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A 43Ca‐nmr study of Ca(II)–DNA interactions

High‐field 43Ca‐nmr is applied to characterize the interactions of calcium ions with double‐helical DNA. Under the conditions examined, 43Ca lineshapes are always Lorentzian and single spin‐lattice relaxation rates are obtained. The measured transverse and longitudinal relaxation rates are, however, not equal, which implies that the relaxation is in the near‐extreme narrowing regime. Relative to the transverse relaxation rate, calcium ions near the DNA exchange rapidly with the bulk solution. The 43Ca linewidths, spin‐lattice relaxation rates, and chemical shifts observed over the course of a titration of DNA with calcium salt are not well described by simple electrostatic models. Deviations are most pronounced at low ratios of calcium to DNA phosphate. In contrast, at higher Ca/P ratios, the changes observed are well described by an electrostatic model based on the Poisson–Boltzmann equation. These results suggest that there is a small class of site‐bound calcium as well as a large background of delocalized calcium electrostatically associated with the DNA. In contrast to previous studies of 25Mg2+–DNA interactions, for which significant site‐binding effects were also indicated, it appears rather easy to displace bound 43Ca2+ by competition with sodium or magnesium cations. Unfortunately, neither these earlier results nor the present work allows a precise quantitation of the extent of site‐bound divalent cation.