IRMPD spectroscopic investigation of gas-phase complexes of deprotonated penicillin G with Ba(2+), Zn(2+) and Cd(2+)

The bare deprotonated penicillin G. and its complexes with the Ba(2+), Zn(2+) and Cd(2+) 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-pi interaction with the phenyl ring. The Zn(Penicillin-H)(+) spectrum is consistent with simple complexation in a similar tridentate conformation, probably without cation-pi 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-pi 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. (C) 2011 Elsevier B.V. All rights reserved.