In situations where ligation must be blocked at the 5’ terminus, 5’-OMe-dT may be used. 5’-OMe modification of a strand of siRNA using 5’-OMe-T can control guide strand selection and targeting specificity.1 5’-Amino-dT terminates an oligonucleotide with a 5’-amino group which may be used for attaching a peptide or a PNA sequence. To avoid polymerase extension at the 3’ terminus, 2’,3’-dideoxynucleoside and 3’-deoxynucleoside CPGs have proved to be effective. 2’,3’- Phosphoramidites are designed to be used with the 5’-phosphoramidites and supports. Since these phosphoramidites have no DMT group, they are not compatible with purification by the DMT-on technique. Ion exchange HPLC or PAGE should be used to purify these dideoxy terminated oligos to ensure that shorter sequences (containing 3’-OH) groups are removed. (3’-Termination can also be effected using a 3’-3’ linkage formed using 5’-supports, or 3’-spacer C3 CPG.)
