ISSN:
0006-3525
Keywords:
Chemistry
;
Polymer and Materials Science
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
Changes in Δν½, the nmr linewidth of 23Na, have been determined during titrations of helical DNA with polyamines (divalent putrescine and trivalent spermidine) and with inorganic cations (Mg2+ and Co(NH3)63+). In each case additions of a multivalent cation (Mz+) to a solution containing NaDNA and NaCl cause decreases in Δν½, 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 23Na is relatively efficient. At a given extent of titration, the binding of a polyamine produces a smaller decrease in Δν½ than does the binding of an inorganic ion of the same valence. The concentration dependence of Δν½ during the course of a titration can be interpreted most simply as a two-state ion-exchange reaction by assuming that the binding of Mz does not alter RB, 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 r°, 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 r°: 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 r° could also reflect specific differences in the effects of the various multivalent cations on RB, if this relaxation rate does change during titration.
Additional Material:
3 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/bip.360250114
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