The behavior of oligonucleotides containing 5-iodouracil,5-bromocytidine, and 5-iodocytidine inconcentrated ammonia is described. 5-Aminouracil and5-aminocytidine are obtained as side productswhen deprotection is performed at 60
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C. Small amounts, if any,of side products are obtained whenammonia deprotection is performed at room temperature. Thebase-pairing properties of these5-halopyrimidines including triple helix are described.