Circular dichroism and ultraviolet absorbance of calf thymus DNA in presence of CH3HgOH.

The changes that one observes upon the addition of CH3HgOH in the circular dichroism spec­ trum and ultraviolet absorbance spectrum of native calf thymus DNA, dissolved in buffered (pH 6.8) solutions of Na2S04 at pNa 2.0, 1.5, 1.0, and 0.0, respectively (pNa= —log [Na+]), are shown to be due to denaturation brought about by the organomercurial interacting with the base moieties of the polymer. The changes are characterized by an extensive shift of both spectra to longer wavelengths, by a decrease of the rotational strength of the long-wavelength positive dichroic absorption band, and by an increase in the UV absorbance at l max • Both the hyperchromicity H* of calf thymus DNA and the normalized decrease of the rotational strength of its long-wavelength positive dichromic band, Q, display cooperativity when plotted against the methylmercury concentra­ tion pM (p M = — log [CH3HgOH] added) at a given salt strength. Rotational strength data, eval­ uated by integration of the area under the positive and negative dichroic absorption bands, have been tabulated for selected values of pNa and pM. They are compared with data available from the literature. In absence of CH3HgOH, and with varying salt strength, native calf thymus DNA exhibits alterations in the long-wavelength positive dichroic absorption band that are interpreted as representing B —*■ C transitions in agreement with currently held views regarding their origin. Similar salt-induced alterations have been noted in the case of denatured DNA; their meaning in terms of DNA geometry remains unclear at this point in time.