Superradiance and Exciton (De)localization in Light-Harvesting Complex II from Green Plants? ² Miguel A. Palacios,* ,‡ Frank L. de Weerd, ‡ Janne A. Ihalainen, § Rienk van Grondelle, ‡ and Herbert van Amerongen ‡ Faculty of Sciences, DiVision of Physics and Astronomy, Department of Biophysics and Physics of Complex Systems, Vrije UniVersiteit, De Boelelaan, 1081, 1081 HV Amsterdam, The Netherlands, and Department of Chemistry, UniVersity of JyVa ¨ skyla ¨ , P.O. Box 35, FIN-40351 JyVa ¨ skyla ¨ , Finland ReceiVed: NoVember 8, 2001 Fluorescence quantum yield and fluorescence lifetime measurements were performed on trimeric light- harvesting complex II (LHCII) from spinach in the temperature range 7-293 K. From the results the radiative rate was calculated, which is related to the amount of delocalization of excitations over different pigments because of intermolecular interactions. The emitting dipole strength of LHCII is very similar to that of unbound Chl a, and it appears to be almost independent of temperature. The apparent increase of the radiative rate upon lowering the temperature can largely be explained by the shrinking of the sample. It is concluded that at all temperatures the amount of exciton delocalization in LHCII is small. Introduction Photosynthesis is the process by which sunlight is converted into chemical energy in the form of organic compounds. The first important steps in this process are the absorption of light by the light-harvesting antenna and the efficient transport of the excited-state energy to the reaction center where a charge separation is initiated. 1 In green plants, more than 50% of the light absorption is accomplished by light-harvesting complex II (LHCII). 2 Since its three-dimensional structure at 3.4 Å resolution became available, 3 intensive investigation and debate concerning the relation between the structure and spectroscopic features have led to a detailed view of the functional properties of LHCII (for a recent review see ref 4). In vivo, this complex exists in a trimeric form, the average number of trimers per PSII reaction center being four. 5 The trimer contains between 36 and 42 chlorophyll molecules (Chl a to Chl b ratio ∼ 1.4) and 9-12 xanthophyll molecules (lutein, neoxanthin, and violaxanthin in the ratio 2:1:0.07-1). The distances between neighboring Chl a and Chl b fall in the range 8.3-10.5 Å, whereas identical pigments are further apart, according to the pigment assignment of Ku ¨hlbrandt et al. 3 Further progress in the elucidation of the pigment identities has been achieved by reconstitution experiments, 6-8 showing that some pigment binding sites can bind both Chl a and Chl b (sites A3 and B3), others bind preferentially Chl b instead of Chl a (A6 and A7), and one binds only Chl a instead of Chl b (B1). 8 Important points of discussion 4,9-18 have been the excitonic interactions in LHCII and related to this the dipole strength of the lowest-energy electronically excited state which can be estimated with different experimental techniques and theoretical modeling. In the presence of strong interactions between pigments, an excitation can be shared by these molecules and become delocalized (excitons), and the dipole strength of each excitonic state can adopt a value that may deviate significantly from that of monomeric pigments. In the case of weak interactions, the excitations are more or less localized on individual molecules, and the dipole strengths of the corre- sponding electronic transitions are close to that of an individual pigment. Not only the intermolecular couplings but also the pigment- protein interactions play a role in the amount of delocalization of the excitation: the distinct environments of pigments bound at various sites lead to differences in their electronic transition energies (different site energies). Variations in the environment of individual pigments (static disorder) lead to the inhomo- geneous broadening of absorption bands, and exciton-phonon interactions lead to additional homogeneous broadening. 13 The extent of delocalization depends on the magnitude of the pigment-pigment coupling compared to the amount of broadening. 19-28 The larger the spread in site energies and the amount of broadening, the less delocalized the excitations are. In the case of LHCII, there is a lack of knowledge about the orientations of the transition dipole moments and therefore also about the sizes of the interaction strengths between the chloro- phylls. However, using the crystal structure and the advances in the identification of the pigments, the interactions between them were estimated, considering dipole-dipole coupling in the point-dipole approximation. 4 These calculations showed that the strongest coupling strength between the Chl aQ y transitions is less than the inhomogeneous width of ∼120 cm -1 and the homogeneous width of ∼185 cm -1 at RT for the Q y transition. 29 From circular dichroism measurements, 9-11 it was concluded earlier that significant excitonic interactions occur among the chlorophylls, but the size of the coupling strength was not determined. Krawczyk et al. 14 measured the Stark spectrum of LHCII and concluded that excitonic interactions are present, although the Chl a absorption bands, including the lowest-energy one, mostly appeared to behave like those of uncoupled Chl’s. ² Abbreviations. PSII, photosystem II; Chl a, chlorophyll a; Chl b, chlorophyll b; fwhm, full width at half-maximum; LHCII, light-harvesting complex II; n, refractive index; OD, optical density; RT, room temperature; ACE, acetone; MeOH, methanol; krad, radiative rate. * Corresponding author. Telephone: (+31) 20 444-7935. Fax: (+31) 20 444-7999. E-mail: miguelan@nat.vu.nl. ‡ Vrije Universiteit. § University of Jyva ¨skyla ¨. 5782 J. Phys. Chem. B 2002, 106, 5782-5787 10.1021/jp014078t CCC: $22.00 © 2002 American Chemical Society Published on Web 05/08/2002