Synthesis and properties of fluorescent NF-kappa B-recognizing hairpin oligodeoxyribonucleotide decoys

Intramolecular fluorescence quenching of cyanine dyes was investigated using a model hairpin oligonucleotide decoy encoding a NF-kappaB p50 subunit binding site. Two types of hairpin oligonucleotides were synthesized: (1) 5'-(6-aminohexyl)- and 3'-(3-aminopropyl)-linked (I); (2) 5'-(6-aminohexyl)- and 3'-[3-(3-hydroxypropyldithio)propyl]-linked (II). Oligonucleotide I was covalently modified using monofunctional either Cy3- or Cy5.5-N-hydroxysuccinimide esters. Using reverse-phase HPLC, mono-and dicyanineamide derivatives of I were isolated. Mono-Cy3-modified derivatives of I, but not the mono-Cy5.5-modified derivatives, showed a 2-fold higher Cy3 fluorescence intensity compared to the free dye. There was no detectable difference in fluorescence between the di-Cy3 derivative of I and the free dye at the same concentration. However, there was a 4-fold quenching of fluorescence in the case of the di-Cy5.5 derivative of the same hairpin oligonucleotide. The quenching of Cy5.5 fluorescence could not be explained by the interaction of Cy5.5 with nucleotide bases as demonstrated by incubating free Cy5.5 dye with oligonuclotides. The quenching effect was further investigated using an oligonucleotide bearing a cleavable 3'-amino-terminated linker bearing an S-S bond (III). After modification of the 5'- and 3'-end of oligonucleotide III with a Cy5.5 monofunctional hydroxysuccinimide ester, a 70-75% quenching of fluorescence was observed. Fluorescence was 100% dequenched after the reduction of S-S bond. The obtained result unequivocally demonstrates that the formation of intramolecular Cy5.5 dimers is the dominant mechanism of fluorescence quenching in symmetric dye-dye hairpin decoy beacons.