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

✍ Scribed by William H. Braunlin; Torbjörn Drakenberg; Lars Nordenskiöld

• Publisher: Wiley (John Wiley & Sons)
• Year: 1987
• Tongue: English
• Weight: 905 KB
• Volume: 26
• Category: Article
• ISSN: 0006-3525
• DOI: 10.1002/bip.360260705

✦ Synopsis

High-field aCa-nmr is applied to characterize the interactions of calcium ions with double-helical DNA. Under the conditions examined, '%a lineahapea 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 ' %a linewidths, spin-lattice relaxation rates, and chemical shifts observed over the c o r n of a titration of DNA with calcium salt are not well described by simple electrostatic models. Deviations are mast pronounced at low ratios of calcium to DNA phosphate. In contrast, a t 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 '3Ca2+ 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.