Journal
PHYSICAL CHEMISTRY CHEMICAL PHYSICS
Volume 21, Issue 13, Pages 6851-6858Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/c9cp00616h
Keywords
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Funding
- Doctoral Programme in Chemistry and Molecular Sciences (CHEMS) at the University of Helsinki
- Swedish Cultural Foundation in Finland
- Sigrid Juselius Foundation
- International Human Frontier Science Program (Long-Term Fellowship) [LT000916-2018-L]
- German Academic Exchange Service-Finnish Academy [287791]
- Magnus Ehrnrooth Foundation
- Academy of Finland [275845]
- Norwegian Research Council through the CoE Hylleraas Centre for Quantum Molecular Sciences [262695, 231571/F20]
- 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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