Structural investigation of oxidized chlorosomes from green bacteria using multifrequency electron paramagnetic resonance up to 330 GHz |
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Authors: | Di Valentin Marilena Malorni Domenico Maniero Anna Lisa Agostini Giancarlo Giacometti Giovanni Vianelli Alberto Vannini Candida Cattaneo Anna Giulia Brunel Louis-Claude Carbonera Donatella |
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Affiliation: | (1) Department of Physical Chemistry, University of Padova, via Loredan 2, 35131, Padova, Italy;(2) Centro Studi Stati Molecolari Radicalici ed Eccitati CNR, via Loredan 2, 35131, Padova, Italy;(3) Department of Structural and Functional Biology, University of Insubria, via J.H. Dunant 3, 21100, Varese, Italy;(4) Center for Interdisciplinary Magnetic Resonance, National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32310, USA |
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Abstract: | Chemical oxidation of the chlorosomes from Chloroflexus aurantiacus and Chlorobium tepidum green bacteria produces bacteriochlorophyll radicals, which are characterized by an anomalously narrow EPR signal compared to in vitro monomeric BChl c.+ [Van Noort PI, Zhu Y, LoBrutto R and Blankenship RE (1997) Biophys J 72: 316–325]. We have performed oxidant concentration and temperature-dependent X-band EPR measurements in order to elucidate the line narrowing mechanism. The linewidth decreases as the oxidant concentration is increased only for Chloroflexus indicating that for this system Heisenberg spin exchange is at least partially responsible for the EPR spectra narrowing. For both species the linewidth is decreasing on increasing the temperature. This indicates that temperature-activated electron transfer is the main narrowing mechanism for BChl radicals in chlorosomes. The extent of the electron transfer process among different BChl molecules has been evaluated and a comparison between the two species representative of the two green bacteria families has been made. In parallel, high frequency EPR experiments have been performed on the oxidized chlorosomes of Chloroflexus and Chlorobium at 110 and 330 GHz in the full temperature range investigated at X-band. The g-tensor components obtained from the simulation of the 330 GHz EPR spectrum from Chlorobium show the same anisotropy as those of monomeric Chl a.+ [Bratt PJ, Poluektov OG, Thurnauer MC, Krzystek J, Brunel LC, Schrier J, Hsiao YW, Zerner M and Angerhofer A (2000) J Phys Chem B 104: 6973–6977]. The spectrum of Chloroflexus has a nearly axial g-tensor with reduced anisotropy compared to Chlorobium and monomeric Chl ain vitro. g-tensor values and temperature dependence of the linewidth have been discussed in terms of the differences in the local structure of the chlorosomes of the two families.This revised version was published online in October 2005 with corrections to the Cover Date. |
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Keywords: | bacteriochlorophyll cations carotenoids Chlorosomes high-field EPR |
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