[Study of DNA-interaction with protons of the medium by means of buffer capacity method]

pH-dependences of buffer capacity of native and denaturated DNA have been obtained. These dependences were expanded into basic functions each corresponding to ionised groups of equal pK. The pH dependence of buffer capacity of native DNA have two narrow g- and s-peaks and are shifted (relatively to nucleosides mixture) to acidic pH values, whereas those of heat denaturated DNA have only one wide assymmetric peak shifted to neutral pH. It was shown that s-and g-peaks of pH dependences of buffer capacity correspond to polymorphic transformation of the double helix (h leads to s transition) and to its disordering (s leads to g transition) respectively. From our data it follows that h leads to s transition at mu = 0.01 M NaCl is observed at temperatures T less than 50 degrees C. The independence of DNA protonation degrees, at which h leads to s leads to g transitions occur, on ionic strength of the solution is, apparently, related to linear dependence of the positions of g- and s-peaks maxima on (lg mu). This points to an important role of protonated bases in double helix destabilisation upon salt elimination. High sensitivity of the buffer capacity method allows us to reveal some differences between states of heat and acid denaturated DNA. Analysing pH dependences of buffer capacity of denaturated DNA it was shown that at neutral pH it contained some double or triple helix regions formed by H-linked bases holding unshared proton. Simms' method was shown to be inapplicable to determine base ionization constants both in native and denaturated DNA.