Binding of 5-fluorotryptamine to polynucleotides as a model for protein-nucleic acid interactions: fluorine-19 nuclear magnetic resonance, absorption, and fluorescence studies.

Fluorine-19 nuclear magnetic resonance (19F NMR), optical absorption, and fluorescence spectroscopy have been used to study the interaction of 5-fluorotryptamine (5FTA) with polynucleotides as a model for protein--nucleic acid interactions. In the presence of DNA, denatured DNA, poly(A), and poly(A).poly(U), the 19F resonance of 5FTA shifted 0.3-0.6 ppm upfield while the presence of poly(I).poly(C) had little effect on the chemical shift. Differences in the 19F chemical shift induced upon changing from H2O to 2H2O indicate differences in the solvent accessibility of 5FTA bound to the various polynucleotides. 19F NMR relaxation experiments were carried out for free 5FTA and in its nucleic acid complexes, and the results were interpreted by using a two correlation time model that included contributions to relaxation from dipolar coupling and chemical shift anisotropy. Values for the internal motion correlation time and the overall motion correlation time are reported. The effect of 5FTA on the melting transition of the double-stranded polynucleotides and on the quenching of 5FTA fluorescence was also studied. The 19F NMR results support the model of partial intercalation of the 5FTA chromophore into the polynucleotides, and the implications for protein-nucleic acid interactions are discussed.