IRMPD spectroscopic investigation of gas-phase complexes of deprotonated penicillin G with Ba2+, Zn2+ and Cd2+

The bare deprotonated penicillin G, and its complexes with the Ba2+, Zn2+ and Cd2+ ions, have been formed by electrospray ionization and studied by IRMPD (infrared multiple photon dissociation) spectroscopy using FT-ICR (Fourier-transform ion cyclotron resonance) mass spectrometry with the FELIX free electron laser infrared light source. The spectra in the 900–2000 cm−1 range have been interpreted by comparison with DFT-computed spectra. The spectrum of deprotonated penicillin G is in good agreement with calculations for a mixture of the two most favorable conformations, and is in good accord with the condensed-phase spectrum, except that the symmetric and antisymmetric carboxylate peaks are split much farther apart in the gas phase. The Ba(Penicillin-H)+ ion is assigned as the simple complex with barium chelated by all three carbonyl oxygens, along with a cation-π interaction with the phenyl ring. The Zn(Penicillin-H)+ spectrum is consistent with simple complexation in a similar tridentate conformation, probably without cation-π interaction, but the agreement with the calculated spectrum is not excellent, and rearrangement is not ruled out. The Cd(Penicillin-H)+ complex probably has the simple tridentate conformation with strong cation-π interaction when sprayed from methanol solution, but rearranges when sprayed from acetonitrile to a new spectroscopically unique conformation, which was not identified with confidence. For this latter complex, the most stable calculated structure found has a deprotonated amide nitrogen and a cadmium–nitrogen covalent bond.

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► IRMPD spectra of deprotonated penicillin G complexes with metal ions.
► Bare penicillinate anion spectrum agrees with calculation.
► Barium(II) complex has predicted structure chelating 3 oxygen sites.
► Zinc(II) complex spectrum suggests similar structure, but rearrangement is possible.
► Cadmium(II) complex rearranges depending on electrospray solvent.