First Experimental Structure of a 1:1 Metal Complex with a PQQ Cofactor Derivative outside Dehydrogenase Enzymes

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.

Crystal structure of a calcium(II)-pyrroloquinoline quinone (PQQ) complex outside a protein environment

Pyrroloquinoline quinone (PQQ) is an important cofactor of calcium- and lanthanide-dependent alcohol dehydrogenases, and has been known for over 30 years. Crystal structures of Ca-MDH enzymes (MDH is methanol dehydrogenase) have been known for some time; however, crystal structures of PQQ with biorelevant metal ions have been lacking in the literature for decades. We report here the first crystal structure analysis of a Ca-PQQ complex outside the protein environment, namely, poly[[undecaaquabis(μ-4,5-dioxo-4,5-dihydro-1H-pyrrolo[2,3-f]quinoline-2,7,9-tricarboxylato)tricalcium(II)] dihydrate], {[Ca3(C14H3N2O8)2(H2O)11]·2H2O}n. The complex crystallized as Ca3PQQ2·13H2O with Ca2+ in three different positions and PQQ3-, including an extensive hydrogen-bond network. Similarities and differences to the recently reported structure with biorelevant europium (Eu2PQQ2) are discussed.

The Earlier the Better: Structural Analysis and Separation of Lanthanides with Pyrroloquinoline Quinone

Lanthanides (Ln) are critical raw materials, however, their mining and purification have a considerable negative environmental impact and sustainable recycling and separation strategies for these elements are needed. In this study, the precipitation and solubility behavior of Ln complexes with pyrroloquinoline quinone (PQQ), the cofactor of recently discovered lanthanide (Ln) dependent methanol dehydrogenase (MDH) enzymes, is presented. In this context, the molecular structure of a biorelevant europium PQQ complex was for the first time elucidated outside a protein environment. The complex crystallizes as an inversion symmetric dimer, Eu2 PQQ2 , with binding of Eu in the biologically relevant pocket of PQQ. LnPQQ and Ln1Ln2PQQ complexes were characterized by using inductively coupled plasma mass spectrometry (ICP-MS), infrared (IR) spectroscopy, 151 Eu-Mössbauer spectroscopy, X-ray total scattering, and extended X-ray absorption fine structure (EXAFS). It is shown that a natural enzymatic cofactor is capable to achieve separation by precipitation of the notoriously similar, and thus difficult to separate, lanthanides to some extent.

Controlling the redox properties of a pyrroloquinolinequinone (PQQ) derivative in a ruthenium(ii) coordination sphere

Difference in the coordination modes of pyrroloquinolinequinone (PQQ), a redox-active heteroaromatic cofactor, in ruthenium(ii) complexes, drastically affects the stability of the metal coordination and the reversibility of the reduction process of the PQQ ligand. Additional metal-ion binding to the PQQ ligand largely raises its 1e⁻-reduction potential.

Uranium exerts acute toxicity by binding to pyrroloquinoline quinone cofactor

Uranium as an environmental contaminant has been shown to be toxic to eukaryotes and prokaryotes; however, no specific mechanisms of uranium toxicity have been proposed so far. Here a combination of in vivo, in vitro, and in silico studies are presented describing direct inhibition of pyrroloquinoline quinone (PQQ)-dependent growth and metabolism by uranyl cations. Electrospray-ionization mass spectroscopy, UV-vis optical spectroscopy, competitive Ca(2+)/uranyl binding studies, relevant crystal structures, and molecular modeling unequivocally indicate the preferred binding of uranyl simultaneously to the carboxyl oxygen, pyridine nitrogen, and quinone oxygen of the PQQ molecule. The observed toxicity patterns are consistent with the biotic ligand model of acute metal toxicity. In addition to the environmental implications, this work represents the first proposed molecular mechanism of uranium toxicity in bacteria, and has relevance for uranium toxicity in many living systems.

(6-Acetyl-1,3,7-trimethyllumazine-κ3O4,N5,O6)bis(triphenylphosphine-κP)copper(I) hexafluoridophosphate

The title compound, [Cu(C₁₁H₁₂N₄O₃)(C₁₈H₁₅P)₂]PF₆, is the third example reported in the literature of a five-coordinated Cu^IP₂NO₂ system. The metal is coordinated to both PPh₃ mol­ecules through the P atoms and to the pyrazine ring of the lumazine mol­ecule through an N atom in a trigonal–planar arrangement; two additional coordinated O atoms, at Cu—O distances longer than 2.46 Å, complete the coordination. The coordination environment can be described as an inter­mediate square-pyramidal/trigonal–bipyramidal (SP/TBP) polyhedron.

Binding modes in metal ion complexes of quinones and semiquinone radical anions: electron-transfer reactivity

9,10-Phenanthrenequinone (PQ) and 1,10-phenanthroline-5,6-dione (PTQ) form 1:1 and 2:1 complexes with metal ions (M (n+)=Sc (3+), Y (3+), Mg (2+), and Ca (2+)) in acetonitrile (MeCN), respectively. The binding constants of PQ--M (n+) complexes vary depending on either the Lewis acidity or ion radius of metal ions. The one-electron reduced species (PTQ(-)) forms 1:1 complexes with M (n+), and PQ(-) also forms 1:1 complexes with Sc(3+), Mg(2+), and Ca(2+), whereas PQ(-) forms 1:2 complexes with Y(3+) and La(3+), as indicated by electron spin resonance (ESR) measurements. On the other hand, semiquinone radical anions (Q(-) and NQ(-)) derived from p-benzoquinone (Q) and 1,4-naphthoquinone (NQ) form Sc(3+)-bridged pi-dimer radical anion complexes, Q(-)--(Sc(3+))(n)--Q and NQ(-)--(Sc(3+))(n)-NQ (n=2 and 3), respectively. The one-electron reduction potentials of quinones (PQ, PTQ, and Q) are largely positively shifted in the presence of M (n+). The rate constant of electron transfer from CoTPP (TPP(2-)=dianion of tetraphenylporphyrin) to PQ increases with increasing the concentration of Sc(3+) to reach a constant value, when all PQ molecules form the 1:1 complex with Sc(3+). Rates of electron transfer from 10,10'-dimethyl-9,9'-biacridine [(AcrH)(2)] to PTQ are also accelerated significantly by the presence of Sc(3+), Y(3+), and Mg(2+), exhibiting a first-order dependence with respect to concentrations of metal ions. In contrast to the case of o-quinones, unusually high kinetic orders are observed for rates of Sc(3+)-promoted electron transfer from tris(2-phenylpyridine)iridium(III) [Ir(ppy)(3)] to p-quinones (Q): second-order dependence on concentration of Q, and second- and third-order dependence on concentration of Sc(3+) due to formation of highly ordered radical anion complexes, Q()--(Sc(3+))(n)--Q (n=2 and 3).

Complex and Charge Transfer between TiO2 and Pyrroloquinoline Quinone

Pyrroloquinoline quinone (PQQ) forms a tridentate complex with coordinatively unsaturated titanium atoms on the surface of approximately 4.5 nm TiO2 particles; an association constant of K = 550 M-1 per Ti(IV)surf has been determined. Low-temperature electron paramagnetic resonance was employed in identification of localized charges and consequently produced radicals and in determination of charge-transfer processes. The photoexcitation of the PQQ-TiO2 complex results in the transfer of conduction band electrons from TiO2 to bound PQQ and the formation of the semiquinone radical. Attaching dopamine (DA) as an electron donor and PQQ as an electron acceptor on the surface of TiO2 results in spatial separation of photogenerated charges; the holes localize on dopamine and electrons on PQQ, with higher yields than for each component separately. In this triad-type assembly (PQQ-TiO2/DA) the PQQ that is bound to the particles acts as a sink for electrons allowing their almost complete scavenging even at temperature as low as 4 K.

Structural, electronic, and magnetic consequences of O-carbonyl vs O-alkoxy ester coordination in new dicopper complexes containing the Cu2(mu-Cl)2 core

The complexes [Cu2(mu-Cl)2(Cl)2(L)2] (L = dialkylpyridine-2,6-dicarboxylate; R = Et, L = depc, 1; R = i-Pr, L = dppc, 2) have been prepared and their magnetic properties studied. The crystal structures of complexes 1 and 2 have been solved. Compound 1 belongs to the P space group with Z = 2, a = 8.3020(10) A, b = 9.2050(10) A, c = 10.065(2) A, alpha = 99.040(10), beta = 100.810(10), and gamma = 106.502(10) whereas 2 belongs to the C2/c space group with Z = 8, a = 11.6360(10) A, b = 25.906(3) A, c = 11.76579(10) A, and beta = 107.900(10). The different alkyl ester substitutes produce substantial structural and electronic differences. The Cu2Cl2 core geometry is planar for 1 whereas it adopts a butterfly shape in the case of 2. Furthermore, in 2 the dppc ligand coordinates only by the carbonyl oxygen atoms whereas in 1 the depc ligand coordinates through carbonyl and alkoxy oxygen atoms. Magnetic susceptibility data show a ferromagnetic coupling between the two Cu(II) centers in both cases (J = 39.9(6) cm(-1) for 1, and J = 51.3(5) cm(-1) for 2) with very weak antiferromagnetic interactions (J ' = -0.59 cm(-1) and -0.57 cm(-1) for 1 and 2, respectively). Theoretical calculations at the extended Hückel level have also been carried out to further understand the electronic nature of complexes 1 and 2.