Characterization of the binding epitope of ciprofloxacin bound to human serum albumin

Aqueous solutions of ciprofloxacin in phosphate buffer were measured by NMR under physiological conditions. The chemical shifts differ substantially compared to earlier investigations at low pH or in DMSO. Protein binding experiments using saturation transfer were optimized to measure proton resonances of ciprofloxacin that are in close proximity to human serum albumin. The relative intensities were mapped on the molecule to define the binding epitope. According to this methodology the cyclopropane ring and the chinolon ring constitute the binding epitope. Competition experiments with increasing amounts of salicylic acid did not change the saturation transfer to the ciprofloxacin protons indicating at least two different binding sites.