Dideoxyadenosine (ddA) is a dideoxyribonucleoside that can only be used to block the 5' end. Use ddC-3' or Spacer C3 to block the 3' end from polymerase extension. ddA is a synthetic analog of deoxyadenosine, in ddA, both the 2’- and 3’-positions of the ribose have a hydrogen (-H) group substituted for the –OH group, whereas in dA, only the 2’-position is so substituted. ddA is added to the 5’-end of an oligo via 5’-to-5’ synthesis, using a 2’,3’-ddA, 5’-phosphoramidite. Purification must be by PAGE purification, since such an oligo will not have a trityl group (necessary for RPC purification).

ddC-3' should be used as a 3’-end blocking moiety (see ddC technical sheet).