Thermal denaturation of Na- and Li-DNA in salt-free solutions

Thermal denaturation of Na- and Li-DNA from chicken erythrocytes was studied by means of scanning microcalorimetry in salt-free solutions at DNA concentrations (Cp) from 4.5 x 10(-2) to 1 x 10(-3) moles of nucleotides/liter (M). Linear dependencies of DNA melting temperature (Tm) vs lgCp were obtained: [formula: see text] for Na- and Li-DNA, respectively. Microcalorimetry data were compared with the results of spectrophotometric studies at 260 nm of DNA thermal denaturation in Me-DNA + MeCl solutions at Cp congruent to (6-8) x 10(-5) M and Cs = 0-40 mM (Me is Na or Li, Cs is salt concentration). It was found that Eqs. (1) and (2) are valid in DNA salt-free solutions over the Cp range 6 x 10(-5)-4.5 x 10(-2) M. Protonation of DNA bases due to the absorption of CO2 from air in Na-DNA + NaCl solutions affects DNA melting parameters at Cs < 4 mM. Linear dependence of Tm on lga+ is found in Na-DNA + NaCl at Cs > 0.4 mM in the absence of contact of solutions with CO2 from air (a+ is cation activity). A dependence of [dTm/dlga+] on Li+ activity was observed in Li-DNA + LiCl solutions at Cs < 10 mM: [dTm/dlga+] increases from 17 degrees-18 degrees at Cs > 10 mM to 28 degrees-30 degrees at Cs congruent to 0.2-0.4 mM. Spectrophotometric measurements at 282 nm show that this effect was caused by protonation of bases in fragments of denatured DNA in neutral solutions. The Poisson-Boltzmann (PB) equation was solved for salt-free DNA at the melting point. The linear dependence of Tm vs lgCp was interpreted in terms of Manning's condensation theory. PB and Manning's theories fit the experimental data if charge density parameter (xi) of denatured DNA is in the range 1.8-2.1 (assuming for native DNA xi = 4.2). Specificity of Li ions in interactions with DNA is discussed.