The effect of methoxy group rotation and hydrogen bonding on the redox properties of ubiquinone

Influence of Polar Mutations on the Electronic and Structural Properties of QA-· in Bacterial Reaction Centers

Reaction centers from Rhodobacter sphaeroides with residue M265 mutated from isoleucine to threonine, serine, and asparagine (M265IT, M265IS, and M265IN, respectively) in the Q_A^-· state are studied by high-resolution electron spin echo envelope modulation (ESEEM) and electron nuclear double resonance spectroscopy methods to investigate the structural characteristics of these mutants influencing the redox properties of the Q_A site. All three mutants decrease the redox midpoint potential (E_m) of Q_A by ∼0.1 V, yet the mechanism for this drop in E_m is unclear. In this work, we examine (i) the hydrogen bonding interactions between Q_A^-· and residues histidine M219 and alanine M260, (ii) the electron spin density distribution of the semiquinone, and (iii) the orientations of the ubiquinone methoxy substituents. ¹³C measurements show no significant contribution of methoxy dihedral angles to the observed decrease in E_m for the Q_A mutants. Instead, ¹⁴N three-pulse ESEEM data suggest that electrostatic or hydrogen bond formation between the mutated M265 side chain and His-M219 N_δ may be involved in the observed lowering of the Q_A midpoint potential. For mutant M265IN, analysis of the proton hyperfine couplings reveals a weakened hydrogen bond network, resulting in an altered Q_A^-· spin density distribution. The magnetic resonance study presented here is most consistent with an electrostatic or structural perturbation of the His-M219 N_δ hydrogen bond in these mutants as a mechanism for the ∼0.1 V decrease in Q_A E_m.

Systematic High-Accuracy Prediction of Electron Affinities for Biological Quinones

Quinones play vital roles as electron carriers in fundamental biological processes; therefore, the ability to accurately predict their electron affinities is crucial for understanding their properties and function. The increasing availability of cost-effective implementations of correlated wave function methods for both closed-shell and open-shell systems offers an alternative to density functional theory approaches that have traditionally dominated the field despite their shortcomings. Here, we define a benchmark set of quinones with experimentally available electron affinities and evaluate a range of electronic structure methods, setting a target accuracy of 0.1 eV. Among wave function methods, we test various implementations of coupled cluster (CC) theory, including local pair natural orbital (LPNO) approaches to canonical and parameterized CCSD, the domain-based DLPNO approximation, and the equations-of-motion approach for electron affinities, EA-EOM-CCSD. In addition, several variants of canonical, spin-component-scaled, orbital-optimized, and explicitly correlated (F12) Møller-Plesset perturbation theory are benchmarked. Achieving systematically the target level of accuracy is challenging and a composite scheme that combines canonical CCSD(T) with large basis set LPNO-based extrapolation of correlation energy proves to be the most accurate approach. Methods that offer comparable performance are the parameterized LPNO-pCCSD, the DLPNO-CCSD(T₀ ), and the orbital optimized OO-SCS-MP2. Among DFT methods, viable practical alternatives are only the M06 and the double hybrids, but the latter should be employed with caution because of significant basis set sensitivity. A highly accurate yet cost-effective DLPNO-based coupled cluster approach is used to investigate the methoxy conformation effect on the electron affinities of ubiquinones found in photosynthetic bacterial reaction centers. © 2018 Wiley Periodicals, Inc.