Base-modified oligodeoxyribonucleotides: using pyrrolo[2,3-d]pyrimidines to replace purines.

Phosphoramidites are first synthesized from suitably protected 7-deazapurine 2'-deoxyribonuclosides that have halogen or alkynyl subsituents at the 7 position. Oligonucleotides containing 7-deazapurines (pyrrolo[2,3-d]pyrimidines) in place of the canonical nucleobases are then prepared by solid-phase synthesis. These oligonucleotides are subsequently allowed to hybridize, and the stabilities of the resulting duplexes are determined. Comparison of T(m) values indicates that the presence of 7-halo or 7-alkynyl substituents increases duplex stability relative to duplexes involving oligonucleotides that contain the corresponding parent purines.