  Mg2+ effect on the thermal stability of the double-stranded polynucleotide
polyIpolyC (IC) under conditions close to physiological ones (0.1M Na+, pH7)
was studied by the differential UV spectroscopy. The initial increase of the
ion concentration ([Mg2+]) leads to the increase of the melting temperature
(Tm) and to the decrease of the absorption hyperchromicity value that is caused
by the helix-coil transition (hmax). Mg2+ interacts to p-electrons of
hypoxanthine or cytosine rings (cation-p-interaction). When [Mg2+] reaches a
critical value of about 10-4 M, Tm decreases and hmax increase to values close
to one observed in the absence of magnesium. It is supposed that at
[Mg2+]=[Mg2+]cr IC transits into a new structural state. With [Mg2+>[Mg2+]cr Tm
increases again and hmax slowly decreases. Key words: polynucleotides, thermal
stability, metal ions.
