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Fluorescence Spectroscopy of Conformational Changes of Single LH2 Complexes

Danielis Rutkauskas ^*, Vladimir Novoderezhkin , Richard J. Cogdell  and Rienk van Grondelle ^*

^* Department of Biophysics and Physics of Complex Systems, Division of Physics and Astronomy, Faculty of Sciences, Vrije Universiteit, 1081 HV Amsterdam, The Netherlands; A. N. Belozersky Institute of Physico-Chemical Biology, Moscow State University, Moscow 119899, Russia; and Division of Biochemistry and Molecular Biology, Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow G12 8QQ, United Kingdom

Correspondence: Address reprint requests to Danielis Rutkauskas, E-mail: danielis{at}nat.vu.nl.

We have investigated the energy landscape of the bacterial photosynthetic peripheral light-harvesting complex LH2 of purple bacterium Rhodopseudomonas acidophila by monitoring sequences of fluorescence spectra of single LH2 assemblies, at room temperature, with different excitation intensities as well as at elevated temperatures, utilizing a confocal microscope. The fluorescence peak wavelength of individual LH2 complexes was found to abruptly move between long-lived quasi-stable levels differing by up to 30 nm. The frequency and size of these fluorescence peak movements were found to increase linearly with the excitation intensity. These spectral shifts either to the blue or to the red were accompanied by a broadening and decrease of the intensity of the fluorescence spectrum. The probability for a particle to undergo significant spectral shift in either direction was found to be roughly the same. Using the modified Redfield theory, the observed changes in spectral shape and intensity were accounted for by changes in the realization of the static disorder. Long lifetimes of the quasi-stable states suggest large energetic barriers between the states characterized by different emission spectra.

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