|
Reaction and energy transfer dynamics in solutions and biological systems
|
We have investigated excited state dynamics in condensed phases.
Reaction and energy transfer dynamics in solutions and biological systems
Site-dependent fluctuations optimize electronic energy transfer in the Fenna-Matthews-Olson protein
Light absorbed by light-harvesting antennae is transferred to the reaction center (RC). The excitation energy transfer (EET) to the RC is known to proceed with nearly perfect quantum yield. However, understanding of EET is still limited at molecular level. Here, we examine the dynamics in the Fenna–Matthews–Olson (FMO) protein by developing an efficient molecular dynamics simulation that can properly describe the electronic properties of the bacteriochlorophylls. We find that the FMO protein consists of sites with heterogeneous fluctuations extending from fast to slow modulation. We also find that efficient EETs are facilitated by site-dependent fluctuations that enhance the resonance condition between neighboring sites with large site-energy differences and circumvent exciton trapping on the pathway to the RC. The effect of underdamped vibrations on the population transfer and dephasing rates is also discussed. Knowledge of site-dependent fluctuations is an important component of understanding optimization of EET in photosynthetic systems.
Saito, Higashi & Fleming, J.Phys.Chem.B (2019).
Quantitative Evaluation of Site Energies and Their Fluctuations of Pigments in the Fenna-Matthews-Olson
Complex with an Efficient Method for Generating a Potential Energy Surface
We develop an efficient method to generate an accurate semi-global potential energy surface of
a molecule in condensed phases with low computational cost. We apply the method to the analysis
of the site energies and their fluctuations of bacteriochlorophyll (BChl) a pigments in
the Fenna-Matthews-Olson (FMO) complex using the density functional properly describing
the ground and excited states of BChl a in solutions in our previous work (J. Phys. Chem. B
2014, 118, 10906-10918). The errors of the potential energies calculated from the present and
QM/MM methods are small: ~1 kcal/mol for both the ground and excited states. The calculated site
energies are in good agreement with the experimentally fitted results. The calculated spectral
density also agrees with the experimentally available data. The spectral densities of BChl 2 and
BChl 5 are much larger than those of the other five sites. The present method is expected to provide
new insights into the efficient excitation energy transfer in light-harvesting antennas.
|
Higashi & Saito, J.Chem.Theory Comput. (2016).
Electronic structures of chlorophyll molecule for excitation energy transfer dynamics
Excited states of chlorophyll a and b in solution by time-dependent density functional theory
The ground state and excited state electronic properties of chlorophyll (Chl) a and Chl b in diethyl ether, acetone, and ethanol solutions are investigated using quantum mechanical and molecular mechanical calculations with density functional theory (DFT) and time-dependent DFT (TDDFT). Although the DFT/TDDFT methods are widely used, the electronic structures of molecules, especially large molecules, calculated with these methods are known to be strongly dependent on the functionals and the parameters used in functionals. Here, we optimize the range-separated parameter, µ, of the CAM-B3LYP functional of Chl a and Chl b to reproduce the experimental excitation energy differences of these Chl molecules in solution. The optimal values of µ for Chl a and Chl b are smaller than the default value of µ and that for bacteriochlorophyll a, indicating the change in electronic distribution, i.e., an increase in electron delocalization, within the molecule. We find that the electronic distribution of Chl b with an extra formyl group is different from that of Chl a. We also find that the polarity of solution and hydrogen bond cause the decrease in the excitation energies and the increase in the widths of excitation energy distributions of Chl a and Chl b. The present results are expected to be useful for understanding the electronic properties of each pigment molecule in a local heterogeneous environment, which will play an important role in the excitation energy transfer in light-harvesting complex II.
Zhu, Higashi, & Saito, J.Chem.Phys. 156, (2022).
Theoretical Study on Excited States of Bacteriochlorophyll a
in Solutions with Density Functional Assesment
The excited-state properties of bacteriochlorophyll (BChl) a in triethylamine,
1-propanol, and methanol are investigated with the time-dependent density
functional theory by using the quantum mechanical and molecular mechanical reweighting
free energy self-consistant field method. It is found that no prevalent density functionals
can reproduce the experimental excited-state properties, i.e., the absorption and
reorganization energies, of BChl a in the solutions. The parameter μ in the rangeseparated
hybrid functional is therefore optimized to reproduce the differences of the
absorption energies in the solutions. We examine the origin of the differences of the
absorption energies in the solutions and find that sensitive balance between contributions
of structural changes and solute−solvent interactions determines the differences. The
accurate description of the excitation with the density functional with the adjusted
parameter is therefore essential to the understanding of the excited-state properties of BChl
a in proteins and also the mechanism of the photosynthetic systems.
|
Higashi, Kosugi, Hayashi, & Saito, J.Phys.Chem. B 118, 10906 (2014).
Excited state intramolecula proton transfer in solution
Direct Simulation of Excited-State Intramolecular Proton Transfer and
Vibrational Coherence of 10-Hydroxybenzo[h]quinoline in Solution
We investigate an ultrafast excited-state intramolecular proton transfer
(ESIPT) reaction and the subsequent coherent vibrational motion of
10-hydroxybenzo[h]quinoline in cyclohexane by the electronically embedded
multi-configuration Shepard interpolation method, which enables us to
generate the potential energy surface of the reaction effectively and thus
carry out a direct excited-state dynamics simulation with low computational
costs. The calculated time scale of the ESIPT and the frequencies and
lifetimes of coherent motions are in good agreement with the experimental
results. The present study reveals the coherent motions are caused by
not only the proton transfer itself but also the backbone displacement
induced by the ESIPT. We also discuss the effects of the solvent on the
dynamics of the coherent vibrational modes.
Higashi & Saito, J.Phys.Chem.Lett. 2, 2366 (2011).
