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Nucleic Acids Research, 2000, Vol. 28, No. 12 2439-2445
© 2000 Oxford University Press

Influence of divalent cations on the conformation of phosphorothioate oligodeoxynucleotides: a circular dichroism study

Siddhesh D. Patil and David G. Rhodes*

Department of Pharmaceutical Sciences, The University of Connecticut, Storrs, CT 06269-2092, USA

Phosphorothioate oligodeoxynucleotides (ODNs) have been extensively investigated in vivo and in vitro for antisense control of gene expression. It has been shown that cellular uptake of phosphorothioate ODNs in some in vitro cell systems increases in the presence of divalent cations. In this work, we analyze the conformation of phosphorothioate ODNs and specific changes induced in it by various divalent cations using circular dichroism (CD) spectroscopy. CD data were obtained with several phosphoro­thioate ODNs in the absence and presence of the divalent cations Mg2+, Ca2+, Sr2+, Ba2+ and Mn2+. All CD spectra indicated stable conformations of the ODNs in solution. The spectra were strongly dependent on ODN sequence and composition. Some ODNs such as T23 and another with ‘random’ distribution of bases showed CD spectra characteristic of B-form DNA. Other ODNs which had at least three consecutive guanines in their sequences exhibited spectra characteristic of parallel G-tetraplexes. CD spectra of antisense ODNs exhibited specific responses to divalent cations. Changes in the conformation were not simply due to ionic strength effects. Mn2+ diminished secondary structure in some ODNs. Group II divalent ions stabilized the parallel G-tetraplexes, and Mg2+ generally had the weakest stabilizing efficiency. Each sequence/ion combination had a specific response so these effects cannot be genera­l­ized. These sequence-dependent, divalent ion-sensitive, and structurally unique solution conformations may be related to ion-mediated ODN uptake.

* To whom correspondence should be addressed. Tel: +1 860 486 5413; Fax: +1 860 486 4998; Email: rhodes@uconnvm.uconn.edu


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