2156
J. Phys. Chem. B 2003, 107, 2156-2161
Direct Observation of Solvation Dynamics and Dielectric Relaxation in the Photosynthetic Light-Harvesting-2 Complex of Rhodopseudomonas acidophila Frank van Mourik,*,† Raoul N. Frese,‡ Gert van der Zwan,§ Richard J. Cogdell,| and Rienk van Grondelle‡ Department of Biophysics and Physics of Complex Systems, Faculty of Sciences, Vrije UniVersiteit, De Boelelaan 1081, 1081HV Amsterdam, The Netherlands, Institut de Physique de la Matie` re Condense´ e, Faculte´ des Sciences, UniVersite´ de Lausanne, CH-1015 Lausanne-Dorigny, Switzerland, Department of Analytical Chemistry and Applied Spectroscopy, Faculty of Sciences, Vrije UniVersiteit, De Boelelaan 1083, 1081HV Amsterdam, The Netherlands, and DiVision of Biochemistry and Molecular Biology, Institute of Biomedical and Life Sciences, UniVersity of Glasgow, G12 8QQ Glasgow, United Kingdom ReceiVed: August 9, 2002; In Final Form: December 21, 2002
We studied the fluorescence dynamics of the light-harvesting-2 complex of Rhodopseudomonas acidophila both with and without an externally applied electric field. The fluorescence emission spectrum of the complex shows (small) spectral shifts of the average emission wavelength on a time scale of ∼300 ps. The shifts are temperature dependent and result from a combination of the (a) dielectric relaxation, or solvation, of the excited states by the protein, which causes a red shift of the emission, and (b) nonradiative loss processes occurring in a subset of the ensemble of complexes. These two processes compete with delocalization and energy transfer around the ring and are shown to be sensitive to an external electric field. The experiments illustrate how nature has prevented the above-mentioned loss processes in vivo by having fast energy transfer around a symmetric structure. Local protein relaxation is slow for the delocalized (or fast-hopping) excited states. However, once localized, localization and (dielectric) relaxation act in a cooperative way. Therefore, at low temperatures (
