Predicting Mössbauer Parameters of Nonheme Diiron Complexes with Density Functional Theory.

Mössbauer spectroscopy provides significant insights into the electronic structure and environment of the metal centers. Herein, we investigate the electronic structures of a set of nonheme diiron complexes by evaluating two key parameters pertaining to Mössbauer spectroscopy, namely, the isomer shift (δ) and quadrupole splitting (|ΔEQ|), using different levels of density functional theory (DFT). The diiron systems investigated here span diverse oxidation states, bridging motifs, and spin coupling patterns, which present a challenging case for theoretical predictions. We demonstrate that the combination of B97-D3/def2-TZVP is an efficient approach in modeling both the δ and |ΔEQ| values with high accuracy for the representative nonheme diiron complexes. We also show that δ is accurately predicted irrespective of the choice of approximate density functional while the |ΔEQ| is sensitive to the level of theory employed. Further investigation shows that the present methodology assessed using synthetic nonheme diiron complexes could be extended to nonheme diiron enzyme active sites, featuring both ferromagnetic and antiferromagnetic coupling between the iron centers.

Elucidating the effect of the ionic liquid type and alkyl chain length on the stability of ionic liquid-iron porphyrin complexes.

The present study is motivated by the long-term objective of understanding how ionic liquids are biodegraded by cytochrome P450, which contains iron porphyrin (FeP) serving as the catalytic center. To this end, the current study is designed to elucidate the impact of types and conformations of ionic liquids on the binding energy with FeP, the key interactions that stabilize the ionic liquid-FeP complex, and how the electron uptake ability of FeP is altered in the presence of ionic liquids. Four classes of ionic liquids are considered: 1-alkyl-3-methylimidazolium, 1-alkyl-pyridinium, 1-alkylsulfonium, and N-methyl-N-alkylpyrrolidinium. The influence of linear alkyl chains of ethyl, butyl, hexyl, octyl, and decyl is examined on the favorable binding modes with FeP, considering two widely different conformations: tail up and tail down with respect to FeP. Electronic structure calculations are performed at the M06 level of theory with the 6-31G(d,p) basis set for C, H, and N atoms, while the Lanl2DZ basis set is employed for Fe. Donor-acceptor interactions contributing to the binding of ionic liquids to FeP are unraveled through the natural bond orbital analysis. The results from this study indicate that the binding energies are dependent not only on the class of ionic liquids but also on the conformations presented to FeP. The propensity of FeP to acquire an electron is significantly enhanced in the presence of ionic liquid cations, irrespective of the type and the alkyl chain length.

Insight into conformationally-dependent binding of 1-n-alkyl-3-methylimidazolium cations to porphyrin molecules using quantum mechanical calculations.

The first step in the biodegradation of imidazolium-based ionic liquids involves the insertion of the -OH group into the alkyl side chain, and it is believed to be triggered by cytochrome P450. However, at present, there is a lack of fundamental understanding of why the hydroxylation process is observed only for longer alkyl chain analogues. As the initial step of the hydroxylation reaction involves the ionic liquid binding to Fe-porphyrin (FeP) - the catalytic center of cytochrome P450, the orientation of ionic liquids presented to FeP is expected to play a crucial role in eventual hydroxylation of the alkyl side chain. In order to elucidate the chain-length dependent binding preferences exhibited by the homologous series of 1-n-alkyl-3-methylimidazolium (n = 2, 4, 6, 8, and 10) [Cnmim]+ cations, a quantum mechanical treatment of the cations in the presence of free base porphyrin (FBP) and FeP is carried out at the B3LYP-D2 and M06 levels. The binding energy of different complexes with FBP and FeP is investigated by considering three vastly different starting relative orientations of the cations with respect to FBP and FeP: tail down, tail up, and interplanar. Our calculations of binding energies reveal that the cation orientations initiated from the tail down conformations (alkyl chain facing the porphyrin molecules) are progressively destabilized as the alkyl chain length increases. The decomposition of the binding energies into various energetic contributions shows that the interaction energy between the cations and porphyrin molecules varies with the cation geometries presented to porphyrin molecules and is the primary determinant of the magnitude of the binding energies. We further demonstrate that the propensity of the cation-FeP complexes to acquire an electron, the next step in the hydroxylation reaction cycle upon substrate binding, is favored independent of the cations and conformations, suggesting that this step is not the reason for the low biodegradability of short alkyl chain bearing cations. Furthermore, the weaker binding of the ionic liquid to FeP is anticipated to facilitate dioxygen binding to FeP, the step following the electron transfer reaction. Overall, the results of the present calculations indicate that the destabilization of the tail down conformations relative to the other two conformations correlates with the experimental results of the chain length-dependent biodegradation of imidazolium-based ionic liquids.