Abstract
We present a computational analysis of the spin-density asymmetry in cation radical states of reaction center models from photosystem I, photosystem II, and bacteria from Synechococcus elongatus, Thermococcus vulcanus, and Rhodobacter sphaeroides, respectively. The recently developed frozen-density embedding (FDE)-diab methodology [J. Chem. Phys., 2018, 148, 214104] allowed us to effectively avoid the spin-density overdelocalization error characteristic for the standard Kohn-Sham density functional theory and to reliably calculate spin-density distributions and electronic couplings for a number of molecular systems ranging from inner pairs of (bacterio)chlorophyll a molecules in vacuum to large proteins including up to about 2000 atoms. The calculated spin densities show a good agreement with available experimental results and were used to validate reaction center models reported in the literature. Here, we demonstrate that the applied theoretical approach is very sensitive to changes in molecular structures and the relative orientation of molecules. This makes FDE-diab a valuable tool for electronic structure calculations of large photosynthetic models effectively complementing the existing experimental techniques.
Highlights
The mechanism of natural photosynthesis can conveniently be divided into several distinct steps [1]: i) light harvesting in antenna proteins, ii) a fast primary electron-transfer event in the reaction center (RC), iii) protein stabilization due to charge separation, and iv) subsequent steps involving the synthesis of new chemical compounds
We present a computational analysis of the spin-density asymmetry in cation radical states of reaction center models from photosystem I, photosystem II, and bacteria from Synechococcus elongatus, Thermococcus vulcanus, and Rhodobacter sphaeroides, respectively
We presented an application of the recently developed Frozen-Density Embedding (FDE)-diab approach to RC models from PSII, PSI, and purple bacteria
Summary
The mechanism of natural photosynthesis can conveniently be divided into several distinct steps [1]: i) light harvesting in antenna proteins, ii) a fast primary electron-transfer event in the reaction center (RC), iii) protein stabilization due to charge separation, and iv) subsequent steps involving the synthesis of new chemical compounds. The degree of the electronic-structure asymmetry in inner pairs and corresponding radical cations, formed during the primary electron donation (for PSI and the BRC) or subsequent charge separation reactions (for PSII), is a frequently discussed issue in the field and received vast attention in the literature Solid-state Photochemically Induced Dynamic Nuclear Polarization Nuclear Magnetic Resonance (photo-CIDNP NMR) gives an access to both molecular properties This methodology can be used to reconstruct spin-density maps of radical pairs inside RCs of plants and bacteria [2,3] and can be applied to whole living cells [15] or even entire intact plants [16]. Reliable calculations of inner pair spin-density distributions may require the consideration of the surrounding protein environment This considerably increases the size of molecular models to be computed and restricts the choice of the electronic structure method to Kohn–Sham Density Function Theory (KS-DFT).
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