The dideoxy chain termination method using deoxy-7-deazaguanosine triphosphate (dc7GTP) in place of dGTP was found to be very useful. Sequencing of a part of the human N-myc gene having 85% GC content is impossible by the original method using dGTP, because of compression of bands. However, the nucleotide sequence of this part was unambiguously determined by analysis of both strands by the modified method. Use of dc7GTP is concluded to improve the dideoxy chain termination method for DNA sequencing.

The replacement of the furanose moiety of DNA by a cyclohexene ring gives a new nucleic acid structure: cyclohexene nucleic acids or CeNA. CeNAs can be obtained by the classical phosphoramidite chemisty starting from protected cyclohexenyl nucleoside building blocks. Incorporation of cylcohexenyl nucleosides in a DNA chain increases the stability of a DNA/RNA hybrid. The complex formed between cyclohexenyl oligoadenylate and its DNA or RNA complement is of similar stability. Circular dichroism (CD) and NMR studies indicate easy conformational adaptation of a cyclohexenyl nucleoside when incorporated in a natural nucleic acid structure. CeNA is stable against degradation in serum and a CeNA/RNA hybrid is able to activate E. Coli RNase H, resulting in cleavage of the RNA strand.

Oligonucleotides constructed of 1',5'-anhydrohexitol nucleoside building blocks (hexitol nucleic acids, HNA) are completely stable towards 3'-exonuclease and form very stable self-complementary duplexes as well as sequence-selective stable duplexes with the natural DNA and RNA. Triple-helix formation has also been observed. These hybridisation characteristics are highly dependcnt on the base sequence and the experimental conKeywords antisense systems -DNA recognitionnucleic acids * oligonucleotides -RNA ditions. When using a phosphate buffer containing 0.1 M NaCI, a homopurine HNA dodecamer gives a ATm of + 1.3 ' C i base pair with DNA as complement and a ATm of +3.O0C/base pair with RNA as complement. These oligomers may therefore be of considerable interest as antisense constructs.

Oligonucleotides incorporating 5-(octa-1,7-diynyl)-2'-deoxycytidine 1a, 5-(octa-1,7-diynyl)-2'-deoxyuridine 2a and 7-deaza-7-(octa-1,7-diynyl)-2'-deoxyguanosine 3a, 7-deaza-7-(octa-1,7-diynyl)-2'-deoxyadenosine 4a were prepared. For this, the phosphoramidites 7, 10, and 13 were synthesized and employed in solid-phase oligonucleotide synthesis. The octa-1,7-diynyl nucleosides 1a- 4a were obtained from their corresponding iodo derivatives using the palladium-assisted Sonogashira cross-coupling reaction. The Tm values demonstrated that DNA duplexes containing octa-1,7-diynyl nucleosides show a positive influence on the DNA duplex stability when they are introduced at the 5-position of pyrimidines or at the 7-position of 7-deazapurines. The terminal alkyne residue of oligonucleotides were selectively conjugated to the azide residue of the nonfluorescent 3-azido-7-hydroxycoumarin ( 38) using the protocol of copper(I)-catalyzed [3 + 2] Huisgen--Sharpless--Meldal cycloaddition "click chemistry" resulting in the formation of strongly fluorescent 1,2,3-triazole conjugates. The fluorescence properties of oligonucleotides with covalently linked coumarin--nucleobase assemblies were investigated. Among the four modified bases, the 7-deazapurines show stronger fluorescence quenching than that of pyrimidines.