The interaction of poly(ethylenimine) with nucleic acids and its use in determination of nucleic acids based on light scattering

For the first time, poly(ethylenimine) (PEI) was used to determine nucleic acids with a light scattering technique using a common spectrofluorometer. The interaction of PEI with DNA results in greatly enhanced intensity of light scattering at 300 nm, which is caused by the formation of the big particles between DNA and PEI. Based on this, a new quantitative method for nucleic acid determination in aqueous solutions has been developed. Under the optimum conditions, the enhanced intensity of light scattering is proportional to the concentration of nucleic acid in the range of 0.01-10.0 microg ml(-1) for herring sperm DNA (hsDNA), 0.02-10.0 microg ml(-1) for calf thymus DNA (ctDNA), 0.02-20.0 microg ml(-1) for yeast RNA (yRNA). The detection limits are 5.3, 9.9, and 13.7 ng ml(-1), respectively. Synthetic samples were determined satisfactorily. At the same time, the light scattering technique has been successfully used to obtain the information on the effects of pH and ionic strength on the formation and the stability of the DNA/PEI complex, which is important in some fields such as genetic engineering and gene transfer. Using ethidium bromide (EB) as a fluorescent probe, the binding of PEI with hsDNA was studied. Both the binding constant of EB with DNA and the number of binding sites per nucleotide decrease with increasing concentration of PEI, indicating noncompetitive inhibition of EB binding to DNA in the presence of PEI. And the association constant of PEI to DNA obtained is 1.2 x 10(5) M(-1). IR-spectra show that PEI interacts with DNA through both the phosphate groups and the bases of DNA and the formation of DNA/PEI complex may cause the change of the conformation of the DNA secondary structure, which is also proved by UV-spectra.