Accelerated Publications Identification of the Upper Exciton Component of the B850 Bacteriochlorophylls of the LH2 Antenna Complex, Using a B800-Free Mutant of Rhodobacter sphaeroides ² M. H. C. Koolhaas, ‡ R. N. Frese, § G. J. S. Fowler, | T. S. Bibby, | S. Georgakopoulou, § G. van der Zwan,* ,‡ C. N. Hunter, | and R. van Grondelle § Department of Physical and Theoretical Chemistry and Department of Physics and Astronomy, Vrije UniVersiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands, and Krebs Institute, Department of Molecular Biology and Biotechnology, UniVersity of Sheffield, Western Bank, Sheffield S10 2UH, U.K. ReceiVed December 11, 1997; ReVised Manuscript ReceiVed February 18, 1998 ABSTRACT: In this paper, we report the circular dichroism (CD) spectra of two types of LH2 -only mutants of Rhodobacter sphaeroides. In the first, only the wild type LH2 is present, while in the second, the B800 binding site of LH2 has been either destabilized or removed. For the first time, we have identified a band in the CD spectrum of LH2, located at ∼780 nm, that can be ascribed to the high exciton component of the B850 band. The experimental spectra have been modeled by theoretical calculations. On this basis, the average interaction strength between monomers in the B850 ring can be estimated to be approximately 300 cm -1 . In addition, we suggest that in LH2 of Rb. sphaeroides the angles made by the Q y transitions of the B850 BChls with respect to the plane of the ring are slightly different from those calculated from the crystal structure of the Rhodopseudomonas acidophila LH2 complex. In photosynthesis, (solar) photons are absorbed by a light- harvesting antenna and are efficiently transported to a reaction center where they are used to drive a charge separation (1, 2). Following the elucidation of the structure of the peripheral light-harvesting complex (LH2 or B800- 850) of Rhodopseudomonas acidophila (3), defining the relationships between the structure, the spectroscopic proper- ties, and the energy transfer dynamics of this complex presents a major challenge. The most prominent absorption band of LH2, around 850 nm, arises from the B850 ring of 18 BChla molecules, and since the publication of the structure, an important issue has been how to describe the absorption and CD features of this ring. Several attempts have since been made to calculate the CD and OD spectra of the B850 ring from the structure (4-6). However, in such calculations, there is one major problem, which originates from the estimate of the amount of dipolar coupling between neighboring pigments which so far has varied between 100 and 700 cm -1 (7-9). Therefore, it is essential that the position of the high-energy exciton band is known for such calculations. However, due to the ² R.v.G. and C.N.H. acknowledge financial support by the Human Frontier Science Program (HFSP). C.N.H. and G.J.S.F. acknowledge support from the BBSRC (U.K.). The Biophysics-VU research is supported by the Netherlands Organization for Fundamental Research (NWO) via the Foundation for Life Sciences (SLW). * Correspondence should be addressed to this author at the Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Nether- lands. Fax: 31 -20 -4447643. E -mail: zwan@chem.vu.nl. ‡ Department of Physical and Theoretical Chemistry, Vrije Univer- siteit. § Department of Physics and Astronomy, Vrije Universiteit. | The University of Sheffield. © Copyright 1998 by the American Chemical Society Volume 37, Number 14 April 7, 1998 S0006-2960(97)03036-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/17/1998