NMR investigation of pefloxacin-cation-DNA interactions:: the essential role of Mg2+

F-19-NMR spectra show that, in the absence of magnesium, pefloxacin binds poorly to DNA and preferentially to single-stranded rather than to double-stranded DNA. In the presence of a fixed concentration of magnesium (10 mM), the competition between double-stranded DNA species and pefloxacin for binding to magnesium is more important with linear DNA than with supercoiled plasmidic DNA. Only the F-19 signals of pefloxacin free and bound to magnesium are observed, but the formation of the ternary complex pefloxacin/Mg2+/DNA is indicated by the decrease in the integration of the proton decoupled F-19 signal at the high concentrations of double-stranded (linear and plasmidic) DNA. The variations of the F-19 NOE demonstrate that this formation depends strongly on the magnesium concentration, the optimum concentration being approximate to 2 mM in the presence of Escherichia coli DNA (4 mM). The F-19 NMR data show that the quinolone ring of pefloxacin has little mobility in the ternary complex. In this complex, pefloxacin could be bound to two magnesium ions, each cation acting as a bridge between this molecule and a phosphate group of the DNA backbone. (C) 1998 Elsevier Science B.V. All rights reserved.