Exciton Dynamics in LH1 and LH2 of Rhodopseudomonas Acidophila and Rhodobium Marinum Probed with Accumulated Photon Echo and Pump-Probe Measurements Stefania S. Lampoura,* ,²,‡ Rienk van Grondelle, ² Ivo H. M. van Stokkum, ² Richard J. Cogdell, § Douwe A. Wiersma, ‡ and Koos Duppen ‡ Department of Physics and Astronomy, Vrije UniVersiteit, De Boelelaan 1081, 1081 HV, Amsterdam, The Netherlands, Ultrafast Laser and Spectroscopy Laboratory, Materials Science Center, UniVersity of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands, and Institute of Biomedical and Life Sciences, DiVision of Biochemistry and Molecular Biology, UniVersity of Glasgow, Glasgow G12 8QQ, UK ReceiVed: June 12, 2000; In Final Form: October 4, 2000 Exciton dynamics in the B850 and B875 bands of isolated complexes of Rhodopseudomonas acidophila (strain 10 050 and 7050) and in the B875 band of isolated complexes of Rhodobium marinum were investigated by means of accumulated photon echo and pump-probe techniques at different temperatures and wavelengths. For all three systems, the optical dephasing time T 2 was found to be very similar: at 4.2 K, T 2 is 116 and 106 ps for the B850 and B875 bands of Rhodopseudomonas acidophila, respectively, and 93 ps for the B875 band of Rhodobium marinum. The rapid dephasing, which displays glassy character, is a consequence of the strong pigment-protein interactions that arise through the rather short distances in these complexes. The observed dephasing time at the red edge of the B850 band of Rhodopseudomonas acidophila at 4.2 K reveals the existence of spectral diffusion in this system. From the wavelength dependence of the pump-probe signal in the B875 LH1 band of Rhodopseudomonas acidophila at 3 K it is concluded that energy transfer between energetically inequivalent LH1 rings occurs on a time scale of several tens picoseconds, while energy trapping takes place in about 250 ps. Introduction The fundamental processes in photosynthesis, underlying the conversion of solar energy into a useful chemical energy, have been extensively studied in many ways. Optical spectroscopic methods have proven to be a powerful tool in these efforts. 1-8 In photosynthesis, two ultrafast events play a decisive role: excitation energy transfer in the light harvesting antenna followed by charge separation in the reaction center (RC). 1,9,10 In most photosynthetic purple bacteria, the antenna system that transfers energy to the RC is composed of two different light harvesting complexes, a core complex surrounding the reaction center, LH1, and a more peripheral complex, LH2. Recently, the structure of the peripheral light harvesting antenna complex, LH2 of Rps. acidophila 11 was resolved to 2.5 Å resolution, by means of X-ray diffraction. This high-resolution crystal structure revealed a densely packed bacteriochlorophyll - protein system. Sandwiched between two concentric cylinders of protein subunits in LH2, there are two parallel rings of Bchl-a molecules, which give rise to absorption bands at 800 nm (B800 band) and 850 nm (B850 band), respectively. The B800 band originates from 9 monomeric Bchl-a molecules with their chlorin planes parallel to the membrane surface, whereas the B850 band contains 18 strongly interacting Bchl-a molecules with their chlorin planes perpendicular to the membrane surface. 11 The close distance of about 9 Å between neighboring B850 Bchl-a molecules leads to large interaction energies of about 250- 300 cm -1 , whereas the much larger distance of 21 Å between adjacent molecules in the B800 band leads to weaker interaction energies of about 20 cm -1 12-14 . The second light harvesting antenna, the core antenna LH1 which surrounds the reaction center, is proposed to consist of 32 Bchl-a molecules, arranged in a circular configuration. 15 This complex gives rise to an absorption band at 875 nm (B875 band). The large interaction energies between nearest neighbor molecules in the B850 and B875 bands lead to a delocalization of the excitation with an extent that is limited by static and dynamic disorder. Despite the various experimental and theo- retical methods that have been applied in order to establish how many Bchl-a molecules (N del ) are involved in the delocalized states, no consensus on this point has been reached: N del for the B850 and B875 bands at room temperature have been estimated to range from one dimmer to the whole ring. 16-26 Another issue that is still under investigation is the role of the protein structures in controlling the Bchl-a exciton state dynamics. Experiments in which the hydrogen bonding pattern of the Bchl-a’s was selectively perturbed by genetic engineer- ing, 27 demonstrated that, besides excitonic effects, the strong protein-pigment interactions also contribute significantly to the red shift of the Q y band of the LH2 complex. Stark spectroscopy 5 has been used to verify this influence of the protein environment on the absorption spectrum of the LH2 B850 band. The dynamics of a chromophore, embedded in a host material, are reflected by the homogeneous line width (Γ hom ) of the absorption or emission spectrum. However, obtaining informa- tion about the dynamics from the line shape of the absorption * To whom correspondence should be addressed. Tel: +31-20- 4447935. Fax: +31-20-4447899. E-mail: lampoura@nat.vu.nl. ² Department of Physics and Astronomy, Vrije Universiteit. ‡ Ultrafast Laser and Spectroscopy Laboratory, Materials Science Center, University of Groningen. § Institute of Biomedical and Life Sciences, Division of Biochemistry and Molecular Biology, University of Glasgow. 12072 J. Phys. Chem. B 2000, 104, 12072-12078 10.1021/jp0021289 CCC: $19.00 © 2000 American Chemical Society Published on Web 11/23/2000