Electrostatic potential modulations on polynucleotides as a function of ionic content: potentiometric determinations.

pK values of ionisation of cytidine residues, in poly C and poly I. poly C, have been determined by potentiometric titration as a function of either ionic strength (Na+) or Mg2+ concentration. The results are interpreted as a consequence of the existence of a local pH, prevailing in the vicinity of the polynucleotides, smaller than that of the bulk solution. This local pH is dependent upon the electrostatic potential psi due to the negatively charged phosphate groups of the polynucleotide. Moreover psi can be modulated through variation of either ionic strength or Mg2+ concentration. At a given pH Mg2+ addition leads to the release of protons which, in the case of complex systems of limited stability, might be detected by the use of the pH-stat method. Possible functional implications for genetic translation systems are examined.