite reagents containing protected fluorescein, e.g. 6-FAM phosphoramidite 2, are widely used for the preparation of fluorescein-labeled oligonucleotides.
Other green dyes include Oregon Green, Tokyo Green, SNAFL, and carboxynaphthofluorescein. These dyes, along with newer fluorophores such as Alexa 488, FluoProbes 488 and DyLight 488, have been tailored for various chemical and biological applications where higher photostability, different spectral characteristics, or different attachment groups are needed.
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