THE EFFECT OF HCL ON THE SOLUTION STRUCTURE OF CALF THYMUS DNA - A COMPARATIVE-STUDY OF DNA DENATURATION BY PROTON AND METAL-CATIONS USING FOURIER-TRANSFORM IR DIFFERENCE SPECTROSCOPY

The interaction of HCl with calf thymus DNA was investigated in aqueou s solution at pH 7-2 with H+/DNA (P) (P:phosphate) molar ratios (r) of 1/80, 1/40, 1/20, 1/10, 1/4, 1/2, and 1, using Fourier Transform (FTI R) difference spectroscopy. Correlations between spectral changes, pro ton binding mode, DNA denaturation, and conformational variations are established. A comparison was also made between the ir spectra of dena turated DNA, in the presence of proton and Cu ions with similar cation concentrations The FTIR difference spectroscopic results have shown t hat at low proton concentrations of r = 1/80 and 1/40 (pH 7-5), no maj or spectral changes occur for DNA, and the presence of H+ results in a n increased base-stacking interaction and helical stability. At higher proton concentrations of r > 1/40, the proton binding to the cytosine and adenine bases begins with major destabilization of the helical du plex. As base protonation progresses, a B to C conformational conversi on occurs with major DNA spectral changes. Protonation of guanine base s occurs al a high cation concentration r > 1/2 (pH < 3) with a major increase in the intensity of several DNA in-plane vibrations. Copper i on complexation with DNA exhibits marked similarities with proton at h igh cation concentrations (r > 1/10), whereas at low metal ion concent rations copper-PO2 and copper-guanine N-7 bindings are predominant. No major DNA conformational transition was observed on copper ion comple xation. (C) 1995 John Wiley & Sons, Inc.