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Probing the Competition among Different Coordination Motifs in Metal–Ciprofloxacin Complexes through IRMPD Spectroscopy and DFT Calculations
journal contribution
posted on 2013-01-07, 00:00 authored by Susanna Piccirillo, Alessandra Ciavardini, Enrico Bodo, Flaminia Rondino, Debora Scuderi, Vincent Steinmetz, Alessandra PaladiniThe vibrational spectra of ciprofloxacin complexes with
monovalent
(Li+, Na+, K+, Ag+) and
polyvalent (Mg2+, Al3+) metal ions are recorded
in the range 1000–1900 cm–1 by means of infrared
multiple-photon dissociation (IRMPD) spectroscopy. The IRMPD spectra
are analyzed and interpreted in the light of density functional theory
(DFT)-based quantum chemical calculations in order to identify the
possible structures present under our experimental conditions. For
each metal–ciprofloxacin complex, four isomers are predicted,
considering different chelation patterns. A good agreement is found
between the measured IRMPD spectrum and the calculated absorption
spectrum of the most stable isomer for each complex. Metal ion size
and charge are found to drive the competition among the different
coordination motifs: small size and high charge density metal ions
prefer to coordinate the quinolone between the two carbonyl oxygen
atoms, whereas large-size metal ions prefer the carboxylate group
as a coordination site. In the latter case, an intramolecular hydrogen
bond compensates the weaker interaction established by these cations.
The role of the metal cation on the stabilization of ionic and nonionic
structures of ciprofloxacin is also investigated. It is found that
large-size metal ions preferentially stabilize charge separated motifs
and that the increase of metal ion charge has a stabilizing effect
on the zwitterionic form of ciprofloxacin.