Selective phosphate protection: A novel synthesis of double-labeled oligonucleotides

[GRAPHICS] A novel, selective labeling of oligonucleotides with two different reporter groups is described. The oligonucleotide is synthesized using a stable 2-( 4-methoxybenzamido)ethyl protection for a selected internucleosidic thiopho sphate (PS) and a labile 2-(N-isopropyl-4-methoxybenzamido)-ethyl for the 3 '-terminal PS and internucleosidic phosphates. The latter group and the bas e protection are removed, and the 3'-terminal PS is labeled. The former pro tection is then cleaved by a prolonged ammonolysis, and the second reporter is introduced at the internucleosidic PS.