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Methods of differential UV and visible spectroscopy are used to study the interaction of Mg2+, Ca2+, Mn2+ and Cu2+ ions with four (4×I)‐ and single‐stranded poly I in solutions with 1 mol·l–1 Na+. Up to concentrations of about 0.1 mol·l–1 Mt2+, Mg2+ and Ca2+ ions do not bind to heteroatoms of hypoxanthine of 4×I. Cu2+ ions interact with N7 and/or N1 and possibly with O6 through the water molecule of the hydrate shell of the ion. It is likely that the interaction with O6 causes enolization of the hypoxanthine, N1 deprotonation and, as a result, this leads to the melting of the four‐stranded helix. In single‐stranded chains, Cu2+ ions induce the formation of compact particles which have the effective radii re ≈ 100 Å. The Mn2+‐induced differential spectra are similar to those observed in the presence of Cu2+ ions but in the case of Mn2+ they occur at concentrations about two orders of magnitude higher. In contrast to the positive cooperativity of the Cu2+ binding to bases of single‐stranded poly I, their interaction with 4×I has the negative cooperativity.

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