Synthesis and high stability of complementary complexes of N-(2-hydroxyethyl)phenazinium derivatives of oligonucleotides.

Two simple methods for the synthesis of oligonucleotides bearing a N-(2-hydroxyethyl)phenazinium (Phn) residue at the 5'- and/or 3'-terminal phosphate groups are proposed. By forming complexes between a dodecanucleotide d(pApApCpCpTpGpTpTpTpGpGpC), a heptanucleotide d(pCpCpApApApCpA), and Phn derivatives of the latter, it is shown that the introduction of a dye at the end of an oligonucleotide chain strongly stabilizes its complementary complexes. The Tmax and the thermodynamic parameters (delta H, delta S, delta G) of complex formation were determined. According to these data, coupling of a dye with the 5'-terminal phosphate group is the most advantageous: delta G(37 degrees C) is increased by 3.59 +/- 0.04 kcal/mol compared to 2.06 +/- 0.04 kcal/mol for 3'-Phn derivatives. The elongation of the linker, which connects the dye to the oligonucleotide, from a dimethylene up to a heptamethylene usually leads to destabilization of the oligonucleotide complex. The complementary complex formed by the 3',5'-di-Phn derivative of the heptanucleotide was found to be the most stable among all duplexes investigated. Relative to the unmodified complex the increase in free energy was 4.96 +/- 0.04 kcal/mol. The association constant of this modified complex at 37 degrees C is 9.5 x 10(6) M-1, whereas the analogous value for the unmodified complex is only 3 x 10(3) M-1.