Lynn Savage, email@example.com
BERLIN – 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
“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