2013 February

Microsolvation of Gas-Phase Proteins with Crown Ethers

Wednesday, 27th February 2013Miscellaneous

Effect of crown ether (CE) microsolvation on the structure of the gas-phase protein cytochrome c. With attached CE the protein retains a compact and more native-like structure while partially extended conformations are observed without CE.

The biological function of proteins is strongly correlated to their three dimensional structure. In recent years, techniques that analyze large biomacromolecules in the gas phase, i.e. after full evaporation of the surrounding solvation shell, attracted increased attention. To date however, it is not clear to which extent the loss oft he solvent affects the overall structural integrity of the molecule. In this context, it is well accepted that the charge state is one of the major determinants for the three dimensional architecture of a protein: high charge states typically exhibit extended conformations that are dominated by the repulsion between like charges while low charge states often adopt a compact structure that, to a certain extent, resembles that present in solution.

Researchers from the Fritz Haber Institute now showed that not only the charge state itself, but also the interaction between charged side chains and the backbone can severely affect the structure of a protein in the gas phase. Using ion mobility mass spectrometry they studied the influence of the non-covalent attachment of crown ether (CE) on the structure of the protein cytochrome c. CEs are known to coordinate and solvate protonated lysine side chains similarly as water molecules do in the condensed phase. Depending on the original charge state of the protein ions, the attachment of crown ether showed very different effects. Cytochrome c ions with high (above 9+) and low (below 6+) charge states progressively increased in size (and collision cross section) upon attachment of multiple CEs – a result that is expected due to the growing size of the complex. Surprisingly, however, protein ions with an intermediate charge state (6 to 9+) underwent a significant compaction of up to 40% when crown ether was added.

The team explained these rather counter-intuitive results by a changed interaction between the protonated side chains and the protein backbone. When the solvent is removed, charged side chains immediately coordinate carbonyl groups that are otherwise involved in the formation of structuring hydrogen bonds. Therefore, significant conformational changes can occur. However, when the protonated side chains are capped with a CE, they are too bulky to interact with backbone carbonyls. As a result, the molecule is much less affected when the solvent is removed and presumably retains a more native like structure.

Original publication: Warnke, S.; von Helden, G.; Pagel, K. Protein Structure in the Gas Phase – the Influence of Side Chain Microsolvation, J. Am. Chem. Soc. 2013, 135, 1177-1180. URL: http://pubs.acs.org/doi/full/10.1021/ja308528d

Highlight: Armstrong, G.; Ion-mobility spectroscopy: Crowning achievement, Nature Chem. 2013, 5 (3), 150.
URL: http://www.nature.com/nchem/journal/v5/n3/full/nchem.1590.html

Cooling down electrons make crystals swing

Friday, 8th February 2013Publication highlights

Schematic representation of the experiment. An extremely short ultraviolet pulse creates hot excited electrons in the semiconductor titanium dioxide. This changes the spatial distribution of the electrons within the lattice, resulting in a shift of the potentials for the atomic cores, i.e., their rest position (central picture). The subsequent cooling of the electrons which takes about 20 femtoseconds further amplifies this effect (right picture). The combined effect of electron excitation and cooling leads to a force on the oxygen atomic cores, resulting in a coherent oscillation within the crystal structure.

At the atomic scale, solids are made of atomic cores, i.e., nuclei with tightly bound electrons, and weakly bound valence electrons. The valence electrons strongly interact with each other and thereby act as a kind of glue that holds the atomic cores together within the crystal. The fundamental properties of a material, e.g., its electrical conductivity, optical properties, or crystal structure, are the result of the continuous interplay between the positions of the atomic cores and the valence electrons.  The investigation these correlations, in particular for complex materials, manifests one of the central topics of modern solid state physics. One experimental approach uses extremely short light pulses to excite the material and observes the response of the crystal lattice to this perturbation. These processes occur on time scales of femtoseconds (1 fs = 10-15 s).

The correlations between electronic and atomic structures are particularly strong for transition metal oxides. For some of them it is even possible to induce a structural phase transition with an optical excitation. A team of researchers from the Fritz Haber Institute, in collaboration with colleagues from the Max Planck Institute for Quantum Optics, the Technical University of Munich, and the University of Kassel have now shown that even a small redistribution of electrons can produce a significant force on the atomic cores in the crystal lattice. They excited the semiconductor titanium dioxide with extremely short ultraviolet light pulses and measured the subsequent changes of the crystals reflectivity. The excitation initially generates a small number of very hot electrons which thereby also redistribute spatially within the crystal: the electron concentration is reduced around the oxygen cores while it is increased around the titanium cores. In consequence, the potential energy surface for the atomic cores, which is due to the valence electron distribution, changes and the rest position of the oxygen cores is shifted relative to the position of the titanium cores. Since these changes occur faster than motion of the atom cores in the crystal, each oxygen core experiences the same force, and all of them start oscillating in phase.

This effect is best understood imagining a ball (oxygen atomic cores) in a bowl (potential surface of the crystal). In the ground state, the ball is in the center at the bottom of the bowl. The excitation of the electrons causes a sudden shift of the bowl, and the ball starts oscillating.

Detailed analysis of the phase of these lattice oscillations and extensive theoretical calculations revealed a surprising effect: it is not only the initial excitation of the electrons that is important for the new rest position of the atomic cores, but also the subsequent cooling of the electrons. The initially hot electrons cool down from several thousand Kelvins to room temperature within about 20 femtoseconds. While the crystals warms up only slightly on those time scales, a

significant change of the spatial redistribution of valence electrons and, in consequence, the rest positions of the atomic cores is observed. Such dependence of the crystal structure on the electronic temperature has been long predicted, and could now be shown experimentally for the first time. The results show how the equilibrium state of a crystal can be extremely sensitive to small changes in the electronic structure. The work is another step towards understanding the complex interactions in transition metal oxides and opens up new ways of designing materials for specific applications.

Original publication:

Elisabeth M. Bothschafter, Alexander Paarmann, Eeuwe S. Zijlstra, Nicholas Karpowicz, Martin E. Garcia, Reinhard Kienberger, and Ralph Ernstorfer
“Ultrafast evolution of the excited-state potential energy surface of TiO2 single crystals induced by carrier cooling”, Phys. Rev. Lett. 110, 067402 (2013).

Contact: Dr. R. Ernstorfer, FHI (ernstorfer@fhi-berlin.mpg.de), Tel: ++49-30-8413 5117