Determination of DNA by its enhancement effect of resonance light scattering by azur A

The interaction of azur A (AA) with DNA in aqueous medium has been studied by measurement of resonance light scattering (RLS), and accordingly a method for DNA determination is proposed with sensitivity at the ng level. At pH 3.29 and ionic strength 0.003, the RLS of AA was greatly enhanced by DNA characterized by the maximum scattering peaks at 398 and 472 nm. Mechanistic studies show that the interaction of AA with DNA involves the salting-out of AA encouraged by the electrostatic attraction of the positive charge of AA molecules with the negative charges of the DNA polyanion, and the salted-out AA molecules bind with DNA in a weak cooperative mode. Scatchard plots were constructed for the interaction equilibrium of AA with double-stranded DNA based on the RLS data, and it was found that each nucleotide residue could bind to four AA molecules with the binding constant being at the 105 level. The enhanced intensity of RLS is proportional to the concentration of calf thymus DNA and fish sperm DNA in the range 0¯2.0 mug ml-1 if 3.0× 10-5 mol l-1 AA was employed. Four synthetic samples were determined satisfactorily.