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).