A comparative study of DNA complexation with Mg(II) and Ca(II) in aqueous solution: major and minor grooves bindings

Abstract

Although structural differences for the Mg-DNA and Ca-DNA complexes are provided in the solid state, such comparative study in aqueous solution has been less investigated. The aim of this study was to examine the bindings of Mg and Ca cations with calf thymus DNA in aqueous solution at physiological pH, using constant concentration of DNA (1.25 or 12.5 mM) and various concentrations of metal ions (2 microM-650 microM). Capillary electrophoresis, UV-visible, and Fourier transform infrared spectroscopic methods were used to determine the cation-binding modes, the binding constants, and DNA structural variations in aqueous solution. Direct Ca-PO(2) binding was evident by major spectral changes (shifting and splitting) of the backbone PO(2) asymmetric stretching at 1222 cm(-1) with K = 4.80 x 10(5) M(-1), whereas an indirect Mg-phosphate interaction occurred (due to the lack of shifting and splitting of the phosphate band at 1222 cm(-1)) with K = 5.6 x 10(4) M(-1). The metal-base bindings were directly for the Mg with K = 3.20 x 10(5) M(-1) and indirectly for the Ca cation with K = 3.0 x 10(4) M(-1). Both major and minor groove bindings were observed with no alteration of the B-DNA conformation.

FIGURE 1

FTIR spectra (top three curves) and difference FTIR spectra [(DNA solution + metal ion) − (DNA solution)] (two bottom curves) of the uncomplexed calf thymus DNA and its Mg and Ca complexes in aqueous solution at pH = 7.4 with various cation/DNA(P) molar ratios in the region of 1800–600 cm⁻¹.

FIGURE 2

Intensity ratio variations for several DNA in-plane vibrations at 1717 (guanine, thymine), 1663 (thymine, guanine, adenine, cytosine), 1609 (adenine), 1492 (cytosine, guanine), and 1222 cm⁻¹ (PO₂ stretch) as a function of Mg and Ca concentrations (different cation-DNA(P) molar ratios).

FIGURE 3

Plots for cation concentrations versus increase of peak height for the Mg and Ca complexes. Incubation of constant concentration of DNA (1.25 mM of phosphate) with various concentrations of cations were carried out in 10 mM Tris-HCl and 4 mM NaCl (pH 7.4), and the mixtures were subjected to capillary electrophoresis. The increase of peak height in cation-DNA was determined by subtracting the peak height of the free DNA from each cation-DNA adduct.

FIGURE 4

Scatchard plot for the Mg-DNA complexes.

FIGURE 5

Scatchard plot for the Ca-DNA complexes.