Determination of the Interaction of Deoxyribonucleate and Magnesium Ions by Means of a Metal Ion Indicator

Abstract

THIS communication presents a study of the binding of magnesium ions by deoxyribonucleate using eriochrome black T as an indicator of magnesium ion concentration. Earlier studies by conductometric and spectrophotometric titration procedures were interpreted as showing that, in the absence of other salts, magnesium was tightly bound by both undenatured and denatured deoxyribonucleic acid¹; this conclusion was based primarily on the presence of breaks, in the titration curves which were, irrespective of the degree of denaturation, stoichiometrically related to the concentration of deoxyribonucleic acid over a wide range of concentrations. However, using similar conductometric procedures, Zubay and Doty² later concluded, from comparison of the increments of conductivity, that under these conditions magnesium was bound tightly by denatured but only loosely by undenatured deoxyribonucleic acid. Titration methods of this type do not of course yield direct information concerning either the extent or tightness of binding. In particular the increments of conductivity found in the conductometric titrations depend¹ not only on the extent of binding but also on the balance of various other factors. In view of these facts and of the importance of ion binding to many aspects of the behaviour of deoxyribonucleic acid, it appeared desirable to obtain further information by methods which give a more direct measure of binding. Results obtained using the metal ion indicator show that magnesium is, in fact, bound even more tightly by undenatured than by denatured deoxyribonucleic acid.