Involvement of Nucleic Bases in the Quenching of the Fluorescence of Acridine by Methylviologen

Abstract

Hybrid molecules 5a-5c having a 9-acridinyl moiety connected to a viologen moiety through alkyl chains of three, six and nine carbons, respectively, were synthesized. The acridine and viologen rings of 5a were stacked in an aqueous solution. The other compounds, 5b and 5c, exist in a conformation separating the acridine and viologen units. Using absorption changes of the acridine chromophore of 5a-5c in the presence of calf thymus DNA, binding constants were determined to be 105–106 M−1 even at high salt concentrations (0.1–0.3 M). The binding affinity and the number of the associated counterions with the DNA complex of 5a were smaller than those of 5b and 5c. This is in agreement with the molecular model consideration in which the methylene chain of 5a was too short to allow interaction of the viologen with the phosphate anions of DNA. Electrochemical study of the viologen units of 5a-5c in the presence of DNA also supported the binding behavior obtained by the spectrophotometric method. Fluorometric study revealed that the quenching of the fluorescence of the acridine by the viologen is enhanced on the DNA matrix.