OLIGONUCLEOTIDE ANALOGS WITH 4-HYDROXY-N-ACETYLPROLINOL AS SUGAR SUBSTITUTE

Modified oligonucleotides incorporating trans-4-hydroxy-N-acetyl-L-pro linol (trans-4-HO-L-NAP) or its D-analogue as sugar substitute were sy nthesised with adenine and thymine as nucleobases. All-adenine oligonu cleotides built from (2S,4S) or (2R,4R)-cis-4-hydroxy-N-acetylprolinol were likewise prepared. Hybridisation studies revealed that heterocom plexes formed between polyU and homochiral trans-4-hydroxy-N-acetylpro linol-based oligomers of the same as well as of opposite chirality (po lyU/trans-DA(13) and polyU/trans-LA(13)*). The former, however, were triple-stranded. Other complexes with ribonucleic acids were polyA/tra ns-LT13 and polyU/cis-LA(13)*. Heteroduplexes with deoxynucleic acids were formed between trans-LA(13) and oligothymidylate. Interaction w as also observed for cis-LA(13) and oligothymidylate, but not with th e D-hydroxyprolinol analogues. Microcalorimetry proved this interactio n to be the formation of a triple-stranded complex. Two heteroduplexes , trans-LA(13)/dT(13) and trans-LA(13)*/polyU, had similar or slightl y increased stability when compared to the natural dA(13)/dT(13) or dA (13)/polyU systems. Microcalorimetry clearly indicated the formation o f a duplex, in contrast to interactions with N-acetylprolinol oligonuc leotides of different stereochemistry. Moreover, the enthalpy change w as of the same magnitude but the association constant was slightly low er. Natural nucleicacids thus clearly prefer hybridisation with L-hydr oxyprolinol oligomers over D-hydroxyprolinol oligomers. For the series investigated, the L-trans oligomers (Figure 1) seem best to mimic nat ural oligonucleotides. These modified oligonucleotides formed homocomp lexes if both strands were of the same chirality, that is, homocomplex es formed between trans-LA and trans-LT* and between trans-DA* and tr ans-DT, reflecting the isochiral pu-py pairing found in natural nucle ic acids. Once more, however, calorimetry proved these to be triplex i nteractions. Heterochiral pairing was not observed between modified ol igonucleotides, but only between modified oligonucleotides and natural polyU. The thermal stabilities of these heterochiral complexes differ ed clearly.