Protonated polynucleotide structures : 15. Metastable protonated polydeoxyribonucleotides.

The CD spectra of three polydeoxyribonucleotide complexes, poly(dG-C). poly(dG-C), poly(dA-G). poly(dT-C) and poly(dA-C). poly(dT-G) have been studied as a function of pH at 25°C. Only poly(dA-G). poly(dT-C) showed large hysteresis, analogous to that observed for poly(dG). poly(dC), while the alternating pur-p-pyr containing polymers reversible titration curves exhibited. The hysteretic form R of poly(dA-G). poly(dT-C) is metastable at neutrality and reverts spontaneously and irreversibly to the neutral from N upon heating to about 60°. A whole titration cycle to pH 2.5 and back to neutrality has to be performed in order to obtain the metastable form R again.