Gas-phase zwitterion stabilization by a metal dication
Robert C Dunbar1, Nick C Polfer, Jos Oomens
1Chemistry Department, Case Western Reserve University, Cleveland, Ohio 44106, USA. rcd@po.cwru.edu
Journal of the American Chemical Society
|November 8, 2007
Summary
Gas-phase amino acids typically favor canonical structures over zwitterions. However, this study experimentally confirms the salt bridge (SB) zwitterionic form in doubly charged barium complexes of tryptophan, ruling out other structures.
Area of Science:
- Physical Chemistry
- Spectroscopy
- Biophysical Chemistry
Background:
- Amino acids exist as zwitterions in solution but typically adopt canonical forms in the gas phase.
- Metal ion complexation can stabilize zwitterionic forms, but this is generally not the most stable configuration for singly charged complexes.
- Computational studies suggested enhanced zwitterionic stability in doubly charged complexes, but experimental verification was lacking.
Purpose of the Study:
- To experimentally investigate the structure of gas-phase doubly charged metal-amino acid complexes.
- To verify the predicted enhanced stability of the zwitterionic (salt bridge) form in such complexes.
- To characterize the barium-tryptophan complex structure using infrared spectroscopy.
Main Methods:
- Utilized multiple-photon infrared dissociation (MP-IRPD) spectroscopy.
- Employed the FELIX free electron laser for precise infrared excitation.
- Analyzed characteristic vibrational frequencies (carboxylate antisymmetric CO stretch and NH3 umbrella mode) to identify ion structures.
Main Results:
- The barium (Ba2+) complex of tryptophan was experimentally confirmed to exist in the salt bridge (SB) zwitterionic form.
- The presence of the nonzwitterionic, charge-solvated form was definitively ruled out.
- Key spectroscopic signatures, including a 1600 cm-1 CO stretch and a 1400 cm-1 NH3 umbrella mode, were identified for the SB structure.
Conclusions:
- Doubly charged metal complexes can stabilize the zwitterionic (salt bridge) form of amino acids in the gas phase.
- This experimental evidence validates computational predictions regarding enhanced SB stability.
- Infrared spectroscopy provides a powerful tool for distinguishing between zwitterionic and nonzwitterionic gas-phase ion structures.
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