Journal of Biological Physics 19: 199-209, 1994. 199 © 1994 KluwerAcaderaic Publishers. Printed in the Netherlands. 23Na NMR Investigation of the Interaction between DNA and Divalent Metallic Cations DAN VASILESCU, SEDDIK ANSISS, and GEORGES MALLET Laboratoire de Biophysique, Universit# de Nice-Sophia Antipolis, Parc Valrose, 06108 Nice Cddex 2, France (Received: 3 October 1993) Abstract. The aim of this study is to follow the thermodynamic behaviour of Na + ions, acting as natural counterions of DNA, in the presence of divalent metal ions, by using the 23Na NMR technique. With the help of the 23Na entropyof fluctuationsconceptintroduced by Lenk, we propose the followingdecreasingsequence: Mg ++, Zn++, Cd ++, Mn ++, and Cu ++, for the magnitudeof divalent metal ions interactionswith DNA phosphate sites. Key words: DNA, 2SNa NMR, divalentcations, Na+ entropyof fluctuations. 1. Introduction The interaction of DNA with divalent metal ions has been extensively studied [1- 12]; the UV spectrophotometry has been widely used as well as the NMR of specific ions Mg ++, Ca ++ . In previous works using the 23Na NMR technique, some of us have reported the results of investigations into the behaviour of Na + ions acting as natural DNA counterions, in the absence, or in the presence, of cationic drugs [6, 13]. Using the 'entropy of fluctuations' concept proposed by Lenk [14, 15], a new vision of Na + behaviour during DNA thermal transconformation has been introduced [16, 17]. The aim of the present work is to use the concept of Na + entropy of fluctuations around DNA and the 23Na NMR technique as a probe for the determination of the relative influence, on DNA phosphate sites, of divalent metallic cations, from the more peripheral Mg ++ to the more binding to bases Cu ++. 2. Materials and Methods 2.I. MATERIALS DNA solutions were prepared from sodium salt of highly polymerized DNA obtained from calf thymus, type D1 (Sigma, St. Louis, MO, U.S.A.). The DNA 10 -2 M was slowly dissolved in NaCl l0 -2 M, prepared from 'suprapur' NaCI (Merk, Darmstadt, Germany). The measured pH was 6.0. Divalent metal ion solu- tions were prepared from corresponding sodium chloride solutions obtained from Merk 'suprapur' (Merk, Darmstadt, Germany) and dissolved in 5 × 10 -3 M NaC1.