Detection of antibiotics in food: Extraction of fluoroquinolones by DNA

The ability of DNA to extract fluoroquinolones from model solutions and real probes of food was demonstrated and investigated quantitatively. The interaction between fluoroquinolones and different types of DNA was studied by equilibrium dialysis. The first application of this direct approach allowed us to determine binding constants and binding stoichiometries in different conditions. The binding of enrofloxacin to heat-denatured DNA (d-DNA) from herring sperm is pH- and magnesium-dependent; the highest fraction of bound drugs was found at pH 7 and a magnesium concentration of 0.5-1 mM. Results for three types of DNA: d-DNA, double-stranded DNA and single-stranded DNA were compared. The unwound DNA showed almost doubled binding constants and stoichiometries, thus indicating preferable interaction of enrofloxacin with single-strand regions of DNA. The binding of other fluoroquinolones (lomefloxacin, ciprofloxacin, norfloxacin, danofloxacin and sarafloxacin) with d-DNA is very similar to that of enrofloxacin: the binding constants are in the range from 0.94x10(5) to 2.40x10(5) M-1, and the stoichiometries range from 4.1 to 6.9 fluoroquinolone molecules per 100 DNA bases. The binding properties were quantitatively the same for extraction of fluoroquinolones from model aqueous solutions and from liquid food (milk). The results indicate the efficiency of DNA for selective extraction of fluoroquinolones from real samples for further analysis. This selective binding also allows us to consider DNA as a natural receptor for development of analytical techniques for fluoroquinolones.