THERMAL-DENATURATION OF NA-DNA 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 (C-p) from 4.5 . 10(-2) to 1 . 10(-3) moles of nucleotides/liter (M). Linear dependencies of DNA melting temperature (T-m) vs lgC(p), were obtained: (1) T-m = 18.3 . lgC(p) + 90.2 (2) T-m = 18.2 . lgC(p) + 95.1 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 C-p similar or equal to (6-8) . 10(-5) M and C-s = 0-40 mM (Me is Na or Li, C-s is salt concentration). It was found that Eqs. (1) and (2) are valid in DNA salt-free solutions over the C-p range 6 . 10(-5)-4.5 . 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 C-s < 4 mM. Linear dependence of T-m on lga(+) is found in Na-DNA + NaCl at C-s > 0.4 mM in the absence of contact of solutions with CO2 from air (a(+) is cation activity). A dependence of [dT(m)/dlga+(]) on Li+ activity was observed in Li-DNA + LiCl solutions at C-s < 10 mM: [dT(m)/dlga(+)] increases from 17 degrees-18 degrees at C-s > 10 mM to 28 degrees-30 degrees at C-s similar or equal 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 T-m vs lgC(p) 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. (C) 1994 John Wiley & Sons, Inc.