The influence of the ionic strength of solution on the protonation of the Micrococcus lysodeicticus DNA molecule was studied by means of circular dichroism, spectrophotometric and potentiometric titration in a wide range of the supporting electrolyte concentration ([NaCl] = 0.5-0.005 M). The concept of local pH determined in Poisson-Boltzman approximation by the electrostatic potential in the monohydrate layer of the macromolecule was used for the interpretation of the data obtained. A phase diagram was constructed in a plot of pHloc vs. log[Na+], which described the polymorphic transformations in the protonated DNA molecule. The comparison of the data obtained with those for the calf thymus DNA showed that the polyelectrolyte invariance of the acid-base equilibrium in natural DNAs is electrostatic in origin and does not depend on the GC content.
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