Chiroptical detection of condensed nickel(II)-Z-DNA in the presence of the B-DNA via porphyrin exciton coupled circular dichroism.

Here, we report a highly sensitive and specific chiroptical detection method of condensed left-handed Z-DNA in the presence of canonical right-handed B-DNA. The selective formation of a left-handed cytosine-guanine oligonucleotide (CG ODN) in the presence of a right-handed adenine-thymine oligonucleotide (AT ODN) was induced by millimolar concentrations of NiCl(2) and confirmed by electronic circular dichroism. The nickel(II) induced B- to Z-DNA transition of the CG ODN was accompanied by the concurrent condensation of the Ni(II)-Z-DNA, as confirmed by resonance light scattering, transmission spectroscopy, and centrifugation. The selective condensation of the CG ODN allowed its separation from the AT ODN using centrifugation. No structural changes were observed for the AT ODN upon addition of Ni(II). Anionic nickel(II) meso-tetra(4-sulfonatophenyl) porphyrin (NiTPPS) spectroscopically detected the left-handed Z-DNA in the Z-DNA/B-DNA mixture via a strong exciton coupled circular dichroism (ECCD) signal induced in the porphyrin Soret band absorption region. The bisignate ECCD signal originates from the assembly of achiral porphyrins into helical arrays by intermolecular interactions with the condensed Z-DNA scaffold. No induced CD signal was observed for the Ni(II)-B-DNA-NiTPPS complex. Hence, an unambiguous spectroscopic recognition of Ni(II) induced condensed Z-DNA in the presence of B-DNA is possible. The sensitivity of this chiroptical method was as low as 5% of the Z-DNA (4.4 μmol base pair concentration) in the presence of 95% B-DNA (80 μmol). Thus, NiTPPS is a highly sensitive probe for applications in biosensing via the CD signal amplification.