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Oxidation by I2 Removal of DMT by ZnBr2

Repeat process with monomer 3, monomer 4, etc.

Repeat process with monomer 3, monomer 4, etc.

Oligonucleotide

▲ FIGURE 9-18 Chemical synthesis of oligonucleotides by sequential addition of reactive nucleotide derivatives. The first (3') nucleotide in the sequence (monomer 1) is bound to a glass support by its 3' hydroxyl; its 5' hydroxyl is available for addition of the second nucleotide. The second nucleotide in the sequence (monomer 2) is derivatized by addition of 4',4'-dimethoxytrityl (DMT) to its 5' hydroxyl, thus blocking this hydroxyl from reacting; in addition, a highly reactive group (red letters) is attached to the 3' hydroxyl. When the two monomers are mixed in the presence of a weak acid, they form a 5' ^ 3' phosphodiester bond with the phosphorus in the trivalent state. Oxidation of this intermediate increases the phosphorus valency to 5, and subsequent removal of the DMT group with zinc bromide (ZnBr2) frees the 5' hydroxyl. Monomer 3 then is added, and the reactions are repeated. Repetition of this process eventually yields the entire oligonucleotide. Finally, all the methyl groups on the phosphates are removed at the same time at alkaline pH, and the bond linking monomer 1 to the glass support is cleaved. [See S. L. Beaucage and M. H. Caruthers, 1981, Tetrahedron Lett. 22:1859.]

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