PQQ

Carbon monoxide (qv), eg, by-product CO from phosphoms manufacture or extracted from synthesis gas, is freed of acidic gases and absorbed in 50—80 wt % KOH at 100—200°C at a partial pressure of Pqq > Pa (>100 psi). The reaction is fairly slow. 

Methoxatin, now known as coenzyme PQQ, was originally obtained from methylotrophic bacteria but is now known to be a mammalian cofactor, for example, for lysyl oxidase and dopamine p-hydroxylase. The first synthesis of this rare compound was accomplished by the route outlined below. In the retrosynthetic analysis both of the heterocyclic rings were disconnected using directly keyed transforms. 

In addition to the oxidation-reduction potentials data, two sets of infrared carbonyl stretching frequencies were correlated with eqs. (2) and (30). Of these, one set, Pqq for 2-substituted 1,4-naphthoquinones, gave significant results, with Pr of about 50. While the other set did not give significant correlation, it contained only four points. Although the sharp difference between pr for vqq and pR for Ep correlations of 2-substituted 1,4-naphthoquinones is worthy of note, it should not be discussed until it is confirmed by further work. 

We now want to estimate the CO coverage when the catalyst is located in a plug-flow reactor with a partial pressure of Pqq = 0-01 bar at T= 1000 K. The desorption energy is estimated to be 147 kj mol and the pre-exponential factor is set to the usual 10 s , while the sticking coefScient is estimated to be 0.2 and independent of temperature. For simplicity we assume that each Ni atom can adsorb a CO molecule. 

Conversion of methanol into formaldehyde by methanol dehydrogenase. A complex array of genes is involved in this oxidation and the dehydrogenase contains pyrroloquinoline quinone (PQQ) as a cofactor (references in Ramamoorthi and Lidstrom 1995). Details of its function must, however, differ from that of methylamine dehydrogenase that also contains a quinoprotein—tryptophan tryptophylquinone (TTQ). 

Alcohol dehydrogenases found in certain microorganisms utilize a pyrroloquino-line quinone (PQQ) or flavin cofactor to pass electrons released upon oxidation of alcohols to the heme electron-acceptor protein, cytochrome c. These membrane-associated alcohol dehydrogenases form part of a respiratory chain, and the energy from fuel oxidation therefore contributes to generation of a proton gradient across... 

There have been a number of reports of electrocatalysis of alcohol oxidation using immobilized PQQ-dependent alcohol dehydrogenases or flavin-containing alcohol dehydrogenases or oxidases with dissolved mediators in solution. Co-immobihzing the mediator with the enzyme is advantageous, as set out in Section 17.1, and several such strategies have been employed for electrocatalytic alcohol oxidation. 

Figure 17.11 Schematic representation of an approach for achieving efficient electrocatalysis of glucose oxidation by glucose dehydrogenase on Au nanoparticles tethered on an Au electrode. The nanoparticles are modified with a PQQ-capped linker that interacts with the unoccupied PQQ site of cofactor-deficient glucose dehydrogenase [Zayats et al., 2005].

Minteer and co-workers have also exploited the broad substrate specificity of PQQ-dependent alcohol dehydrogenase and aldehyde dehydrogenase from Gluconobacter species trapped within Nahon to oxidize either ethanol or glycerol at a fuel cell anode [Arechederra et al., 2007]. Although the alcohol dehydrogenase incorporates a series of heme electron transfer centers, it is unlikely that many enzyme molecules trapped within the mediator-free Nahon polymer are electronically engaged at the electrode. 

Pqq - 1 atm) with various concentrations of cosolvents added led to significant quenching of by donor solvents and gave linear Stern-Volmer type plots (e.g., f°/ f versus [THF]) with slopes (KSy) of 34 1, 26 1 and 16 1 M 1 for THF, diglyme and cyclohexene, respectively. In contrast, photolysis in 2,5-dimethyltetrahydrofuran led to quantum yields comparable to those observed in hydrocarbon solutions, an observation which reinforces the view that the ability to coordinate may be important to the quenching process. 

This transformation was demonstrated (14) by evaluating changes in the UV, IR and NMR spectra and comparing these to the spectra of authentic samples of each cluster (15., 16.). Quantum yields for the photoisomerization depicted in Equation 17 were found to be notably dependent both on the CO concentration and on the A rr. Although the resulting optical changes were the same for different A rr, the quantum yields in CO saturated cyclohexane ranged from < 10 at 405 run to 4.9 x 10 at 313 nm. Furthermore, 2 varied linearly from 1.2 x 10" at Pqq " 0.0 to 4.9 x 10 at P q - 1.0 atm for 313 nm photolysis in cyclohexane. 

As dehydrogenases (DH) are widely distributed enzymes, a number of studies have been carried out with these biocatalysts. For example, Willner el al. [20] have used a PQQ-monolayer functionalized gold electrode for the catalytic oxidation of NADH in the presence of Ca2+. In this scheme, the pyrrolo-quinoline quinine co-factor, PQQ, was covalently linked, as before for the GOx system [15, 20, 21], to the Au electrode,... 

M. Yamada, M. Elias, K. Matsushita, C.T. Migita, and O. Adachi, Escherichia coli PQQ-containing quinoprotein glucose dehydrogenase its structure comparison with other quinoproteins. Biochim. Biophys. Acta Proteins Proteomics 1647, 185-192 (2003). 

J. Okuda and K. Sode, PQQ glucose dehydrogenase with novel electron transfer ability. Biochem. Biophys. Res. Commun. 314, 793-797 (2004). 

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