BIOPHYSICAL THEORY AND MODELING |
Quantitative Vibrational Dynamics of Iron in Carbonyl Porphyrins
Bogdan M. Leu 1, Nathan J. Silvernail 2, Marek Z. Zgierski 3, Graeme R. A. Wyllie 2, Mary K. Ellison 2, W. Robert Scheidt 4, Jiyong Zhao 5, Wolfgang Sturhahn 5, E. Ercan Alp 5 and J Timothy Sage 1*
1 Northeastern University
2 University of Notre Dame
3 National Research Council of Canada
4 Notre Dame University
5 Argonne National Laboratory
* To whom correspondence should be addressed. E-mail: jtsage{at}neu.edu.
Submitted on July 20, 2006
Revised on September 26, 2006
Accepted on 4 January 2007
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Abstract |
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We use nuclear resonance vibrational spectroscopy (NRVS) and computational predictions based on DFT to explore the vibrational dynamics of 57Fe in porphyrins that mimic the active sites of histidine-ligated heme proteins complexed with carbon monoxide. NRVS yields the complete vibrational spectrum of a Mössbauer isotope, and provides a valuable probe that is not only selective for protein active sites but quantifies the mean squared amplitude and direction of the motion of the probe nucleus, in addition to vibrational frequencies. Quantitative comparison of the experimental results with DFT calculations provide a detailed, rigorous test of the vibrational predictions, which in turn provide a reliable description of the observed vibrational features. In addition to the well-studied stretching vibration of the Fe-CO bond, vibrations involving the Fe-imidazole bond and the Fe-Npyr bonds to the pyrrole nitrogens of the porphyrin contribute prominently to the observed experimental signal. All of these frequencies show structural sensitivity to the corresponding bond lengths, but previous studies have failed to identify the latter vibrations, presumably because the coupling to the electronic excitation is too small in resonance Raman measurements. We also observe the FeCO bending vibrations, which are not Raman active for these unhindered model compounds. The observed Fe amplitude is strongly inconsistent with three-body oscillator descriptions of the FeCO fragment, but agrees quantitatively with DFT predictions. Over the past decade, quantum chemical calculations have suggested revised estimates of the importance of steric distortion of the bound CO in preventing poisoning of heme proteins by carbon monoxide. Quantitative agreement with the predicted frequency, amplitude, and direction of Fe motion for the FeCO bending vibrations provides direct experimental support for the quantum chemical description of the energetics of the FeCO unit.
Key Words:
NRVS, heme protein, iron carbonyl porphyrin, vibrational spectroscopy
