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Structures of Bare and Hydrated [Pb(AminoAcid-H)]+ Complexes Using Infrared Multiple Photon Dissociation Spectroscopy

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journal contribution
posted on 06.10.2011 by Michael B. Burt, Sarah G. A. Decker, Chad G. Atkins, Mark Rowsell, André Peremans, Travis D. Fridgen
Infrared multiple-photon dissociation (IRMPD) spectroscopy was used to determine the gas-phase structures of deprotonated Pb2+/amino acid (Aa) complexes with and without a solvent molecule present. Five amino acid complexes with side chains containing only carbon and hydrogen (Ala, Val, Leu, Ile, Pro) and one with a basic side chain (Lys) were compared. These experiments demonstrated that all [Pb(Aa-H)]+ complexes have Pb2+ covalently bound between the amine nitrogen and carbonyl oxygen. The nonhydrated complexes containing Ala, Val, Leu, Ile, and Pro are amine-deprotonated, whereas the one containing Lys is deprotonated at its carboxylic acid. The difference is attributed to the polar and basic side chain of lysine, which helps stabilize Pb2+. IRMPD spectroscopy was also performed on the monohydrated analogues of the [Pb(Aa-H)]+ complexes. The [Pb(Aa-H)H2O]+ complexes, where Aa = Ala, Val, Leu, and Ile, exhibited two N–H stretches as well as a carboxylic acid O–H and a PbO-H stretch. Hence, their structures are monohydrated versions of the amine-deprotonated [Pb(Aa-H)]+ complexes where a proton transfer has occurred from the lead-bound water to the deprotonated amine. The IRMPD spectrum and calculations suggest that [Pb(Pro-H)H2O]+ has a hydrated carboxylate salt structure. The structure of [Pb(Lys-H)H2O]+ was also carboxyl-deprotonated, but Pb2+ is bound to the carbonyl oxygen and the amine nitrogen, with one of the protons belonging to the water transferred to the basic side chain. This results in an intramolecular hydrogen bond that does not absorb in the region of the spectrum probed in these experiments. The IRMPD spectra and structural characterizations were confirmed and aided by infrared spectra calculated at the B3LYP/6-31+G(d,p) level of theory and 298 K enthalpies and Gibbs energies using the MP2(full)/6-311++G(2d,2p) method on the B3LYP geometries.