Share several ways of labeling an oligonucleotide with fluorescein like 6-FAM/6-HEX/6-TET phosphoramidite?

The choice of label is diversified further, depending on the spectral requirements. 5'-fluorescein-CE phosphoramidite, derived from the single isomer 6-carboxyfluorescein, 5'-hexachlorofluorescein-CE phosphoramidite (HEX) and 5'-tetrachlorofluorescein-CE phosphoramidite (TET) can all be used to efficiently label an oligonucleotide at the 5'-end. Labeling the 3'-end of an oligo with fluorescein can be achieved using 3'-Fluorescein CPGs. Standard cleavage and deprotection with ammonium hydroxide liberates the fluorescein-labeled oligo when using any of these supports.

Figure 1. Chemical structure for 6-HEX phosphoramidite [5'-Hexachlorofluorescein phosphoramidite].