Fully modified oligonucleotides were synthesised from the 3-O-phosphoramidites of monomethoxytritylated trans-3-hydroxy-N-[(N-6-benzoyladenin-9-yl)-acetyl]-prolinol [(2S,3R) and (2R,3S) series], trans-3-hydroxy-N-[(thymin-1-yl)-acetyl] [(2S,3R) and (2R,3S) series], and cis-3-hydroxy-N-[(N-6-benzoyl-adenin-9-yl)-acetyl]-L-prolinol (2R,3R). Remarkably, as well the L-trans (2R,3S) as the D-trans (2S,3R) all-adenine oligonucleotides are capable of hybridisation with complementary DNA and RNA. With modified ail-thymine trans-oligomers no complexation with natural nucleic acids was observed. However, complex formation between two modified strands of the same sense of chirality does occur with formation of a triple stranded complex. The all-thymine oligonucleotides with trans-3-HO-N-acetylprolinol backbone are capable of hybridisation with trans-4-HO-N-acetylprolinol oligoadenylates of the same enantiomeric form in both the D and the L series, and inversely, the all-adenine oligonucleotide with the trans-3-HO conformation hybridises with the trans-4-HO oligothymidylates. While the former interactions have a triple stranded origin, the latter are 1:1 interactions. No interactions were noticed upon mixing oligonucleotide analogues of different sense of chirality. Modified mixed trans-3-HO A,T sequences display no hybridisation with complementary nucleic acids, nor homocomplex formation. The L-cis all-adenine oligonucleotide hybridises with its RNA complement. Several complexes were investigated by circular dichroism and microcalorimetry. In conclusion, the 3-hydroxy-N-acetylprolinol system represents an example of homochiral oligonucleotides built up from two enantiomeric forms and hybridizing both with natural nucleic acids. (C) 1997 Elsevier Science Ltd.