Invited feature article
Absorption-energy calculations of chlorophyll a and b with an explicit solvent model

https://doi.org/10.1016/j.jphotochem.2017.10.003Get rights and content
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Highlights

  • The absorption energies of chlorophylls a and b in organic solvents were calculated by time-dependent density functional theory.

  • The observed absorption energies were best reproduced when the CAM-B3LYP-related parameter μ = 0.14.

  • The parameter μ = 0.14 can be used for absorption-energy calculations for Chla and Chlb.

Abstract

The absorption energies of the Qy bands of chlorophylls a and b (Chla and Chlb) in organic solvents (acetone, diethyl ether, and ethanol) were calculated by combining molecular dynamics simulations and quantum mechanical/molecular mechanical approaches with an explicit solvent model. It was found that excitation-energy calculations by time-dependent density functional theory (DFT) using the CAM-B3LYP functional, following DFT geometry optimizations with the B3LYP functional, accurately reproduced the differences between the observed absorption energies of Chla/Chlb in each solvent. The calculated energies were within a root-mean-square deviation of 0.0014 eV from the observed values when the CAM-B3LYP-related parameter μ, which is associated with the long-range correlation, was set to 0.14. Calculations using μ = 0.14 also reproduced the observed transition-dipole strengths of the Qy-band moments, and the observed absorption spectra, with linewidths of ∼0.05 eV. These results suggest that μ = 0.14 can be used for absorption-energy calculations for Chla and Chlb.

Abbreviations

Chl
chlorophyll
LHC
light harvesting complex
FMO
Fenna Matthews-Olson
QM
quantum mechanics
MM
molecular mechanics
DFT
density functional theory
TD-DFT
time-dependent density functional theory
HF
Hartree Fock
GAFF
generalized Amber force field
RMSD
root-mean-square deviation
FWHM
full-width-at-half-maximum

Keywords

Photosynthesis
Light harvesting complex
Inhomogeneous broadening
Long-range correction
Ligand coordination
Site energy

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