This phosphoramidite allows fluorescein (FAM) to be introduced into any position in the oligonucleotide sequence during synthesis by the phosphoramidite method (in the middle of the chain, at the 5- and 3-terminus). The reagent is a conjugate of deoxythymidine phosphoramidite and 6-isomer of FAM. Modification is performed during oligonucleotide synthesis by substituting standard dT phosphoramidite with fluorescein-dT phosphoramidite. This modification does not affect exonuclease or polymerase activity. For modification with fluorescein at the 5-terminus use FAM phosphoramidite, 6-isomer. Coupling takes 10 minutes. Standard deprotection conditions using ammonium hydroxide can be used; deprotection time depends on oligonucleotide composition and nucleobase protecting groups (deprotection for 17 h at 55 °C removes all protecting groups from standard nucleobases). AMA (solution of 30% ammonium hydroxide/40% aqueous methylamine 1:1 v/v) can be used with ~5% of non-fluorescent side product forming. To avoid formation of the side product, start deprotection with ammonium hydroxide (30 min at room temperature), then add an equal volume of 40% aqueous methylamine and continue deprotection as required with AMA (e.g. 10 min at 65 °C).
