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May 2008

Follow that Ion

Scientists study the fate of charged particles after soft landing

Soft landing technique and proton loss.
Scientists studied the kinetics of proton loss and ion desorption from substrates following a soft landing. The three curves above represent the change in ions with +2 charge (top), +1 (middle), and zero (bottom). Enlarged View

Results: When a large molecule with several extra protons softly lands on a surface, little happens to the molecule, but a lot happens to the protons, according to scientists at Pacific Northwest National Laboratory. Some of the protons, positively charged subatomic particles, dissipate on impact. Others stay with the molecule through the landing and only slowly leave the scene.

"This is the first detailed study of the kinetics of proton loss and ion desorption from substrates following the soft landing," said Dr. Julia Laskin, principal investigator at PNNL. The unique nature of the paper resulted in it being chosen for December 13, 2007, cover of Journal of Physical Chemistry C.

Why it matters: The kinetics of proton loss and ion desorption provides clues to scientists about phenomena associated with soft-landing of complex ions on surfaces. Soft landing is a unique method for preparing extremely pure, uniform layers of molecules on selected materials and creating materials for bio-inspired green energy sources.

Methods: The researchers began by generating a peptide, a portion of a protein, with two additional protons in the electrospray ion source. The additional protons allowed researchers to follow the evolution of molecules with one, two, or no additional protons during and after the deposition. The team selected a cyclic Gramicidin S peptide as a model system for this study.

Then, the Gramicidin S was placed onto a specially prepared surface using the soft-landing technique. Using a Fourier transform ion cyclotron resonance mass spectrometer coupled with a high-energy cesium beam, the scientists gently probed the surface both when the ion was deposited and for several hours afterwards.

The team found that the ions produced by soft-landing of Gramicidin S on the surface underwent a series of changes. For example, ions with two protons rapidly lost one proton, resulting in an increase of ions with one proton that continued for 2 to 3 hours after the deposition was finished. Neutral molecules were created when the ions lost both protons on impact.

Finally, the team developed a model to show how quickly the deposited ions lose protons and escape from the substrate. The results from the experiments could be reproduced quite well with the kinetic model, which considers charge reduction and thermal desorption of different species from the surface.

What's Next? The team plans to extend this research to other molecules and surfaces. In particular, charge reduction processes in catalytic systems are of great interest.

Acknowledgments: This work was supported by a grant from the Chemical Sciences Division of DOE's Office of Basic Energy Sciences and the Laboratory Directed Research and Development Program at PNNL.

The work was conducted by Omar Hadjar, Jean H. Futrell, and Julia Laskin of PNNL. This work was performed at DOE's Environmental Molecular Sciences Laboratory, a national scientific user facility at PNNL. This work is part of PNNL's work to develop tools and understanding required to control chemical and physical processes in complex multiphase environments.

Citation: Hadjar O, JH Futrell, and J Laskin. 2007. "First Observation of Charge Reduction and Desorption Kinetics of Multiply Protonated Peptides Soft Landed onto Self-Assembled Monolayer Surfaces." Journal of Physical Chemistry C 111(49):18220-18225.  doi:10.1021/jp075293y.

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