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Physical Sciences Division
Research Highlights

June 2008

Science of Metal-Protein Complex Could Help Generate Energy

Recent study shows cobalt is highly efficient at moving electrons

Results: In the spinach and other plants in your garden, metal-packed proteins capture photons from sunlight and move protons and electrons through a chain of reactions, creating chemical energy for the plants and energy for those who eat them. Using a model system Pacific Northwest National Laboratory scientists Julia Laskin and postdoctoral fellow Zhibo Yang and Professor Ivan Chu from the University of Hong Kong determined for the first time how much energy it takes to move electrons and  protons in a metal-protein complex, advancing scientific frontiers.

Why it matters: Fundamental research on proton and electron transfers provides key scientific insights on such major societal problems as the conversion of solar energy into chemical energy to provide alternative fuel for cars, homes, and industries.

Methods: The team started with a solution containing a ternary complex shown in the cartoon. The first of three partners in the complex was a metal. Iron, cobalt, and manganese were tested. The second part was salen, a molecule that mimics common metal centers in metalloproteins. The third part was a peptide, representing an active site of a protein.

The solution was then ionized, and a mass spectrometer was used to isolate the ion corresponding to the complex and collide it with a special kind of target.

When the ions bounce off the special surface designed for these experiments, part of their kinetic energy is converted into internal energy, which causes the ions to fragment or dissociate into characteristic pieces. There are three competing pathways for dissociation: the peptide can lose an electron, gain a proton, or leave the complex as a neutral molecule. Researchers used detailed modeling of their experimental data to determine which pathway Nature determined as the winner in the competition.

They found that the preferred pathway depends on the electronic properties of the metal center. When cobalt or iron were used, more complexes followed the electron transfer pathway, while manganese favored proton transfer and simple separation of the metal-salen complex from the model protein. The team also observed that the competition between proton transfer and electron transfer in complexes containing cobalt is determined by the differences in reaction entropies. This indicates that rearrangements in the structure of the metal-protein complex influences the course of the reaction.

"This work takes us to a broader field of very complex electron and proton transfer processes in metalloproteins," said Laskin. In addition, this research proves that fragmentation induced by collisions with surface is a very valuable method for studying metal-protein complexes.

Much of the research was done using specialized  instrumentation at the Department of Energy's EMSL, a national scientific user facility at PNNL.

What's next? The team is studying proteins with the amino acids tryptophan and tyrosine next. Tryptophan, part of ongoing research, is of interest because of its inability to give up a proton. Tyrosine is of interest because once it loses an electron, it immediately sheds a proton. Researchers want to know how these amino acids, which are active in the metal centers of the metalloproteins,  effect proton and electron transfer.

Acknowledgments: This work was funded by the Chemical Sciences Division, Office of Basic Energy Sciences, U.S. Department of Energy and by the University of Hong Kong and Research Grant Council of Hong Kong, Special Administrative Region. PNNL's Interfacial and Condensed Phase Summer Research Institute provided travel support.

This work supports PNNL's mission to strengthen U.S. scientific foundations for innovation by developing tools and understanding required to control chemical and physical processes in complex multiphase environments.

Citation: Laskin J, Z Yang, and IK Chu. 2008. "Energetics and Dynamics of Electron Transfer and Proton Transfer in Dissociation of MetalIII(salen)-Peptide Complexes in the Gas Phase." Journal of the American Chemical Society 130:3218-3230.

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