title: Exploring the Light-Capturing Properties of Photosynthetic Chlorophyll Clusters Using Large-Scale Correlated Calculations.
abstract: Chlorophylls are light-capturing units found in photosynthetic proteins. We study here the ground and excited state properties of monomeric, dimeric, and tetrameric models of the special chlorophyll/bacteriochlorophyll (Chl/BChl) pigment (P) centers P700 and P680/P870 of type I and type II photosystems, respectively. In the excited state calculations, we study the performance of the algebraic diagrammatic construction through second-order (ADC(2)) method in combination with the reduced virtual space (RVS) approach and the recently developed Laplace-transformed scaled-opposite-spin (LT-SOS) algorithm, which allows us, for the first time, to address multimeric effects at correlated ab initio levels using large basis sets. At the LT-SOS-RVS-ADC(2)/def2-TZVP level, we obtain vertical excitation energies (VEEs) of 2.00-2.07 and 1.52-1.62 eV for the P680/P700 and the P870 pigment models, respectively, which agree well with the experimental absorption maxima of 1.82, 1.77, and 1.43 eV for P680, P700, and P870, respectively. In the P680/P870 models, we find that the photoexcitation leads to a π → π* transition in which the exciton is delocalized between the adjacent Chl/BChl molecules of the central pair, whereas the exciton is localized to a single chlorophyll molecule in the P700 model. Consistent with experiments, the calculated excitonic splittings between the central pairs of P680, P700, and P870 models are 80, 200, and 400 cm(-1), respectively. The calculations show that the electron affinity of the radical cation of the P680 model is 0.4 V larger than for the P870 model and 0.2 V larger than for P700. The chromophore stacking interaction is found to strongly influence the electron localization properties of the light-absorbing pigments, which may help to elucidate mechanistic details of the charge separation process in type I and type II photosystems.
authors.affiliation: Department of Chemistry, University of Helsinki , P.O. Box 55 (A. I. Virtanens plats 1), FIN-00014 Helsinki, Finland.|Department Chemie, Technische Universität München , Lichtenbergstraße 4, Garching, Germany.
authors.forename: Carl-Mikael
authors.initials: CM
authors.lastname: Suomivuori
authors.affiliation: Ruhr-University at Bochum , Universitätsstraße 150, 44801 Bochum, Germany.
authors.forename: Nina O C
authors.initials: NO
authors.lastname: Winter
authors.affiliation: Ruhr-University at Bochum , Universitätsstraße 150, 44801 Bochum, Germany.
authors.forename: Christof
authors.initials: C
authors.lastname: Hättig
authors.affiliation: Department of Chemistry, University of Helsinki , P.O. Box 55 (A. I. Virtanens plats 1), FIN-00014 Helsinki, Finland.
authors.forename: Dage
authors.initials: D
authors.lastname: Sundholm
authors.affiliation: Department Chemie, Technische Universität München , Lichtenbergstraße 4, Garching, Germany.
authors.forename: Ville R I
authors.initials: VR
authors.lastname: Kaila
mesh_terms: D002734:Chlorophyll / Q000737:chemistry*; D004579:Electron Transport; D008027:Light; D010788:Photosynthesis; D045331:Photosystem I Protein Complex / Q000737:chemistry* / Q000378:metabolism; D045332:Photosystem II Protein Complex / Q000737:chemistry* / Q000378:metabolism; D017434:Protein Structure, Tertiary; D011789:Quantum Theory; D016992:Rhodobacter / Q000378:metabolism; D013816:Thermodynamics
chemical_list: D045331:Photosystem I Protein Complex; D045332:Photosystem II Protein Complex; D002734:Chlorophyll