Conformational aspects and reactivity of DNA. Effects of manganese and magnesium ions on interaction with DNA.

Abstract

The complex formation of manganese(II) and magnesium(II) with DNA has been investigated by ultraviolet absorption and circular dichroism (CD) measurements.G · C‐rich DNA shows a difference spectrum in the presence of manganese(II) but not with magnesium(II). The lowering of the melting temperature observed at Mn2+/DNA‐phosphorus ratios > 1 increases with increasing G · C content but is diminished in the presence of higher ionic strength.The circular dichroism spectra of native DNA are greatly affected by manganese(II). Increasing Mn2+ concentration decrease the CD maximum of G · C‐rich DNA at 270 nm while Mg2+ does not change the CD curve. Contrary to the ordered double helical form the CD spectra of denatured DNA are unaltered. The manganese(II)‐induced perturbation of the circular dichroism of native DNA becomes increased with increasing G · C content and reaches a saturation level around 50 mol % guanine‐plus‐cytosine.The results show that manganese(II) is preferably bound to G · C pairs besides the phosphate groups. The CD spectrum of protonated G · C‐rich DNA is strongly changed by manganese(II) which indicates competition between the divalent metal ion and the proton at the binding site. In agreement with the behavior of methylated DNA the proton displacement by Mn2+ from G · C pairs suggests that Mn2+ coordinates with the N‐7 position of guanine. The spectral changes of the circular dichroism depend also on the shielding of phosphate groups by sodium ions. The results of the circular dichroism which indicate G · C selective conformational changes of the DNA are interpreted by tilting of the bases of locally Mn2+ chelated regions. The possible functional role of the differences observed in the complexing of DNA by Mn2+ and Mg2+ are discussed.