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The Low-Energy Forms of Photosystem I Light-Harvesting Complexes: Spectroscopic Properties and Pigment-Pigment Interaction Characteristics

Roberta Croce ^* , Agnieszka Chojnicka , Tomas Morosinotto , Janne A. Ihalainen ^¶, Frank van Mourik ^||, Jan P. Dekker , Roberto Bassi  ^|| and Rienk van Grondelle 

^* Department of Biophysical Chemistry, Groningen Bimolecular Sciences and Biotechnology Institute, University of Groningen, Groningen, The Netherlands; Istituto di Biofisica del CNR c/o ITC, Trento, Italy; Division of Physics and Astronomy, Vrije Universiteit, Amsterdam, The Netherlands; Université Aix-Marseille II, LGBP, Département de Biologie, Marseille, France; ^¶ Laboratoire de Spectroscopie Ultrarapide, ISIC, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland; and ^|| Dipartimento Scientifico e Tecnologico, Università di Verona, Verona, Italy

Correspondence: Address reprint requests to Roberta Croce, Fax: 31-50-3634800; E-mail: R.Croce{at}rug.nl.

In this work the spectroscopic properties of the special low-energy absorption bands of the outer antenna complexes of higher plant Photosystem I have been investigated by means of low-temperature absorption, fluorescence, and fluorescence line-narrowing experiments. It was found that the red-most absorption bands of Lhca3, Lhca4, and Lhca1–4 peak, respectively, at 704, 708, and 709 nm and are responsible for 725-, 733-, and 732-nm fluorescence emission bands. These bands are more red shifted compared to "normal" chlorophyll a (Chl a) bands present in light-harvesting complexes. The low-energy forms are characterized by a very large bandwidth (400–450 cm^–1), which is the result of both large homogeneous and inhomogeneous broadening. The observed optical reorganization energy is untypical for Chl a and resembles more that of BChl a antenna systems. The large broadening and the changes in optical reorganization energy are explained by a mixing of an Lhca excitonic state with a charge transfer state. Such a charge transfer state can be stabilized by the polar residues around Chl 1025. It is shown that the optical reorganization energy is changing through the inhomogeneous distribution of the red-most absorption band, with the pigments contributing to the red part of the distribution showing higher values. A second red emission form in Lhca4 was detected at 705 nm and originates from a broad absorption band peaking at 690 nm. This fluorescence emission is present also in the Lhca4-N-47H mutant, which lacks the 733-nm emission band.