Study of the interaction between nucleic acid and oxytetracycline-Eu3+ and its analytical application

In Tris-HCl buffer (pH = 8.50), nucleic acid can enhance the fluorescence intensity of oxytetracycline (OTc)-Eu3+ complex and the enhancement of DNA is far larger than that of RNA. The electrostatic binding of the phosphate group of nucleic acids with Eu3+ in OTc-Eu3+ complex could decrease the nonradiative energy loss through O-H vibration of H2O molecule in Eu3+ complex. Therefore, the fluorescence of the system would be enhanced by nucleic acids. The enhancement of DNA comes from the groove binding and the electrostatic binding. It is the hydrophobic microenvironment of DNA groove that makes the fluorescence intensity of OTc-Eu-DNA system far stronger than that of OTc-Eu-RNA system. The above conclusion of the interaction mechanism between OTc-Eu3+ complex and nucleic acids can explain the effect of the factors such as salt concentration, KH2PO4, solvent polarity, etc., on the fluorescence intensity of the system. A fluorescence method for the selective determination of DNA in the presence of RNA was developed in this report. Under the optimum conditions, the enhanced fluorescence intensity is in proportion to the concentration of DNA in the range 3.0 x 10(-8)-5.0 x 10(6)g/ml for fish sperm DNA, 7.0 x 10(-8)-5.0 x 10(-6)g/ml for calf thymus DNA. The limits of detection are 11.2 and 15.1 ng/ml, respectively. The number of binding sites of nucleotide residue of DNA and RNA are 0.323 and near 0, while the association constants of DNA and RNA to OTc-Eu3+ complex are 7.5 x 10(5) and 6.5 x 10(5), respectively. These results indicate that every three base pairs of DNA can bind two molecules of OTc-Eu3+ complex with 1:1 molar ratio of Eu3+ to OTc, while the base group of RNA does not bind with OTc-Eu3+ complex. (C) 2002 Elsevier Science B.V. All rights reserved.