Direct Observation of Doming Modes and Dynamics of Model Heme Compounds
Dennis D. Klug1, Marek Z.
Zgierski1, John S. Tse1, Zhenxian
Liu2, James R. Kincaid3, Kazimierz
Czarnecki3, and Russell J. Hemley2
1Institute for Molecular
Sciences, National Research Council of
2Geophysical
Laboratory, Carnegie Institution of Washington,
3Department
of Chemistry,
An outstanding important problem in the understanding of
physiological functions is that of protein dynamics. In particular, the
transportation of oxygen via the heme protein is
controlled by the dynamics of this type of molecule. There have been numerous
studies with the goal of characterizing the dynamics of heme
compounds that are models for myoglobin but a
definitive characterization of the dynamics and of specific vibrational
modes has been challenging. The most important vibrational
mode and the one that has received the most intensive study is that of the
"doming" mode in which an iron atom moves out of the plane of the poriphyrin molecular plane while the periphery of this ring
moves in the opposite direction. This mode is expected as infrared active but
could not be observed experimentally for a long time because of its low
frequencies and quite weak absorptions.
In our experiments, we have used a
unique combination of synchrotron far-infrared and high-pressure techniques
together with state-of-the-art quantum chemistry methods to characterize the
dynamics of model heme proteins. To directly detect
the “doming” vibrations, the compound was mounted in a diamond anvil cell
equipped with very thick steel gaskets (up to 500 microns thick) and large hole
(up to 800 microns in diameter). Spectra were obtained on beamline U2A that
is ideally suited for these types of experiments. The use of pressure as an
experimental parameter was essential since the pressure dependence could be
used to assign and verify the origin of each vibrational
mode. In addition, the well-defined sample geometry allowed the determination
of absorption intensities that also gave critical information for comparison
with theory and for identification of vibrational
modes.
The results of our experiments and
theoretical calculations yielded definitive assignments of the low-frequency
vibrations in model heme compounds and in particular
for the long sought "doming" mode. This combination of techniques should
help to provide an understanding of the dynamics of heme
proteins and could now be successfully applied to a wide class of biological
materials.
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The model heme molecule showing atom displacements for the doming mode |
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