Structure and stability of complexes formed by nucleic acids. I. Binding of acridine to DNA

Nuclear magnetic resonance spectra of acridine have been measured in aqueous methanol solutions over a wide concentration range in the presence and absence of dissolved DNA. In solutions containing DNA the acridine spectra show a marked line broadening and intensity decrease at temperatures lower than 50°C. These line‐shape changes can be associated with two types of binding interactions: (1) a tight, irrotational binding of the acridine at low acridine:phosphate ratios and (2) a weaker, rotationally less restrictive binding at high acridine concentrations. At temperatures above 50°C. a marked line narrowing is noted for the acridine spectrum and is attributed to an increase in mobility of the bound acridine as the DNA complex undergoes a helix–coil transition. A loose association of acridine molecules with the purine and pyrimidine bases in heat‐denatured DNA is indicated by chemical shift changes in the acridine spectrum. The NMR measurements also show that the presence of acridine in denatured DNA solutions greatly reduces renaturation of the DNA.