23Na-nmr investigations of counterion exchange reactions of helical DNA

✍ Scribed by W. H. Braunlin; C. F. Anderson; M. Thomas Record Jr.

• Publisher: Wiley (John Wiley & Sons)
• Year: 1986
• Tongue: English
• Weight: 646 KB
• Volume: 25
• Category: Article
• ISSN: 0006-3525
• DOI: 10.1002/bip.360250114

✦ Synopsis

Changes in Av,, the nmr linewidth of =Na, have been determined during titrations of helical DNA with polyamines (divalent putrescine and trivalent spermidine) and with inorganic cations (Mg2+ and Co(NH3) %+I. In each case additions of a multivalent cation (Mz+ ) to a solution containing NaDNA and NaCl cause decreases in A V ~, which is a population-weighted average of contributions from nuclei in bound and free environments. Thus, the binding of Mz+ to DNA displaces sodium ions from regions where the quadrupolar relaxation of WNa is relatively efficient. At a given extent of titration, the binding of a polyamine produces a smaller decrease in Av, than does the binding of an inorganic ion of the same valence. The concentration dependence of Av, during the course of a titration can be interpreted most simply as a two-state ionexchange reaction by assuming that the binding of Mz+ does not alter R g , the average relaxation rate of sodium nuclei that remain bound. On the basis of this assumption, the initial linear portions of titration curves can be analyzed to determine upper bounds for P , the number of sodium ions bound per DNA phosphate in the absence of any competing counterion. Analyzing the titration curves for the four multivalent competitors leads to a range of upper-bound estimates for 4 : 0.5-0.8. The differences in these estimates could indicate that polyamines displace fewer sodium ions from DNA than do their smaller inorganic counterparts. Alternatively, the range in upper-bound estimates for 4 could also reflect specific differences in the effects of the various multivalent cations on RB, if this relaxation rate does change during titration.