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Solvation Effect of Bacteriochlorophyll Excitons in Light-Harvesting Complex LH2

V. Urbonien ^*, O. Vrublevskaja , G. Trinkunas , A. Gall , B. Robert  and L. Valkunas  

^* Department of General Physics and Spectroscopy, Vilnius University, Vilnius, Lithuania; Institute of Physics, Vilnius, Lithuania; Service de Biophysique des Fonctions Membranaires, Departement de Biologie Joliot-Curie/Commissariat a l'energie atomique and Centre National de la Recherche Scientifique/URA2096, CEA Saclay, Gif-sur-Yvette, France; and Department of Theoretical Physics, Vilnius University, Vilnius, Lithuania

Correspondence: Address reprint requests to L. Valkunas, Tel.: 370-5-266-1640; E-mail: leonas.valkunas{at}ff.vu.lt.

We have characterized the influence of the protein environment on the spectral properties of the bacteriochlorophyll (Bchl) molecules of the peripheral light-harvesting (or LH2) complex from Rhodobacter sphaeroides. The spectral density functions of the pigments responsible for the 800 and 850 nm electronic transitions were determined from the temperature dependence of the Bchl absorption spectra in different environments (detergent micelles and native membranes). The spectral density function is virtually independent of the hydrophobic support that the protein experiences. The reorganization energy for the B850 Bchls is 220 cm^–1, which is almost twice that of the B800 Bchls, and its Huang-Rhys factor reaches 8.4. Around the transition point temperature, and at higher temperatures, both the static spectral inhomogeneity and the resonance interactions become temperature-dependent. The inhomogeneous distribution function of the transitions exhibits less temperature dependence when LH2 is embedded in membranes, suggesting that the lipid phase protects the protein. However, the temperature dependence of the fluorescence spectra of LH2 cannot be fitted using the same parameters determined from the analysis of the absorption spectra. Correct fitting requires the lowest exciton states to be additionally shifted to the red, suggesting the reorganization of the exciton spectrum.