Binding of Europium(III) to a non-nucleosidic phenanthroline linker in DNA.

Eu(III) complexes of DNA containing a non-nucleosidic linker, a derivative of 1,10-phenanthroline-2,6-dicarboxamide (Q), are studied with the goal of forming novel lanthanide ion binding sites that are incorporated in the backbone of DNA. One oligonucleotide is short and unstructured (TTTQTTT (QT6)) and the other (5'-AGCTCGGTCAQCGAGAGTGCA-3' (SQ)) is studied both in single-stranded form and in the presence of a partially complementary DNA strand. Luminescence spectroscopy studies show that Eu(III) binds to SQ, QT6, AQB, or QB 1100-, 56-, 23-, or 27-fold more tightly, respectively, than to a simple 1,10-phenanthroline-2,6-dicarboxamide ligand (Q1). Direct excitation and phenanthroline sensitized luminescence spectroscopy supports binding of Eu(III) to the phenanthroline linker in QT6 and in double-stranded DNA formed from SQ and partially complementary sequences that place Q in a bulge-like position. Eu(III) hydration numbers range from 3 to 5 when bound to the phenanthroline moiety in modified DNA, consistent with binding to the tetradentate linker and, in some cases, coordination to other groups in the DNA. Thermal melting experiments show that Q in a bulge-like structure stabilizes double-stranded DNA and that Eu(III) binding does not markedly affect the stability of the duplex.