Fluorescein

4 ,5 -Dibromofluorescein (DBF) is a fluorone dye. Fluorone dyes are hydroxyl-xanthene dyes (homologues of fluorescein) and have been the most widely used class of organic fluorophores for labeling and sensing biomolecules. DBF is routinely used as a ligand for studying the properties of protein using spectral analysis.

Fluorescein is a synthetic organic compound available as a dark orange/red powder slightly soluble in water and alcohol. It is widely used as a fluorescent tracer for many applications.Fluorescein is a fluorophore commonly used in microscopy, in a type of dye laser as the gain medium, in forensics and serology to detect latent blood stains, and in dye tracing. Fluorescein has an absorption maximum at 494 nm and emission maximum of 521 nm (in water). The major derivatives are fluorescein isothiocyanate (FITC) and, in oligonucleotide synthesis, 6-FAM phosphoramidite.Fluorescein also has an isosbestic point (equal absorption for all pH values) at 460 nm. Fluorescein is also known as a color additive (D C Yellow no. 7). The disodium salt form of fluorescein is known as uranine or D CYellow no. 8.The color of its aqueous solution varies from green to orange as a function of the way it is observed: by reflection or by transmission, as can be noticed in bubble levels, for example; in which fluorescein is added as acolorant to the alcohol filling the tube in order to increase the visibility of the air bubble contained within (thus enhancing the precision of the instrument). More concentrated solutions of fluorescein can even appear red. Chemical and physical properties he fluorescence of this molecule is very intense; peak excitation occurs at 494 nm and peak emission at 521 nm.Fluorescein has a pKa of 6.4, and its ionization equilibrium leads to pH-dependent absorption and emission over the range of 5 to 9. Also, the fluorescence lifetimes of the protonated and deprotonated forms of fluorescein are approximately 3 and 4 ns, which allows for pH determination from nonintensity based measurements. The lifetimes can be recovered using time-correlated single photon counting or phase-modulation fluorimetry. Derivatives[edit]Fluorescein isothiocyanate and 6-FAM phosphoramidite 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 ahydrogen 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 esterfunctional group attached to the fluorescein core, creating NHS-fluorescein, forms another common amine-reactive derivative, yielding more stable amide adducts. A carboxyl group added to fluorescein makescarboxyfluorescein, and carboxyl, fluorescein, and succinimidyl ester groups compose carboxyfluorescein succinimidyl ester. 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. 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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