Substituted Quinoline Quinones as Surrogates for the PQQ Cofactor: An Electrochemical and Computational Study

A perspective on the role of lanthanides in biology: Discovery, open questions and possible applications

Because of their use in high technologies like computers, smartphones and renewable energy applications, lanthanides (belonging to the group of rare earth elements) are essential for our daily lives. A range of applications in medicine and biochemical research made use of their photo-physical properties. The discovery of a biological role for lanthanides has boosted research in this new field. Several methanotrophs and methylotrophs are strictly dependent on the presence of lanthanides in the growth medium while others show a regulatory response. After the first demonstration of a lanthanide in the active site of the XoxF-type pyrroloquinoline quinone methanol dehydrogenases, follow-up studies showed the same for other pyrroloquinoline quinone-containing enzymes. In addition, research focused on the effect of lanthanides on regulation of gene expression and uptake mechanism into bacterial cells. This review briefly describes the discovery of the role of lanthanides in biology and focuses on open questions in biological lanthanide research and possible application of lanthanide-containing bacteria and enzymes in recovery of these special elements.

Synthetic modeling of the structure and function of the rare-earth dependent methanol dehydrogenase cofactor

Historically, rare-earth ions have been considered irrelevant to biology. Recently, the active sites of certain methanol dehydrogenase (MDH) enzymes have been shown to contain a redox-inactive, rare-earth (RE) cation coordinated by the redox-active pyrroloquinoline quinone (PQQ) cofactor. Importantly, it was demonstrated that rare earths were essential for the growth of certain methylotrophs that incorporated the XoxF-MDH. In this chapter, we summarize the optimized synthesis of a previously published rare-earth complex that serves as a model of the active site of this RE-containing MDH enzyme. The structure and reactivity of the metalated complex, [La(L_QQ)(NO₃)₃] are also discussed. [La(L_QQ)(NO₃)₃] catalytically oxidizes the test alcohol substrate, p-methylbenzyl alcohol, ^4MeBnOH, to p-methylbenzaldehyde, ^4MePhCHO, in the presence of a base (2,6-lutidine) and a terminal oxidant (ferrocenium hexafluorophosphate) with ~17 turnovers. By studying this synthetic model, we have developed a body of evidence about both the reactivity and the mechanism of dehydrogenation of alcohols as a molecular analogue to a native, rare-earth dependent enzyme.

Pyrroloquinoline Quinone Aza-Crown Ether Complexes as Biomimetics for Lanthanide and Calcium Dependent Alcohol Dehydrogenases*

Understanding the role of metal ions in biology can lead to the development of new catalysts for several industrially important transformations. Lanthanides are the most recent group of metal ions that have been shown to be important in biology, that is, in quinone‐dependent methanol dehydrogenases (MDH). Here we evaluate a literature‐known pyrroloquinoline quinone (PQQ) and 1‐aza‐15‐crown‐5 based ligand platform as scaffold for Ca²⁺, Ba²⁺, La³⁺ and Lu³⁺ biomimetics of MDH and we evaluate the importance of ligand design, charge, size, counterions and base for the alcohol oxidation reaction using NMR spectroscopy. In addition, we report a new straightforward synthetic route (3 steps instead of 11 and 33 % instead of 0.6 % yield) for biomimetic ligands based on PQQ. We show that when studying biomimetics for MDH, larger metal ions and those with lower charge in this case promote the dehydrogenation reaction more effectively and that this is likely an effect of the ligand design which must be considered when studying biomimetics. To gain more information on the structures and impact of counterions of the complexes, we performed collision induced dissociation (CID) experiments and observe that the nitrates are more tightly bound than the triflates. To resolve the structure of the complexes in the gas phase we combined DFT‐calculations and ion mobility measurements (IMS). Furthermore, we characterized the obtained complexes and reaction mixtures using Electron Paramagnetic Resonance (EPR) spectroscopy and show the presence of a small amount of quinone‐based radical.

Cerium(iv) complexes with guanidinate ligands: intense colors and anomalous electronic structures

A series of cerium(iv) mixed-ligand guanidinate–amide complexes, {[(Me₃Si)₂NC(NⁱPr)₂]_xCe^IV[N(SiMe₃)₂]_3−x}⁺ (x = 0–3), was prepared by chemical oxidation of the corresponding cerium(iii) complexes, where x = 1 and 2 represent novel complexes. The Ce(iv) complexes exhibited a range of intense colors, including red, black, cyan, and green. Notably, increasing the number of the guanidinate ligands from zero to three resulted in significant redshift of the absorption bands from 503 nm (2.48 eV) to 785 nm (1.58 eV) in THF. X-ray absorption near edge structure (XANES) spectra indicated increasing f occupancy (n_f) with more guanidinate ligands, and revealed the multiconfigurational ground states for all Ce(iv) complexes. Cyclic voltammetry experiments demonstrated less stabilization of the Ce(iv) oxidation state with more guanidinate ligands. Moreover, the Ce(iv) tris(guanidinate) complex exhibited temperature independent paramagnetism (TIP) arising from the small energy gap between the ground- and excited states with considerable magnetic moments. Computational analysis suggested that the origin of the low energy absorption bands was a charge transfer between guanidinate π orbitals that were close in energy to the unoccupied Ce 4f orbitals. However, the incorporation of sterically hindered guanidinate ligands inhibited optimal overlaps between Ce 5d and ligand N 2p orbitals. As a result, there was an overall decrease of ligand-to-metal donation and a less stabilized Ce(iv) oxidation state, while at the same time, more of the donated electron density ended up in the 4f shell. The results indicate that incorporating guanidinate ligands into Ce(iv) complexes gives rise to intense charge transfer bands and noteworthy electronic structures, providing insights into the stabilization of tetravalent lanthanide oxidation states.

A series of cerium(iv) mixed-ligand guanidinate-amide complexes, {[(Me₃Si)₂NC(NⁱPr)₂]_xCe^IV[N(SiMe₃)₂]_3−x}+ (x = 0−3), was prepared by chemical oxidation and studied spectroscopically and computationally, revealing trends in 4f/5d orbital occupancies.

The chemical and physical properties of tetravalent lanthanides: Pr, Nd, Tb, and Dy

The thermochemistry, descriptive chemistry, spectroscopy, and physical properties of the tetravalent lanthanides (Pr, Nd, Tb and Dy) in extended phases, gas phase, solution, and as isolable molecular complexes are presented.

Density Functional Theory as a Predictive Tool for Cerium Redox Properties in Nonaqueous Solvents

Two methods to correlate and predict experimental redox potentials for cerium complexes were evaluated. Seventeen previously reported cerium complexes were computed using DFT methods in both the Ce^III and Ce^IV oxidation states with a dichloromethane solvent continuum. In the first computational approach, the ΔG^o(Ce^IV/Ce^III) was determined for each of the compounds and these values were correlated with the experimental E_1/2 values measured in dichloromethane, referenced to the ferrocene/ferrocenium couple. The second method involved correlating the energies of the Ce^IV LUMOs (lowest unoccupied molecular orbitals) with the experimental redox potentials, E_1/2. The predictive capabilities of these two correlative methods were tested using a new cerium hydroxylamine complex, Ce(ODiNOx)₂ (ODiNOx = bis(2-tert-butylhydroxylaminatobenzyl) ether). All 18 complexes studied in this paper were combined with the 15 complexes determined in acetonitrile from a previously published correlation by our group. These sets of data allowed us to develop two methods for predicting the redox potential of cerium complexes regardless of the solvent for the experimental measurement.