Structure-Redox Response Correlation in a Few Select Heme Systems Using X-ray Absorption Spectroelectrochemistry

Influence of the Fe(II) Spin State on Iron-Ligand Bonds in Heme Model Iron-Porphyrin Complexes with 4, 5 and 6 Ligands

In this work heme models with four [Fe(II)(P)], five [Fe(II)(P)Im], [Fe(II)(P)O2] and six ligands [Fe(II)(P)(Im)O2], where P=porphyrin, with different spin states (ms=5, 3 and 1) of the iron atom were investigated using relativistic-corrected quantum chemistry methods (PW6B95-D3-DKH/jorge-TZP-DKH). Dependence of the iron-ligand bond properties on (i) spin state and (ii) number of ligands were analyzed using natural bond orbital analysis, electron density topology, electrostatic potential and electron localization function. It is shown that reversible binding of O2 is possible in case of formation of semicoordination bond between Fe(II) and imidazole. Binding of the fifth and sixth ligand from the energetic and orbital points of view is more favorable for the triplet Fe(II) state. At the same time for the six-coordinated complex [Fe(II)(P)(Im)O2] interconversion of Fe(II) electrons of valent 3d orbital from quintet to triplet and vice versa is possible under thermal fluctuations (energy barriers less than 2 kcal/mol).

Binding mechanism and antioxidant activity of piperine to hemoglobin

Piperine (PIP) is the most active main component in pepper. The interaction of small molecules with biomolecules leads to structural and functional changes. In this study, the binding mechanism and antioxidant activity of PIP with hemoglobin (Hb) are presented using spectroscopic and computational methods. Results showed that the redox activity of PIP on Hb showed concentration dependence. Fluorescence and isothermal titration calorimetric experiments showed that the Hb-PIP system had a static quenching mechanism at a single binding site. The addition of PIP caused a slight perturbation to the secondary structure of Hb by structural analysis. The structural stability of the Hb-PIP binding system was demonstrated by molecular dynamics simulations, and molecular docking and thermodynamic constants confirmed that the electrostatic interaction force was dominant in the energy contribution of the system. Research results are conducive to the potential use of PIP in related meat products.

Enzymatic X-ray absorption spectroelectrochemistry

The ability to observe the changes that occur at an enzyme active site during electrocatalysis can provide very valuable information for understanding the mechanism and ultimately aid in catalyst design. Herein, we discuss the development of X-ray absorption spectroscopy (XAS) in combination with electrochemistry for operando studies of enzymatic systems. XAS has had a long history of enabling geometric and electronic structural insights into the catalytic active sites of enzymes, however, XAS combined with electrochemistry (XA-SEC) has been exceedingly rare in bioinorganic applications. Herein, we discuss the challenges and opportunities of applying operando XAS to enzymatic electrocatalysts. The challenges due to the low concentration of the photoabsorber and the instability of the protein in the X-ray beam are discussed. Methods for immobilizing enzymes on the electrodes, while maintaining full redox control are highlighted. A case study of combined XAS and electrochemistry applied to a [NiFe] hydrogenase is presented. By entrapping the [NiFe] hydrogenase in a redox polymer, relatively high protein concentrations can be achieved on the electrode surface, while maintaining redox control. Overall, it is demonstrated that the experiments are feasible, but require precise redox control over the majority of the absorber atoms and careful controls to discriminate between electrochemically-driven changes and beam damage. Opportunities for future applications are discussed.