Research Highlights
June 2005
"Soft-landing of Peptide Ions on Self-assembled Monolayer Surfaces"
Innovative research conducted during the Pacific Northwest National Laboratory's 2004 Summer Institute focused on controlled deposition of peptide ions on surfaces using mass spectrometry, research that holds promise for highly specific modifi cation of surfaces and development of new technology relevant for a variety of applications in material science and biotechnology. The Summer Institute program pairs visiting graduate and faculty members with recognized PNNL scientists to conduct laboratory and theoretical investigations. This project featured the teaming of PNNL scientists Julia Laskin and Jean Futrell, with Purdue University's Henry Bohn Hass Distinguished Professor, Graham Cooks, and graduate student Jormarie Alvarez.
Soft landing (SL) is defined as the intact capture in the condensed phase (surfaces of solids or liquids) of mass-selected polyatomic ions. A related process of depositing atomic ions at surfaces was used as early at 1942 to collect uranium isotopes for the Manhattan Project. Previous research by Cooks' group suggested that SL of mass-selected polyatomic ions provides a means for highly specific deposition of molecules of any size and complexity on surfaces using only a small fraction of material utilized in standard synthetic approaches.
The research conducted at PNNL was the first fundamental study of SL of peptide ions on selfassembled monolayer (SAM) surfaces. The team developed a novel experimental approach that utilized a specially designed Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) configured for studying interactions of large ions with surfaces. The instrument was modified to enable in situ analysis of modified surfaces by combining 2 keV Cs+ secondary ion mass spectrometry with FT-ICR detection of the sputtered ions (FT-ICR-SIMS). This allowed the researchers to conduct the first systematic study of several factors that affect SL of peptide ions on SAM surfaces. Peptide ions are particularly attractive as model systems that can provide important insights on the behavior of soft landed proteins. In addition, fundamental principles derived from studies on interaction of protonated peptides with hydrophobic surfaces provides important insights related to transport of biomolecules through membranes in living organisms.
The research showed for the first time that peptide ions retain at least one proton after SL on FSAM or HSAM surfaces. The special characteristics of the FT-ICR apparatus, which is housed at the Environmental Molecular Sciences Laboratory, a national DOE user facility located at PNNL, enabled quantitative investigation of the effect of the initial kinetic energy of peptide ions on SL. The team found that in the range of collision energies from 0 to 150 eV SL results in deposition of intact ions on surfaces. Most of the findings, such as the decrease in SL with increase in collision energy and its dependence on the properties of the surface, are consistent with an ion-polarizable molecule model for the initial stage of soft landing on SAM surfaces.
The team inferred that ions are oriented with their charged residues to the surface to maximize the binding energy, while ion loss involves rotation of the ion on the surface that results in larger separation distances and weakens the ion-surface interaction. Larger SL efficiencies observed for doubly protonated as compared to singly protonated ions is in good agreement with this model. These finding indicate a clear pathway for controlled deposition of large ions on surfaces.
This and other compelling research resulted from PNNL's first Summer Research Institute, conducted May-August 2004, and involved 14 graduate students, 1 undergraduate student, and 1 faculty member from universities across the United States and from Europe.
Publications
B. Gologan, J. R. Green, J. Alvarez, J. Laskin and R. G. Cooks. "Ion/Surface Reactions and Ion Soft-Landing," Physical Chemistry Chemical Physics, 7, 1490-1500 (2005)
J. Alvarez, R. G. Cooks, S. E. Barlow, D. J. Gaspar, J. H. Futrell and J. Laskin. "Preparation and in situ Characterization of Surfaces Using Soft-Landing in a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer," Anal. Chem., 77, 3452-3460 (2005)
J. Alvarez, J. H. Futrell and J. Laskin. "Soft-Landing of Peptides onto Self-Assembled Monolayer Surfaces," J. Phys. Chem A, submitted