ИСТИНА

Ethidium bromide (EB) is well-known fluorescence probe widely used for identification of native double-stranded DNA molecules in aqueous solutions because of sharp increase of EB fluorescence intensity (I) due to intercalation of EB between the bases of the DNA. It was noted that the addition of positively charged species (polyamines, or spermine, and spermine analogues) to a solution of (DNA-EB) complex was accompanied with a decrease of I. This phenomenon has been explained by a displacement of intercalated EB cation from DNA double helix as a result of competitive binding of the positively charged species since the fluorescence intensity of free EB in a aqueous solution is negligible. Nevertheless the authors did not prove experimentally the squeezing of EB molecules out. We have studied an interaction of (DNA-EB) complex with a numerous positively charged macromolecules (PC) of different chemical nature.We have founded that the decrease of I at the first part of the curve is a result of adverse conformational changes of the environment of EB intercalated in the ternary complex (DNA-EB-PC) caused by electrostatic binding of the PC. The more pronounced decrease of I observed at the second part of the curve is due to accumulation of free, practically non-fluorescent EB in the solution.