Vibrational circular dichroism spectroscopy and the effects of metal ions on DNA structure

The effects of manganese (II), nickel (II), chromium (III), and platinum (II) (cisplatin) on the double helical structure of selected oligonucleotides and DNA as detected by vibrational circular dichroism spectroscopy (VCD) are reviewed. For (dG–dC)20, the VCD spectra displayed unambiguously the transition from the normal right-handed B-form to the left-handed Z-from at the highest concentration of Mn2þ (8.5 [Mn]/[P]). For poly(rU)·poly(rA)·poly(rU), the double-triplesingle helix transition induced by nickel (II) was clearly observable at increasing temperatures. For calf thymus DNA, chromium (III) at 0.6 [Cr]/[P] produced a 4-fold increase of the main VCD couplets, which are characterized as c-type spectra. These unusual features are due to condensed particles with dimensions comparable to the wavelength of the probing light and with regular arrangements of DNA double-helices inside the aggregates. c-Type spectra were also observed for DNA with manganese (II) between 55 and 60 8C at 2.4 [Mn]/[P]. Between 2.4 and 10 [Mn]/[P], intense and indistinct VCD features appeared above the DNA melting point indicating aggregation induced by Mn2þ ions at elevated temperature in a relatively wide range of experimental conditions, whereas DNA condensation with Mn2þ ions occurs only in a very narrow range of concentration and temperature. The complexes formed by coordination of cisplatin with d(CCTGGTCC)·d(GGACCAGG) and d(CCTCTGGTCTCC)·d(GGAGACCAGAGG) yielded very detailed VCD spectra, which distinctly confirmed the G4pG5 and G6pG7 lesion sites, respectively. The octamer slowly isomerized to form other adducts, which was not the case for the dodecamer. q 2003 Published by Elsevier B.V.