Determination of DNA and its binding to protamine using potentiometric polyion sensors

Polyion-sensitive membrane electrodes based on the ion-exchanger dinonylnaphthalene sulfonate (DNNS) have previously been developed and used as an end-point detector for the titrimetric determination of heparin, low molecular weight heparins, and pentosan polysulfate (PPS). Herein, we report a similar detection methodology for double stranded DNA (dsDNA (500 bp) and dsDNA (100 bp)) in buffered solution at μg/mL levels using protamine as a titrant. Further, it is shown that the titration data can be used to estimate the binding constants between protamine and dsDNA. The equilibrium constant Keq and the number of protamine binding sites (n) per mole of DNA are determined by recasting the data in the form of a Scatchard plot.