4.6 Article

Absorption shifts of diastereotopically ligated chlorophyll dimers of photosystem I

Journal

PHYSICAL CHEMISTRY CHEMICAL PHYSICS
Volume 21, Issue 13, Pages 6851-6858

Publisher

ROYAL SOC CHEMISTRY
DOI: 10.1039/c9cp00616h

Keywords

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Funding

  1. Doctoral Programme in Chemistry and Molecular Sciences (CHEMS) at the University of Helsinki
  2. Swedish Cultural Foundation in Finland
  3. Sigrid Juselius Foundation
  4. International Human Frontier Science Program (Long-Term Fellowship) [LT000916-2018-L]
  5. German Academic Exchange Service-Finnish Academy [287791]
  6. Magnus Ehrnrooth Foundation
  7. Academy of Finland [275845]
  8. Norwegian Research Council through the CoE Hylleraas Centre for Quantum Molecular Sciences [262695, 231571/F20]
  9. Finnish Grid and Cloud Infrastructure [urn:nbn:fi:research-infras-2016072533]

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The light-harvesting chlorophyll (Chl) molecules of photosynthetic systems form the basis for light-driven energy conversion. In biological environments, the Chl chromophores occur in two distinct diastereotopic configurations, where the alpha and beta configurations have a magnesium-ligating histidine residue and a 17-propionic acid moiety on the opposite side or on the same side of the Chl ring, respectively. Although beta-ligated Chl dimers occupy conserved positions around the reaction center of photosystem I (PSI), the functional relevance of the alpha/beta configuration of the ligation is poorly understood. We employ here correlated ab initio calculations using the algebraic-diagrammatic construction through second order (ADC(2)) and the approximate second-order coupled cluster (CC2) methods in combination with the reduced virtual space (RVS) approach in studies of the intrinsic excited-state properties of alpha-ligated and beta-ligated Chl dimers of PSI. Our ab initio calculations suggest that the absorption of the alpha-ligated reaction-center Chl dimer of PSI is redshifted by 0.13-0.14 eV in comparison to the beta-ligated dimers due to combined excitonic coupling and strain effects. We also show that time-dependent density functional theory (TDDFT) calculations using range-separated density functionals underestimate the absorption shift between the alpha- and beta-ligated dimers. Our findings may provide a molecular starting point for understanding the energy flow in natural photosynthetic systems, as well as a blueprint for developing new molecules that convert sunlight into other forms of energy.

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