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The “dark” side of XAS illuminates the electron-transfer process

Nov 2010
Lynn Savage,

A variety of techniques are used to probe biologically interesting molecules, but little is known about the way in which these molecules interact at the most basic level within aqueous solutions. Using x-ray absorption spectroscopy (XAS), however, a team of European researchers has found a way to reveal a more complete picture of the electron-transfer process.

“The electronic structure of metallo-porphyrins in heme proteins (hemoglobin, myoglobin, cytochrome C, vitamin B12, etc.) are at the core of their chemical reactivity and, therefore, their biological functions,” said Emad F. Aziz of Helmholtz-Zentrum Berlin für Materialien und Energie (HZB). According to Aziz, the binding of diatomic and triatomic ligands, such as O2, NO, H2O and N3, is governed by charge transfer and related properties. Such processes take place via the empty orbital of the transition metal of the heme-active center. These orbitals are of the d-type character, and direct access to this state is through soft x-ray absorption spectroscopy.

Aziz and his colleagues from HZB and from École Polytechnique Fédérale de Lausanne in Switzerland and Université Bordeaux in Talence, France, have long studied the various components of hemoglobin, including hemin, its iron-based core.

XAS works by generating detectable fluorescence from the probed molecules by x-rays. The spectra of the returned light are indicative of the nature of the bonding d-type orbitals as well as the behavior of the excited electrons in the targeted states, providing a microscopic picture of the function of the probed atom or molecule.

The investigators performed their x-ray studies at the Bessy II (Berliner Elektronenspeicherring-Gesellschaft für Synchrotronstrahlung) synchrotron. They placed hemin samples in solution and circulated the solution through stainless steel tubing set up at the facility’s U41-PGM beamline. After shooting monochromatic soft x-rays at the moving sample through a 150-nm-thick silicon nitride membrane, they captured the resulting fluorescence emissions with a gallium arsenide phosphide photodetector. Alternatively, they also used a silica diode to record fluorescence emissions.

“Dark channel” fluorescence-yield x-ray absorption spectroscopy provides insight into the electron-transfer process in biologically important chemicals. Courtesy of Emad F. Aziz.

Curiously, the researchers found that some substances struck by the beam emitted no fluorescence, resulting in a dip peak dubbed a “dark channel.” They reasoned that the lack of fluorescence results from interactions between closely related energies in the iron center of heme and water molecules. The interactions, they believe, cause extinctions of some or all of the fluorescence because of competition between the fluorescence yield of the solute (such as hemin or other molecule of interest) to that of the solvent (such as water or ethanol), as well as to an electron transfer from the solvent material to the solvent.

Therefore, whenever the spectrum shows dark channels, each dipped peak indicates a femtosecond-scale moment when electrons are on the move.

Importantly, the technique permits them to measure this electron transfer process at room temperature, at normal pressure and in the heme’s natural aqueous environment.

“It is of fundamental importance to gain a detailed understanding of the structural and dynamic properties of these materials under realistic conditions,” Aziz said. “The goal is to perform a systematic study of the electronic structure and the dynamic behavior of materials in solution and at liquid-solid interfaces. Furthermore, it is opening the door for radiation attosecond biology and chemistry in solution.”

aqueous solutionsBasic ScienceBerliner Elektronenspeicherring-Gesellschaft für SynchrotronstrahlungBessyBiophotonicsBioScanchemicalscytochrome Cdark channeldiatomic ligandsEcole Polytechnique Fédérale de Lausanneelectron transferEmad F. Azizgallium arsenide phosphideHelmholtz-Zentrum Berlin für Materialien und Energieheme proteinsheminhemoglobinHZBimagingmetalloporphyrinsmyoglobinNewsphotodetectorsSensors & Detectorssoft x-raystriatomic ligandsUniversité Bordeauxvitamin B12x-ray absorption spectroscopyXAS

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